Circuits for Using High Power LED s
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http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/ Home Sign Up! Browse Community Submit All Art Craft Food Games Green Home Kids Life Music Offbeat Outdoors Pets Photo Ride Science Tech Circuits for using High Power LED's by dan on January 8, 2007 Table of Contents Author: dan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 License: Attribution Non-commercial Share Alike (by-nc-sa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Intro: Circuits for using High Power LED's . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 step 1: Overview / Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 step 2: Power LED performance data - handy reference chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 step 3: Direct Power! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 step 4: The humble resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 step 5: $witching regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 step 6: The new stuff!! Constant Current Source #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 step 7: Constant current source tweaks: #2 and #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 step 8: a little micro makes all the difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 step 9: another dimming method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 step 10: the analog adjustable driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 step 11: an *even simpler* current source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Related Instructables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Advertisements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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Transcript of Circuits for Using High Power LED s
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
Home Sign Up! Browse Community Submit
All Art Craft Food Games Green Home Kids Life Music Offbeat Outdoors Pets Photo Ride Science Tech
Circuits for using High Power LED'sby dan on January 8, 2007
Table of Contents
Author: dan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
License: Attribution Non-commercial Share Alike (by-nc-sa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Intro: Circuits for using High Power LED's . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 1: Overview / Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 2: Power LED performance data - handy reference chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 3: Direct Power! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 4: The humble resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 5: $witching regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
step 6: The new stuff!! Constant Current Source #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
step 7: Constant current source tweaks: #2 and #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
step 8: a little micro makes all the difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
step 9: another dimming method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
step 10: the analog adjustable driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
step 11: an *even simpler* current source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Related Instructables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
Author: dan
Author: dan MonkeyLectricDan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products. He also writes aDIY column for Momentum magazine.
License: Attribution Non-commercial Share Alike (by-nc-sa)
Intro: Circuits for using High Power LED'sHigh-power LED's: the future of lighting!
but... how do you use them? where do you get them?
1-watt and 3-watt Power LED's are now widely available in the $3 to $5 range, so i've been working on a bunch of projects lately that use them. in the process it wasbugging me that the only options anyone talks about for driving the LED's are: (1) a resistor, or (2) a really expensive electronic gizmo. now that the LED's cost $3, itfeels wrong to be paying $20 for the device to drive them!
So I went back to my "Analog Circuits 101" book, and figured out a couple of simple circuits for driving power LED's that only cost $1 or $2.
This instructable will give you a blow-by-blow of all the different types of circuits for powering Big LED's, everything from resistors to switching supplies, with some tips onall of them, and of course will give much detail on my new simple Power LED driver circuits and when/how to use them (and i've got 3 other instructables so far that usethese circuits). Some of this information ends up being pretty useful for small LED's too
here's my other power-LED instructables, check those out for other notes & ideas
step 1: Overview / PartsThere are several common methods out there for powering LED's. Why all the fuss? It boils down to this:1) LED's are very sensitive to the voltage used to power them (ie, the current changes a lot with a small change in voltage)2) The required voltage changes a bit when the LED is put in hot or cold air, and also depending on the color of the LED, and manufacturing details.
so there's several common ways that LED's are usually powered, and i'll go over each one in the following steps.
Parts
This project shows several circuits for driving power LED's. for each of the circuits i've noted at the relevant step the parts that are needed including part numbers thatyou can find at www.digikey.com . in order to avoid much duplicated content this project only discusses specific circuits and their pros and cons. to learn more aboutassembly techniques and to find out LED part numbers and where you can get them (and other topics), please refer to one of my other power LED projects.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
step 2: Power LED performance data - handy reference chartBelow are some basic parameters of the Luxeon LED's which you will use for many circuits. I use the figures from this table in several projects, so here i'm just puttingthem all in one place that i can reference easily.
Luxeon 1 and 3 with no current (turn-off-point):white/blue/green/cyan: 2.4V drop (= "LED forward voltage")red/orange/amber: 1.8V drop
Luxeon-1 with 300mA current:white/blue/green/cyan: 3.3V drop (= "LED forward voltage")red/orange/amber: 2.7V drop
Luxeon-1 with 800mA current (over spec):all colors: 3.8V drop
Luxeon-3 with 300mA current:white/blue/green/cyan: 3.3V dropred/orange/amber: 2.5V drop
Luxeon-3 with 800mA current:white/blue/green/cyan: 3.8V dropred/orange/amber: 3.0V drop (note: my tests disagree with spec sheet)
Luxeon-3 with 1200mA current:red/orange/amber: 3.3V drop (note: my tests disagree with spec sheet)
Typical values for regular "small" LED's with 20mA are:red/orange/yellow: 2.0 V dropgreen/cyan/blue/purple/white: 3.5V drop
step 3: Direct Power!Why not just connect your battery straight to the LED? It seems so simple! What's the problem? Can I ever do it?
The problem is reliability, consistency & robustness. As mentioned, the current through an LED is very sensitive to small changes in the voltage across the LED, and alsoto the ambient temperature of the LED, and also to the manufacturing variances of the LED. So when you just connect your LED to a battery you have little idea howmuch current is going through it. "but so what, it lit up, didn't it?". ok sure. depending on the battery, you might have way too much current (led gets very hot and burnsout fast), or too little (led is dim). the other problem is that even if the led is just right when you first connect it, if you take it to a new environment which is hotter or colder,it will either get dim or too bright and burn out, because the led is very temperature sensitive. manufacturing variations can also cause variability.
So maybe you read all that, and you're thinking: "so what!". if so, plow ahead and connect right to the battery. for some applications it can be the way to go.
- Summary: only use this for hacks, don't expect it to be reliable or consistent, and expect to burn out some LED's along the way.
- One famous hack that puts this method to outstandingly good use is the LED Throwie.
Notes:
- if you are using a battery, this method will work best using *small* batteries, because a small battery acts like it has an internal resistor in it. this is one of the reasons theLED Throwie works so well.
- if you actually want to do this with a power-LED rather than a 3-cent LED, choose your battery voltage so that the LED will not be at full power. this is the other reasonthe LED Throwie works so well.
step 4: The humble resistorThis is by far the most widely used method to power LED's. Just connect a resistor in series with your LED(s).
pros:- this is the simplest method that works reliably- only has one part- costs pennies (actually, less than a penny in quantity)
cons:- not very efficient. you must tradeoff wasted power against consistent & reliable LED brightness. if you waste less power in the resistor, you get less consistent LEDperformance.- must change resistor to change LED brightness- if you change power supply or battery voltage significantly, you need to change the resistor again.
How to do it:
There are a lot of great web pages out there already explaining this method. Typically you want to figure out:- what value of resistor to use- how to connect your led's in series or parallel
There's two good "LED Calculators" I found that will let you just enter the specs on your LED's and power supply, and they will design the complete series/parallel circuitand resistors for you!
http://led.linear1.org/led.wizhttp://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng
When using these web calculators, use the Power LED Data Handy Reference Chart for the current and voltage numbers the calculator asks you for.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
if you are using the resistor method with power LED's, you'll quickly want to get a lot of cheap power resistors! here's some cheap ones from digikey: "Yageo SQP500JB"are a 5-watt resistor series.
step 5: $witching regulatorsSwitching regulators, aka "DC-to-DC", "buck" or "boost" converters, are the fancy way to power an LED. they do it all, but they are pricey. what is it they "do" exactly? theswitching regulator can either step-down ("buck") or step-up ("boost") the power supply input voltage to the exact voltage needed to power the LED's. unlike a resistor itconstantly monitors the LED current and adapts to keep it constant. It does all this with 80-95% power efficiency, no matter how much the step-down or step-up is.
Pros:- consistent LED performance for a wide range of LED's and power supply- high efficiency, usually 80-90% for boost converters and 90-95% for buck converters- can power LED's from both lower or higher voltage supplies (step-up or step-down)- some units can adjust LED brightness- packaged units designed for power-LED's are available & easy to use
Cons:- complex and expensive: typically about $20 for a packaged unit.- making your own requires several parts and electrical engineering skillz.
One off-the-shelf device designed specially for power-led's is the Buckpuck from LED Dynamics. I used one of these in my power-led headlamp project and was quitehappy with it. these devices are available from most of the LED web stores.
step 6: The new stuff!! Constant Current Source #1lets get to the new stuff!
The first set of circuits are all small variations on a super-simple constant-current source.
Pros:- consistent LED performance with any power supply and LED's- costs about $1- only 4 simple parts to connect- efficiency can be over 90% (with proper LED and power supply selection)- can handle LOTS of power, 20 Amps or more no problem.- low "dropout" - the input voltage can be as little as 0.6 volts higher than the output voltage.- super-wide operation range: between 3V and 60V input
Cons:- must change a resistor to change LED brightness- if poorly configured it may waste as much power as the resistor method- you have to build it yourself (oh wait, that should be a 'pro').- current limit changes a bit with ambient temperature (may also be a 'pro').
So to sum it up: this circuit works just as well as the step-down switching regulator, the only difference is that it doesn't guarantee 90% efficiency. on the plus side, it onlycosts $1.
Simplest version first:
"Low Cost Constant Current Source #1"
This circuit is featured in my simple power-led light project.
How does it work?
- Q2 (a power NFET) is used as a variable resistor. Q2 starts out turned on by R1.
- Q1 (a small NPN) is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing.
- The main current flow is through the LED's, through Q2, and through R3. When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2.Turning off Q2 reduces the current through the LED's and R3. So we've created a "feedback loop", which continuously monitors the LED current and keeps it exactly atthe set point at all times. transistors are clever, huh!
- R1 has high resistance, so that when Q1 starts turning on, it easily overpowers R1.
- The result is that Q2 acts like a resistor, and its resistance is always perfectly set to keep the LED current correct. Any excess power is burned in Q2. Thus for maximumefficiency, we want to configure our LED string so that it is close to the power supply voltage. It will work fine if we don't do this, we'll just waste power. this is really theonly downside of this circuit compared to a step-down switching regulator!
setting the current!
the value of R3 determines the set current.
Calculations:- LED current is approximately equal to: 0.5 / R3- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3. choose a resistor value at least 2x the power calculated so the resistor does not get burninghot.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
so for 700mA LED current:R3 = 0.5 / 0.7 = 0.71 ohms. closest standard resistor is 0.75 ohms.R3 power = 0.25 / 0.71 = 0.35 watts. we'll need at least a 1/2 watt rated resistor.
Parts used:
R1: small (1/4 watt) approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)R3: large (1 watt+) current set resistor. (a good 2-watt choice is: Panasonic ERX-2SJR series)Q2: large (TO-220 package) N-channel logic-level FET (such as: Fairchild FQP50N06L)Q1: small (TO-92 package) NPN transistor (such as: Fairchild 2N5088BU)
Maximum limits:
the only real limit to the current source circuit is imposed by NFET Q2. Q2 limits the circuit in two ways:
1) power dissipation. Q2 acts as a variable resistor, stepping down the voltage from the power supply to match the need of the LED's. so Q2 will need a heatsink if thereis a high LED current or if the power source voltage is a lot higher than the LED string voltage. (Q2 power = dropped volts * LED current). Q2 can only handle 2/3 wattbefore you need some kind of heatsink. with a large heatsink, this circuit can handle a LOT of power & current - probably 50 watts and 20 amps with this exact transistor,but you can just put multiple transistors in parallel for more power.
2) voltage. the "G" pin on Q2 is only rated for 20V, and with this simplest circuit that will limit the input voltage to 20V (lets say 18V to be safe). if you use a differentNFET, make sure to check the "Vgs" rating.
thermal sensitivity:
the current set-point is somewhat sensitive to temperature. this is because Q1 is the trigger, and Q1 is thermally sensitive. the part nuber i specified above is one of theleast thermally sensitive NPN's i could find. even so, expect perhaps a 30% reduction in current set point as you go from -20C to +100C. that may be a desired effect, itcould save your Q2 or LED's from overheating.
step 7: Constant current source tweaks: #2 and #3these slight modifications on circuit #1 address the voltage limitation of the first circuit. we need to keep the NFET Gate (G pin) below 20V if we want to use a powersource greater than 20V. it turns out we also want to do this so we can interface this circuit with a microcontroller or computer.
in circuit #2, i added R2, while in #3 i replaced R2 with Z1, a zener diode.
circuit #3 is the best one, but i included #2 since it's a quick hack if you don't have the right value of zener diode.
we want to set the G-pin voltage to about 5 volts - use a 4.7 or 5.1 volt zener diode (such as: 1N4732A or 1N4733A) - any lower and Q2 won't be able to turn all the wayon, any higher and it won't work with most microcontrollers. if your input voltage is below 10V, switch R1 for a 22k-ohm resistor, the zener diode doesn't work unless thereis 10uA going through it.
after this modification, the circuit will handle 60V with the parts listed, and you can find a higher-voltage Q2 easily if needed.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
step 8: a little micro makes all the differenceNow what? connect to a micro-controller, PWM or a computer!
now you've got a fully digital controlled high-power LED light.
the micro-controller's output pins are only rated for 5.5V usually, that's why the zener diode is important.
if your micro-controller is 3.3V or less, you need to use circuit #4, and set your micro-controller's output pin to be "open collector" - which allows the micro to pull down thepin, but lets the R1 resistor pull it up to 5V which is needed to fully turn on Q2.
if your micro is 5V, then you can use the simpler circuit #5, doing away with Z1, and set the micro's output pin to be normal pull-up/pull-down mode - the 5V micro canturn on Q2 just fine by itself.
now that you've got a PWM or micro connected, how do you make a digital light control? to change the brightness of your light, you "PWM" it: you blink it on and offrapidly (200 Hz is a good speed), and change the ratio of on-time to off-time.
this can be done with just a few lines of code in a micro-controller. to do it using just a '555' chip, try this circuit. to use that circuit get rid of M1, D3 and R2, and their Q1 isour Q2.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
step 9: another dimming methodok, so maybe you don't want to use a microcontroller? here's another simple modification on "circuit #1"
the simplest way to dim the LED's is to change the current set-point. so we'll change R3!
shown below, i added R4 an a switch in parallel with R3. so with the switch open, the current is set by R3, with the switch closed, the current is set by the new value of R3in parallel with R4 - more current. so now we've got "high power" and "low power" - perfect for a flashlight.
perhaps you'd like to put a variable-resistor dial for R3? unfortunately, they don't make them in such a low resistance value, so we need something a bit morecomplicated to do that.
(see circuit #1 for how to choose the component values)
step 10: the analog adjustable driverThis circuit lets you have an adjustable-brightness, but without using a microcontroller. It's fully analog! it costs a little more - about $2 or $2.50 total - i hope you won'tmind.
The main difference is that the NFET is replaced with a voltage regulator. the voltage regulator steps-down the input voltage much like the NFET did, but it is designed sothat its output voltage is set by the ratio between two resistors (R2+R4, and R1).
The current-limit circuit works the same way as before, in this case it reduces the resistance across R2, lowering the output of the voltage regulator.
This circuit lets you set the voltage on the LED's to any value using a dial or slider, but it also limits the LED current as before so you can't turn the dial past the safepoint.
I used this circuit in my RGB Color Controlled Room/Spot lighting project.
please see the above project for part numbers and resistor value selection.
this circuit can operate with an input voltage from 5V to 28V, and up to 5 amps current (with a heatsink on the regulator)
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
step 11: an *even simpler* current sourceok, so it turns out there's an even simpler way to make a constant-current source. the reason i didn't put it first is that it has at least one significant drawback too.
This one doesn't use an NFET or NPN transistor, it just has a single Voltage Regulator.
Compared to the previous "simple current source" using two transistors, this circuit has:
- even fewer parts.- much higher "dropout" of 2.4V, which will significantly reduce efficiency when powering only 1 LED. if you're powering a string of 5 LED's, perhaps not such a big deal.- no change in current set-point when temperature changes- less current capacity (5 amps - still enough for a lot of LED's)
how to use it:
resistor R3 sets the current. the formula is: LED current in amps = 1.25 / R3
so for a current of 550mA, set R3 to 2.2 ohmsyou'll need a power resistor usually, R3 power in watts = 1.56 / R3
this circuit also has the drawback that the only way to use it with a micro-controller or PWM is to turn the entire thing on and off with a power FET.
and the only way to change the LED brightness is to change R3, so refer to the earlier schematic for "circuit #5" which shows adding a low/high power switch in.
regulator pinout:ADJ = pin 1OUT = pin 2IN = pin 3
parts:regulator: either LD1585CV or LM1084IT-ADJcapacitor: 10u to 100u capacitor, 6.3 volt or greater (such as: Panasonic ECA-1VHG470)resistor: a 2-watt resistor minimum (such as: Panasonic ERX-2J series)
you can build this with pretty much any linear voltage regulator, the two listed have a good general performance and price. the classic "LM317" is cheap, but the dropoutis even higher - 3.5 volts total in this mode. there are now a lot of surface mount regulators with ultra-low dropouts for low current use, if you need to power 1 LED from abattery these can be worth looking into.
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
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Comments50 comments Add Comment view all 248 comments
Orikson says: Feb 10, 2010. 10:31 AM REPLYHey there,
First of all, I guess the symbol for Q2 is wrong. You write, that you are using a N-channel FET, but in the scematics there are p-channel FET symbols!
Also I'm trying to build this circuit, but it isn't working as I expected. I'm tried a 100k and 1M for R1, 0.85 Ohm (5 watt) for R3 (should be around 600mA),white power LED (3,4 - 3,6V @ 700mA), BD548C for Q1 and IRL540N for Q2 because I already have them. The power supply was a regulated powersupply.I set it to 5 Volts and set the current limiter to 100 mA which I increased slowly up to 600mA.
The problem is, that the voltage wasn't regulated by this circuit! In the end the LED got 4 V at 550mA!
So where is the problem? My Q1 and Q2 seems to have nearly the same technical data, don't they?
mjhilger says: Mar 24, 2010. 9:11 AM REPLYQ1 & R3 set up a clasic constant current supply (by def voltage will vary and is not regulated). The base emitter junction drop is approximately 0.7 volts,as the author noted, this voltage will vary as temp varies (temp variation of diode junstion - exp of absolute kelvin temp). The current is set by 0.7/R3. He set the circuit to effect the gate voltage and allow the FET to regulate the power (heat), the R3 power drop is constant i^2 * r, but the FET power willdepend on the current, led voltage drop and supply voltage P(fet) = (V+ - V(led) - 0.7) * (0.7 / R3). As you noted the drop of the LED is not as listed,welcome to the real world. This is usually a result of heat build up within the LED, but could be normal for your LED. While this circuit is not the most efficient from a power view, it is easily one of the best to understand and sets the current with a minimum ofcomponents. This is a good circuit and should perform well with a wide power supply tolerance. Just monitor the heat dissapated by the FET.
xsmurf says: Dec 6, 2007. 7:59 PM REPLYIn the FET circuit, can I dim the leds by changing R1 to a potentiometer or multi-position switch?
dan says: Dec 6, 2007. 8:19 PM REPLYno. see the dimming options in the following steps
http://www.instructables.com/id/Pulse-Width-Modulated-LED-Torch/?utm_source=pdf&utm_campaign=related
http://www.instructables.com/id/Pulse-Width-Modulated-LED-Torch/?utm_source=pdf&utm_campaign=related
http://www.instructables.com/id/Pulse-Width-Modulated-LED-Torch/?utm_source=pdf&utm_campaign=related
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
gopskochi says: Mar 3, 2010. 11:25 PM REPLYcant i adjust the brightness by connecting a pot instead of R1, pls can u brief the theory behind how R3 adjust the current in the led.
bobbyg says: Jan 6, 2008. 6:52 AM REPLYDear dan, I am designing a light for argicultural applications I want to use 36 k2s red 700 ma 2 watt at 75 lm and 10 royal blue at 620mw 1500maappx 7 watts I am disturbed because they are charging me almost $300 for the materials and only $100 of it is actually leds the drivers are so much Iwant to do a 24 x 24" box w/6" overhang where do I start?
gopskochi says: Mar 3, 2010. 11:22 PM REPLYcan i adjust led brightness by making R1 a variable resistor, also i am using a lead acid battery, which will not be connected to any charging circuits whenlighting leds. will the brightness remain constant for a few hours as the battery discharges.
lamikam says: Feb 24, 2010. 4:46 PM REPLYHi. I want to use circuit #5 example, but I bought some RGB leds from Hong Kong (cheap) but they are all common anode instead of common cathode.How would I change circuit #5 to handle this?
Thanks!
osmana says: Feb 20, 2010. 12:15 PM REPLYhi all,i want to drive 12x3w power LED's. LED's forward voltages are 3.2V. Total i need 38.4V. How can i drive it.
conntaxman says: Jan 27, 2010. 9:16 PM REPLYI want to 3watt led, forward voltage 3.5/4 at 700ma draw.Will a 7806 IC work.I want to put the led in series.guess the cir is just the basic.Also what is themost of the same leds that you could run in series. It will be of a 12 dc source.TksJohn
andybuda says: Jan 20, 2010. 10:36 AM REPLYi want to make a few leds that get charged up in the day and switch on at night ... i need a solar cell photo cell, relay switch and rechargable battries anyideas how to connect the thing up its to light up a plant at night and charge by day...
doctek says: Feb 2, 2008. 12:48 PM REPLYHow did you make these measurements? One way I can think of is to put appropriate current limiting resistors in series with a led and measure the voltagedrop across the led. Is that indeed what you did or is there a much cleverer method I'm missing?
Nyxius says: Jan 17, 2010. 5:24 PM REPLYLED are incapable of limiting current on it's own. Therefore, voltage drop across the LED is dependent on total potential and total load in series with theLED. Too much potential and not enough load means that the voltage drop across the LED will be high enough to "fry" the LED. This is because LEDhave very small impedance compared to regular light bulbs. The voltages listed on the spec sheet are recommended drops that will ensure long lastinglifetime.
kreston says: Dec 12, 2009. 4:04 AM REPLY Hi there. I have a 70watt single led. In the item description it says 24-28V 2800mA
I'm planning to use LD1585CV with 30v input (220v to 30v transformer with bridge rectifier)like in this webpage : www.electronics-lab.com/projects/power/016/index.html
do you think its ok?
conntaxman says: Oct 31, 2009. 8:58 AM REPLYHello Dan and everyone else.Im trying to find a cir, to run 2 of these 3 watt leds, and I looked all over.I will be useing a 12v car battary for the source power.Iknow that the voltage can go up to 14vdc.here is the spec.for the 3 watt led.voltage in 3.0v-3.27v Current 700ma - brightness 80 Lumes - color temp 6700k-.This is going to be used when the power goes of in the house.I want to put one of these in each room for light.I was thinking of just putting 5 in seires andthat would be 15 volts.I know about the brightness will varry,but I dont think that it would be that much.Also I know I would need a heat sink on these.So Iguess I would like to use 3-4-5 of these in one light.Thanks for the help and I did read many of the cir. here.This is not my trade,And that is why I ask for help.LOL.I did build a wind-mill from this site also.This is a good site,with many smart people.and Friendly people.Thank you all and have a nice day [or] night.Johnny
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
mrmarshall1 says: Sep 29, 2009. 6:58 PM REPLYdoes anyone know where I can get a driver for this
http://cgi.ebay.com/1x-High-Quality-30W-White-1800-Lumen-Save-Power-LED-F_W0QQitemZ270459275669QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3ef8a10995&_trksid=p3286.c0.m14#ht_1990wt_950
led? i want it to run off 120 volts if possible, but 12 to 24 volts is fine to. if you could do this it would be greatly appreatiated for my first Instructable.
Re-design says: Sep 27, 2009. 5:19 AM REPLYGreat info. I've got it bookmarked so I can really study it later.
Mezbah says: Sep 16, 2009. 8:16 AM REPLYHi Dan Thanks a lot for the instructrable. Your circuits are very nice. I would like to know if I can drive, say about 200 white high bright LED directly from 220AC by any of your design of this instructrable? What modification, if any I need for that. The idea is to drive as much LED as possible and practicable for mynew LED tube light project. Thanks alot in advance and have a good day
DarkRage3 says: Sep 1, 2009. 9:52 AM REPLYWhile I'm not one to normally take the quick and easy way out, however I was hoping to get a suggestion on a SIMPLE LEDsetup for a light-box I'mconstructing for my son. Space is very limited and there are currently multiple variables that are up in the air because of my lack of knowledge in the subject.The box would have at least two settings. I'm completely new to LEDs and have only rudimentary, and mostly theoretical knowledge of circuits. Normally I'ddo extensive research for a project like this, but time is not on my side right now. ANY help would be appreciated.
Possible parts I've been looking into include:
-random DC power adapters ranging from about 5V - 19V
-a wide range of LEDs (because of my lack of knowledge I can't choose)
TOP CONTENDERS:
--5mm InGaN White Slow Burning* wavelength(nm): 6000~12000KVf(V): Typ.3.2 Max.3.6 mcd:400~1000half intensity angle(deg):85°~100°http://ledz.com/led.datasheet/599PWC.pdf
--5mm White Slow Burning* wavelength(nm): 6000~12000KVf(V): Typ.3.2 Max.3.6 mcd:1000~2000half intensity angle(deg):60°~70°http://ledz.com/led.datasheet/560PWD.pdf
The exact number of LEDs to be used is also up in the air because I'm uncertain of the brightness and uniform distribution of light. Best guess, (based on thenumber of LEDs on a prefab. LED strip I found) 66 - 108 LEDs
I'm attempting to use the two diffuser screens of an old projection TV (it seems like it should work well with the lights shining through the sides, toward thecenter) and an evenly white frosted glass from an old cabinet (looks just like the real light-box diffuser screens) so the number could be considerably less,however I have no single LEDs of comparable power to test it.
Again I'm unaccustomed to asking for help, but ANY at all would be greatly appreciated, Thank you.
The Lightning Stalker says: Aug 28, 2009. 12:02 AM REPLYThese circuits are great. If you want to interface it to a microcontroller, you should really use a logic level FET. Otherwise make sure that the turn-on voltageis low enough. A lot of commonly available MOSFETs don't fully turn on until like 8V. That's why the IRF540 mentioned by gerojalo won't work. It will work,just not with a microcontroller or a lower input voltage.
starwarts says: Aug 21, 2009. 1:41 AM REPLYHi Dan, Can a TIP122 (BJT NPN with VBE of 2.5V) be used inplace of the NFET? What needs to be modified? Thanks
Peregrine7 says: Aug 13, 2009. 5:02 PM REPLYHi Dan,Thank you very much for so much good info here. However, I'm still at a loss on how to proceed with my project. I was wondering if you could help me fill inthe gaps. I think the analog adjustable driver would be the best option in my case.
I'm doing a circuit that will use LEDs in car taillights. So the input power will be a standard automotive 12V battery, which usually has a voltage that variesbetween 11V -14V. The LEDs will have 2 stages: full brightness when in stop light mode, limited brightness when in parklight mode. There'll be 4 identicallights, so let's concentrate on only 1:
The driver for each light will accommodate 2 series (in parallel) of 4 LEDs in each serie. So 8 LEDs total. The following are specs and parts I already have onorder:
Regulator LM317Red 1W LED, 2.1v-2.3v, 300mA
4 of the above LEDs in serie is (2.2 x 4) + 3 = 11.8v necessary to support the LM317 regulator.Since there will be 2 of those series in parallel, it will require 1.25v / (0.300A + 0.300A) = 2.08ohms R1 resistor (I believe closest is 2.1).
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
1) Can you help me fill in the gaps for the rest of the parts in your circuit (R2, R3, R4, Q1, C1) ?
2) Do you think this would be the best solution based on my above specs? The 2 stage brightness is what complicates my circuit. The R1 would be enough ifI only wanted to run them at full brightness. But since I also need them to switch to limited brightness, I think that's what your circuit can accomplish. Initially,I thought I was gonna just do 2 inputs on the regulator separated by diodes, and one input would be with some resistor that would cause the LEDs to bedimmer, but I'm not sure if that would work because I don't know how the regulator would affect that.
Thank you in advance!
rhosman says: Aug 12, 2009. 12:49 PM REPLYif I changed Q2 to something like http://www.fairchildsemi.com/ds/FD%2FFDB52N20.pdf and added a Bridge Rectifier that was 120V 20A rated I should beable to hook this circuit up directly to a US power main? Maybe through a light socket?
popeyescu says: Aug 10, 2009. 3:59 AM REPLYHi Dan,
it seems that the link to "555 circuit" doesn't work. Do you have a schema/link for this circuit?I found another schema for 555: http://www.dprg.org/tutorials/2005-11a/index.htmlIs it good?
Thanks
TS84 says: Aug 8, 2009. 12:30 PM REPLYDan can u help me,.... if i'm using IRF530 and 2N3904 for this circuit.is it works fine?can you give me advise because your listed parts are hard to find......
jasmclbs says: Jul 22, 2009. 8:53 PM REPLYHi Dan, Can you kindly help me with the components' selection? Do you mind sharing with me the part no. of Q1 and Q2 that you have used? As I amadopting circuit #5, what will be the preferred R1 and R3 values for an 5V output from my MCU I/O pin. The voltage supply to the LEDs will be +5V too. Formy LEDs, I am using OSRAM LUW W5AM which has a IF of 350mA and VF of 3.2V. (White) Please kindly advise. Thank you in advance.
soovui says: Jul 15, 2009. 6:09 PM REPLYDear friend,
I like to make use of this circuit for my LEDs about 18 LEDs in a row. It groups into 3 LEDs in serial and 4 parallel (3 LEDs in serial) in a row of 18 LEDs.Each LED is 1 Watt x 18 = 18watts in a row and also LED is 3 Watts x 18watts = 54 watts in a row.
Anyone can help me on what is the right resistors value for LED circuit 18watts and 54watts?
Your advise and help is very much appreciated.Thanks and best regards,
Stuart
alah says: Jul 10, 2009. 11:56 AM REPLYIt´s good to use your brains doing something like this, but if you prefer just using your hands you can buy a simple, up to 1A controller, ( CAT4101 )for just$1.56 at futureelectronics. You will only need one resistor. Please read and UNDERSTAND the datasheet.
bliz23 says: Jun 28, 2009. 4:05 PM REPLYDan, I've looked at everything I can think of but something is still wrong. Used your pwm circuit #5 with r3 set at 0.6 ohms (read with multimeter). The ledruns with about 505 ma of current. This is kinda strange, but i'm cool with it. So I want to increase the led current, so i decrease the resistance to 0.45 (themultimeter toggles between .4 and .5) but the current only increases to about 580ma. Theoretically it should increase to ~670 ma. The microcontrollerproduces 5v output. Any ideas on why or how to correct this? Also the leds voltage drop is about 3.2v at 580 ma and the power supply gives out about 5.5vat the load i'm working with. Any help would be nice, thanks. R1: 100k R3: 0.6 or 0.45 Q1: ? Q2: IRF510
bliz23 says: Jun 29, 2009. 8:07 PM REPLYI've been trying to play with the led's more: I have the microcontroller part separate from the rest of the circuit board. I connect the two parts together withtwo separate wires, one for ground and one to 'turn' on Q2 (the +5 pin). When i disconnect the wires on the microcontroller side, and touch just the pluswire, the led will light up and when i let go the led stays lit up. But when i touch both wires, the led will stop lighting up. Now i know there is somecapacitance in Q2, which makes sense as to why it stays lit up when i stop touching the wires, but why does it light up in the first place? Also i tried tosee if i had a natural voltage drop between me and the ground, which could cause this, and when using the multimeter, i got like 50mV drop and the ledturned off. Some very strange things are happening, if anyone has any ideas, please let me know, this is starting to drive me kinda crazy. But i figure ifnothing works out, i could just make the wire into some touch pad and see if people can light it up, almost like a magic trick, but it would be cooler to useit like i wanted to
bliz23 says: Jul 3, 2009. 10:11 PM REPLYI'm beginning to think that my power supply is not constant enough. I may need more capacitance in my smoothing section of my power supplybecause I can notice my leds flickering, which would cause the non ideal ratio, i'm hoping. Anyways, I'm going to try to find a better capacitor andhope that works, i'll let you know how it works, if anyone is reading this
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
saurabhvarma says: Jun 24, 2009. 1:30 AM REPLYHi, I am new to the world of electronics and you seem to be a person who knows a lot more than what I do, So I have a quick question for you... I have a toyRC and I want to replace it's boring and dull headlights with bright white LEDs. I want to use typical 8mm White LEDs, I think they fall in the category asbelow: Typical values for regular "small" LED's with 20mA are: green/cyan/blue/purple/white: 3.5V drop The car runs on brand new 4 x 1.2V rechargeablecells 2800 mAh (a custom upgrade from 600 mAh) The car has 2 motors. I have already soldered the new LEDs to the circuit, but they somehow don't lightup, whereas if i attach the old lights, they work just fine. What do I do? Also, while the car is turning, it tends to get jerky and it feels like the motors are notgetting enough voltage because the car seems to be running at full load at that time... I think 3V for the drive motor and 1.5V for the direction motor and 7 Vfor the LEDs. Later, I plan to use LEDs for the taillights as well which will work only when the car is reversing (2 x 2V 5 mm Red LEDs) Load on Battery:Forward: Drive Motor + 2 White LEDs (3V + 7V i.e. 3.5V+3.5V) Reverse: Drive Motor + 2 Red LEDs (3V + 4V i.e. 2V+2V) Forward while changing direction:Drive Motor + Direction Motor + 2 White LEDs (3V + 1.5 V + 7V) Reverse while changing direction: Drive Motor + Direction Motor + 2 Red LEDs (3V + 1.5 V+ 4V) So is there a way i could have regulated voltage to all the electric components so that the car doesn't slow down or gets jerky (as if it's switchingon/off) while taking turns etc? or at least tell me what do I need to have the LEDs work with having to upgrade the battery : (
FJMSoft says: Jun 3, 2009. 11:04 PM REPLYHi Dan. I want to drive 3W RGB led (1W per color) 350mA per color 2.2~2,8v red 3,0~3,8v green and blue. Can you teach me how to calculate R3 for Circuit5 and can you give me some easy to find substitutes for Q2? Can I use somethin like TIP31? Thanks a lot.
Wesley666 says: Jun 2, 2009. 5:16 PM REPLYhow much power will these work with before parts start burning out? It would be nice if this worked with 120v or 240v.
nyeskus says: May 28, 2009. 2:17 AM REPLYThank you Dan, it works very well
torptorp says: May 15, 2009. 6:54 AM REPLYDan can you help me understand your math for step 6"setting the current!
the value of R3 determines the set current.
Calculations:- LED current is approximately equal to: 0.5 / R3- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3. choose a resistor value at least 2x the power calculated so the resistor does notget burning hot.
so for 700mA LED current:R3 = 0.5 / 0.7 = 0.71 ohms. closest standard resistor is 0.75 ohms.R3 power = 0.25 / 0.71 = 0.35 watts. we'll need at least a 1/2 watt rated resistor."How are you coming up with the R3 number? If I was trying to set my current at 800mA then R3=0.5/0.8=0.625ohms? correct?R3 Power=0.25/.625=0.4 Watts So go Minimum 1/2 Watt but I would go safe with a 1Watt?
BTW my wife doesn't think my new hobby is the greatest. But I really thank you for your great instructables! I have built 2 ultimate headlamps and justordered a load of parts for "other experiments" I really appreciate the depth or research on the control circuits. I hate having to pay almost $20 for the bucks.Now I won't have too. =)The most fun I'm having is getting creative with the heatsinks and enclosure methods for a finished product. I have a number of Ideas currently in the works.
kim gregersen says: May 13, 2009. 2:33 AM REPLYHello! First off all, great circuit, thanks for showing. I have a question. I want to build a string of 5 3w led. Could i use a sistem like yours? If sow, what powersuply would i need?? and what components should i buy? The leds run at 700ma and have a forward Voltage of 3.5V~4.0V Sorry about all the questions buti'm not all that good in electronics! Thanks!!!
tiledz says: May 9, 2009. 11:30 AM REPLYSorry for my question im a noobs in led. i want to use 12 x 3.2v 300ma led on a single 12v power supply. Im thinking about 4 group of 3 led in serie with a8.2 ohm resistor. then the 4 group will be wired in parallel to the 12vdc power supply. My cricuit total is 1170.7ma. I already have a 12v 2amp power supply,will this fit or i need a smaller one? i can eassyly find a 12v 1.2 amp here. Thanks!
gerojalo says: May 8, 2009. 5:23 AM REPLYall i could manage to get hold of was an IRF540A for Q2 ... and a BC549C for Q1 ..... would these be ok ?
nukte says: Sep 4, 2007. 5:24 PM REPLYCan't find the specified voltage regulator anywhere. Do you have the specs for it or do you have a substitute number? Thanks
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
xsmurf says: Dec 9, 2007. 1:42 PM REPLYI believe you can use any voltage regulator. An LM317 for example will handle up to 1.5A. Just find one that matches your specs. Linear and TI bothmake some.
Jugfet says: May 2, 2009. 3:47 AM REPLYIf using a an LM317 be sure to get the right one ie LM317T as there are 'L' and 'M' versions that have significantly lower ratings.
geeklord says: Mar 31, 2009. 6:47 PM REPLYOkay, so I'm tryin to figure out how this works, because I always do that. I can't decide if that's a curse or a blessing :D. So Q1's base is "monitoring" thecurrent going through the LED's, the FET, and R3. If the current through that part of the circuit is too high, it pulls that gate of the FET low, lowering thecurrent through the LED's. Right?
geeklord says: Apr 12, 2009. 7:05 PM REPLYwow, I didn't think I would receive all of that, thanks! I learned some new stuff today.
jonathanjo says: Apr 12, 2009. 11:17 AM REPLYHow It Works
The gate of Q2 is held near its threshold voltage (around 1 - 2 V) by the potential divider formed by R1 and C-E of Q1. But as the voltage on the gaterises, the resistance of D-S of Q2 falls (perhaps as low as 0.02 ohm). As that happens, the voltage on the base of Q1 rises, which turns on Q1, pullingthe voltage on the gate of Q2 down. Which feeds back by increasing the D-S resistance, which lowers the voltage on the base of Q1. It reachesequilibrium quickly.
In practice, both transistors will be near their turn-on points.
I built this with the same FET (FQP 50N06L), a similar NPN (the very common 2n3904), and a single 3 W LED from Avago (ASMT-MYE0).
The FET has gate threshold of 1.0-2.5 V, the 2n3904 has base saturation voltage of 0.65-0.95 V, and the LED has a forward drop of 3.6-4.3 V, current of700 mA. (Values from datasheets.)
If the supply voltage is enough (above the forward voltage of the LEDs + the turn-on voltage of Q1), then the base of Q1 will basically be at its turn-onvoltage (measured at about 550 mV in my circuit). It depends a little on the supply voltage (see later), but mostly on the properties of Q1. It doesn'tdepend on R1 at all (tried a few values 100K, 150K etc).
This is the crux of the design: the voltage on the base of Q1 is held constant, R3 is constant, so the current is constant. As all the current (except for anegligible amount) is going through the LEDs, the D-S of the FET, and R3, we can control the current through the LEDs by the value of R3.
So now we know that R3 will have all the LED current, and drop 550 mV. Per Dan's calculation, a 1R resistor will give 550 mV = 550 mA * 1R, thus 0.55V * 0.55 A = 0.3 W.
It should be pointed out that the power used by the resistor is dependent on the turn-on voltage of Q1 and the required current. The power dissipated bythe FET is dependent on how much we have to drop the supply voltage to the forward drop voltage of the LED chain.
Attached below are the scope traces for 100R, 10R and 1R setting resistors. The top trace is the voltage at the gate of the FET, the bottom trace is thebase of the NPN. We see some start-up bounce, then the gate falls while the base rises until they are in equilibrium. It's quicker if R3 is lower.
The start-spike for 10R is shown expanded in the fourth trace. It shows a gate voltage 'shoulder', followed by a fast rise, and damped oscillation to thesupply voltage. At this point Q1 is off with Vbase at zero. With 10R, it takes a further 75 usec unti Vbase starts rising. Sorry but I don't know why that is!(Both Q1 and Q2 have switching times in nanoseconds, so my guess is that capacitive effects of the LED or the circuit in general are charging up.)
At 1R, the current should be 550 mA (confirmed on power supply) and the 1 Watt resister got warm. The FET got very hot, and the LED got finger-burning hot. (It should have had a heatsink like they told us as it has an external operating temperature of only 95 C. It lasted about 5 minutes.)
Lastly is graph of measured Vbase against Vsupply.
Hope that's of some use.
Kind regards.Jonathan
http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
dan says: Apr 12, 2009. 1:45 PM REPLYgood summary and scope charts!
minor point: the circuit depends *a little* on R1. If you use 10k, Vbe on Q1 must be a little higher to turn off Q2. maybe 0.1V higher, so you do get a10% or 20% reduction in R3 power losses is you use R1 of 100k or 200k. That's also why i spec the 2N5088BU - it has a slightly lower turn-on pointthan 2N3904 so again reduces R3 losses by 10%.
jonathanjo says: Apr 12, 2009. 3:22 PM REPLYThanks Dan.
You're right of course: I've just tried it now with R1 = 12K, and it drives up the Q1 base voltage to 631mV. When R1 was 100K or 150K I got thesame 550mV value.
gigi_boeru says: Apr 6, 2009. 3:21 AM REPLYhow can I use the 555 circuit? i don't understand from that link... can you post the complete circuit (555 & this led driver)? thank you
ddickenson says: Mar 10, 2009. 1:29 PM REPLYDoes anyone smarter than me know how to adjust the calculations to use 350mA LED's? I am using "Luxeon I Star (hex) High Power LED Red Lambertian350 mA" I forget the part number but I need to calculate this for either 4 or 8 of these led's in series.
gnodev says: Feb 1, 2009. 12:26 PM REPLYDo you know about the ZXLD1350? This simple integrated circuit costs about $1.50 (small quantity) and has a complete LED driver in it for 350mA LEDs. Ituses switching technology, so it is efficient. It can be regulated with a variable resistor, or with PWM. And you only need about 4 other components (0.3 Ohmresistor, protection zener, small coil 50 to 200 microH, capacitor) to use it.
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