Circuit Protection,Tips, andTroubleshooting

Fall 2014ECE 445

Becoming a Good Design

Engineer• Understand the problem• Understand the constraints you have• Understand previous approaches

to the problem at hand

• End goal of senior design:– Solve problems you haven’t been faced with

and be innovative

• But first: background knowledge is required

CircuitConnections

• Pay attention to polarity

Longer Lead: Positive

Electrolytic capacitor

Shorter Lead: Negative

BADGOOD

Ratings

• Determine part ratings• All components are rated• Capacitors:

– If the voltage across the capacitor is going to be 50 [V], should you use a capacitor that is rated to 50 [V]?

• Resistors: maximum power dissipation P = i2 * R or P = v2/R

– Example:– P = (5e-3)2 * 1e4– i.e. the power dissipated

without burning it out = 250 mW

From Newark Electronics

Data Sheets• By reading the data

sheet:– Provide pin-layouts– Device ratings– Potential applications

Voltage CurrentLimiting• Fuses: typically allow for

passage of “normal” current• A fuse will “blow”

above its current rating

• Diodes:• Conduct when V>0.7 V

• Best solution: use both diodes and fuses

Fuses

Voltage/CurrentProtection

Examples

Fuses and Diode Combination Current Limiting Resistor

Current across resistor typically = 20 mA

Device Polarity

• The longer length is the (+) terminal

• Capacitor

Polarity: tantalum or electrolytic

(+)(-)

The bar indicates cathode

(+) (-)

(+)• Diode

(+)(-)

No polarity: ceramic or polyester

(-)

http://electrapk.com/zener-diode/http://academic.evergreen.edu/projects/biophysics/technotes/electron/leds.htm/

Reverse PolarityProtection

Below are 2 different configurations to ensure correct voltage polarity to the circuit.

“Diode Bridge”Circuit will operate under either polarity, but will have higher losses

Circuit will not operate with incorrect polarity

Wikipedia

PowerSupplyBypass/ Decoupling

Capacitors• Bypass capacitors• Takes noise to

ground• Rephrased:

basically shunts AC signals to ground

• Typical values range from: 0.1 uF

• Larger size capacitors for higher supply voltages

*Picture http://www.physics.udel.edu/~nowak/phys645/The_bipolar_transistor_files/image013.jpg

Earth Ground vs “Ground”

Earth Ground Floating Ground

• Green terminal is earth ground

• Black terminals are signal grounds

• Know the difference!

Potentiometers

• Variable resistors– Example is a

trimpot with R=10 K Ohms

– From (a) to (c),R1 = 6 K Ohms

– From (b) to (c),R2 = 4 K Ohms (a) R1 = 6k

To Rest of Circuit (c)

(b) R2 = 4k

ResistorCodes

METRIC-TO-AWG CONVERSION TABLE

Metric Size mm2

0.5

0.8

1.0

2.0

3.0

5.0

8.0

13.0

19.0

32.0

52.0

AWGSize

20

18

16

14

12

10

8

6

4

2

0Resistor Value Chart

• Reading Surface Mount Resistors• A resistor

marked 332 is3.3 kilo-ohms

• 3K3 is 3.3 kilo-ohms

• Know wire gauges

Wire Gauges

• Wire gauge is a standard for the size of the wire (proportional to current rating)

• Typical wire in lab is 22 AWG (American Wire Gauges) – 52.9 mΩ/meter– 7 A for short wiring– 0.92 A for power

transmissionhttp://www.powerstream.com/Wire_Size.htm

• Breadboard in the lab can only use 22 AWG or smaller, otherwise it will damage the clips

http://www.how-to-wire-it.com/romex-cable.html

DrivingHigh CurrentLoad

• Most microprocessor/TTL can drive <20mA, that is approximately an LED.– Interface microprocessor I/O with a gate.

Let the gate break instead of the microprocessor!

• Methods– Relays

• may wear out and have delays

– Transistor• fast switching but have current limit

– H-bridge• allows forward and reverse current• good for motors

http://www.acroname.com/robotics/info/articles/drivers/drivers.html

Troubleshooting Steps (1/2)

1. Check supply voltage using the multimeter– Is power plugged in? Is any switch off? Is the fuse blown?

Are all the breadboard bus strips connected to VDD/GND?

2. Probe signal at intermediate stages or individual function blocks I/O– Equipment available:

• For digital signals: Logic Analyzer, LEDs• For analog signals: Oscilloscope, Voltmeter, Spectrum

Analyzer

3. Check interconnections– Is anything mis-wired? Are any wires loose? Are any contacts

bad? Is any signal floating?

Troubleshooting Steps (2/2)

4. Double check the design– Check the pin diagram– Check that you have the correct datasheet for the part

number– Re-analyze the logic, go through some calculation– Ensure correct polarity (refer to the next slide)

5. Faulty devices/breadboard (Last resort if all else fails!)– Replace/rewire one part at a time, test after every

change– Isolate the parts under test from the rest of the circuit

References

• http://www.intersil.com/data/an/ an1325.pdf• http://en.wikipedia.org/wiki/Diode

• http://en.wikipedia.org/wiki/ Fuse_(electrical)

• http://www.learnabout-electronics.org/ resistors_07.php

• http://www.rbeelectronics.com/ wtable.htm• Previous ECE 445 Lecture Slides

• Staff of the ECE Electronics Shop,

Dan Mast, Mark Smart, Skot Wiedmann