Dummy Load & Power Indicator - Fists CW Club · Dummy Load & Power Indicator Often a good dummy...

QRPLoad+ Ver 1.0 October 2019

Dummy Load & Power Indicator

Often a good dummy load and being able to quickly check output power while building or check a radio is a

very useful thing to have. Many of the cheap units are not very reliable and very questionable power ratings.

In steps the QRPLoad+ module.

The Module is a very simple and so reliable dummy load that has a couple of extra features to increase its

usability. First of all it’s a decent dummy load that gives a good 50 ohm load rated at 16 watts so is just right

for QRP radios. The load is made up from 8 * 2 Watt resistors and will withstand much higher power levels for

short test’s. the Extra features are a RF detector diode circuit has been added that allows the use of a standard

volt meter (DMM) to read the RF output voltage and by use of the handy chart that comes with the module a

fast way of confirming or messuring output power while you make adjustments. The next feature is a visual RF

out indicator LED, the LED will light with under 50 milliwatts of RF up to the full capacity of the load.

The circuit may seem a bit strange in that I am taking the output voltage from the centre of the resistive

network, why? Well if you short the output at the voltage terminals the transmitters output is not shorted, it

still sees a load which at worst is 2:1 SWR so no damage should result. If we take the tap from directly across

the load resistors (as most circuits show) there is a real risk of shorting the transmitters output and hence

damage to the radio. Makes more sense now doesn’t it! The formula and charts have been adapted to suit my

configuration

The Kit.

The kit comprises of the following:- Tick these parts off when fitted

1 QRPLoad+ PCB approx. 65 x 40 mm (Extra Thick 2mm heavy Duty Board)

R1 -R8 100 Ohm 2 Watt resistors

C2 10pF disk capacitor

C3 0.01uF disk capacitor

D1 & D2 1N4148 diode

D3 1N5711 RF diode

RF IND Red LED (Can be mounted in the case instead of on the board)

The module will fit into the well-known British Mint tin with a BNC connector at one end and 2 x 4mm banana

sockets for the DVM the other

The PCB and circuit


Construction is very simple but a suggestion is to mount the resistors standing clear of the PCB by a few mm, I

slide a 5 pence piece under each resistor as I soldered it in place, this gave a small gap between the resistor

and board to allow for cooling.

There are two capacitors, both look similar. C1 is a 10nF (0.01uF) disk, it may be marked ‘103’ the other

capacitor is a 10pF disk and may be marked ‘10’, make sure you do not get the two mixed.

D1 and D2 are both 1N4148’s they are light brown with a black band. On the PCB you will see the silk screen

print shows one end with a band, the black band on the diode needs to be at the end with the band on the

PCB symbol. The two diodes are pointing in opposite directions. D3 is a 1N5711, this is a special diode that has

been selected for its low forward volt drop. D3 is blue with a black band at one end. The direction of this diode

is also shown on the PCB.

Testing the module before use.

First measure the resistance on the input to the dummy load, it should be 50 ohms give or take a few ohm’s.

Next you can check the RF output level indicator is working correctly, using just a DC input not RF at this stage,

the output voltage should be Vout=(Vin-0.2)/2

If you put 5v DC across the input then Vout will be 5-0.2/2 which is 2.4Volts, if you tested it off your 13.8v

supply you should measure :- 13.8-0.2 =13.6/2 = 6.8 Volts. If you do test it by connecting the input to 13.8V DC

the load will draw about 280mA from the supply and consume 8 watts of power, the resistors will start to get

warm at this power level so you may find that they are getting hot to the touch, be careful not to burn

yourself.

You can NOT test the RF LED this way, that will only respond to RF signals not DC.

If your module passes these tests its ready to be put into service.

Find a nice little case for it, a metal case is best but it can be fitted into a plastic box if you prefer. Many use

the good old mint tin as the module will fit in there with room to spare.

I Hope you find the QRPLoad+ useful and that it gives many years of good service.

73 Paul M0BMN

Power lookup charts (PDF copy’s available via email request to [email protected])


The Minty Load DIY Version

The minty load is basically a QRP+ dummy load with

a RF detector circuit that allows a standard Voltmeter

to be used to measure RF power out, ideal for anyone

adjusting homebrew QRP Transmitters. The Load is

designed for HF use but will still be usable on 2m

(145MHz)

The Load is built in one of the well-known small

mint tins. If you have no luck finding them near you

then search eBay/Amazon for ‘Hinged Tins’ . You will

find loads!

Simple to build but a very useful bit of kit for anyone

that build’s radio transmitters.

I have produced look up tables that can be used for quick and easy power checks.

So let’s have a look how to build the ‘Minty Load’

First let’s get the parts together,

1 x Tin of Mints

1 x BNC Socket

1 x Black 4mm Socket

1 x Red 4mm Socket

1 x 1N5711 Diode

1 x 0.01 uF Capacitor

8 x 100 ohm Resistors (2 Watt)

Ok so lets get started!

First step , remove or eat any mints! (Check the date, these can be old tins) Next step drilling the box.

You need to take your time with this step, its key to making a nice finished

unit, the box is very thin metal and easy to distort and dent. Put masking

tape around the side of the box so you can mark it easier and the drill bit

will not be so likely to wander. On one side you will need to make a hole for

the BNC socket, when deciding where to make the hole consider that

normally the lid will be closed so measure the case side of the case under

the closed lid, mine was about 13mm. Mount the BNC half way across the

box, take care, the BNC will just squeeze in the space we have.

Next make the holes for the 4mm Sockets, these are a little more forgiving

but still not a lot of room for mistakes, I mounted my socket’s 20mm in from the edge of the box on each side.


When making the holes apply only light pressure or you will damage the box. Also wear protective eye wear

and if you can some protective gloves, the box is very thin metal and you could cut yourself. If you do damage

the box a quick trip to the local shop will get you a new one! I used a drill to make small holes first and then a

hand reamer to enlarge them to suit.

Once done clean up the edges of the holes and mount the BNC socket and 4mm sockets.

Well done, that’s the difficult part done. The rest is easy.

We now need to cut and form the resistors

to make the load, there are 8 of them and

the all need to be cut and shaped to the

details below, each different shape as been

called Type A,B, or C.

The top two resistors (Type A) in the photo

have been cut with leads to 10mm each

side.

The second pair (Type B) have one leg cut to 10mm and the other leg bent 90 degrees at 5mm from the

resistor body, the rest of this bent leg is uncut.

The final 4 resistors (Type C) have one leg cut to 10mm but the other leg is again folded to 90 degrees just

5mm from the body and then bent again a further 10mm from the first bend, look at the photo here and the

other ones later in the instructions if you’re not clear on this.

Now we need to form the resistors into a network.

First build the 4 resistors that form the outer network,

take 2 of the Type C resistors and 2 Type B, looking at

the photo on the right build up this part of the

network.

Now Fit these into the box.

Tin the end of the BNC socket first. Also tin the ‘Feet’ of

the end 2 resistors. The resistors should solder easy to

the bottom of the box, its tin coated and accepts

soldering very well.

Next solder the other resistors together as in the photo (above right).

Now add these resistors to the network.


Make sure you solder each of the ‘feet’ to the bottom

of the box.

Now join the resistors in the

middle of the network. Use

the off cuts to do this.

Now the dummy load part should be complete, use a meter to check all is well and

you can read 50 ohms at the BNC socket. You should find 50 ohm’s +/- 1 ohm .

We can now move onto the parts that allow us to measure the voltage and hence the power of the transmitter

connected. Fit the 0.01uF capacitor across the solder lugs on the 4mm sockets, One important point here is

that the black 4mm socket needs a wire from its lug to the bottom of the box to make the ground connection, I

used the lead from the capacitor for this. Next connect the diode between the centre of the resistor network

and the lug on the red socket. Make sure the black band on the diode is pointing to the socket lug.

It should look like this.

That’s the Minty Load built now how to use it…

If you just wanted a dummy load for a QRP

transmitter you’re done! Connect the transmitter

to the BNC socket and you have a good match for

your transmitter, I would recommend only going

up to about 10 watts for any length of time as

the resistors will get HOT! , higher power is ok for

a short time but the network is rated at a max of

16 watts in total.

But the Minty load can be much more use than

that, lets see how to use it to check the power

output from your transmitter projects.


Using the minty Load

The RF energy fed into the load will produce a DC voltage via the low volt drop diode and capacitor, this

voltage can be measured across the two 4mm terminals, just a simple DC volt meter or a standard DMM can

be used for this, if tuning for maximum power just look for the highest voltage you can get at the terminals,

just for a quick reference 5 watts would be just over 11V.

For more accurate measurements you have a number of options, I have made credit card size lookup tables

that can be stored inside the minty load box and a chart to be stuck to the bottom of the box too. You can, if

you are so inclined calculate the power yourself armed with a calculator. The formula for this is below;-

Power= ((Volts x 1.414)+0.2)^2/50

So for example, if we find we have 13.5 volts on the terminals

Power = ((13.5 x 1.414)+0.2))^2/50

Power= (19.089+0.2)^2/50

Power= 373.065/50

Power= 7.44 watts

If you want an easier way I have put lookup tables below, small ones for putting into the minty load lid and to

one to stick under the bottom of the box, also larger ones for shack use. If you measured a voltage of 3.6V

then look for 3 volts on the left hand column, next look across the top for the nearest part volt to the value

measured, in this example closest value would be 0.5v. Where the two values intersect is the power output in

watts. So we look at 3.5v and we get just over ½ watt

If you do manage to damage the Minty load by applying 100 Watts it likely to destroy the small diode, at a

push you can use a small signal diode such as a 1N4148, the voltage drop across this diode is higher than the

low voltage type I have used so readings will be a bit lower but on a bank holiday weekend it will get you

going.

To finish your new load you may want to put a nice label on the front of the tin I have designed one for mine

and printed and laminated it before sticking it to the lid.

Good luck with the Minty Load.

73 Paul M0BMN

Print these out and keep them inside the Minty Load’s lid or read on………


Improving the Minty Load .

So far we have built a very useful bit of shack equipment. You can stop here if you wish but consider this extra

bit of work.

I often find I am building radio transmitters and have the load connected to them to keep them happy but

don’t always want to have to keep my meter connected to it to check I have RF. I had used a simple LED to

indicate RF in the past and so decided to add a simple LED RF detector to the load, this made a very

worthwhile addition to the project.

The new circuit looks like this.

I mounted this little extra bit of circuit in the lid of

the load, BECAREFUL drilling the hole for the LED or

you will distort the lid and spoil the appearance of

your load. I popped the led through the hole and

soldered one leg to the inside of the lid. That

should be the short lead of the LED.

The 1st diode is soldered one leg to the lid and the

other end (That’s the end with the band on ) to the

floating leg of the LED. This floating leg also has the

2nd diode soldered to it (again the banded end). You

will have one side of the 2nd diode just sticking out,

trim this short and connect a small thin wire to it.

Solder one end of the 10pf capacitor to the input to

the load network, the point where the BNC

connects to the resistors. Attach the floating wire

from the diode to the other leg, you may want to

put a little insulating tape over the bare ends.

That’s it now when RF is applied the led will light, I find that with a standard red 5mm LED that power levels

around 50mW will light the LED.

I have tested the load up to 50 watts for short periods of time without problem, it may take more but I would

worry about the diodes and LED, remember the load is designed for no more than 15 watts really.

The led mounted with diodes on the lid.

The picture here show

where I drilled my 5mm

hole, 13mm from the side

and 16mm down from the

hinged edge of the lid.

You could use a epoxy glue

to hold it in place although

since one leg is soldered to

the lid it stay’s in place well anyway.

I have included some ‘Art work’ for the front of the tin, feel free to use your own if you don’t like mine, also I

have enclosed an artwork for the bottom of the box with the power charts printed.


This is the artwork for the rear

This is the art work for the front, This one for

the led version.

This is the art work for the front, This one for

the standard version.

If using the led version my advice would be to use a craft knife and cut out INSIDE the green box on the

artwork (just above the words Power Input. (Only cut INSIDE the green line)

Now use a black marker and colour the area around the led on the lid as above photo

I laminated these images and after cutting out the green square for the led used very thin double sided tape to

fix it to the lid, I also stuck the chart image to the bottom of the box, here is my finished unit in use

Power

Input

+

0v

Voltage O/P

QRP+ Load & Power Detector

15 WATTS MAX Minty Load

Power

Input

+

0v

Voltage O/P

QRP+ Load & Power Detector

15 WATTS MAX Minty Load


You can see that the LED is brightly lit. The measured voltage is

4.8v

Using the chart on the back (or the large ones if you wish) find the nearest value on the chart. And read the

value

We have a little over 4.75 volts

but under 5v so the power is 1

watt

I compared it to my old but

trusted QRP power meter and it

that reads bang on 1W so I’m

happy.


Voltage to Power tables for shack use.

Dummy Load & Power Indicator - Fists CW Club · Dummy Load & Power Indicator Often a good dummy...

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