Friday, 27 January 2017

A 'Brute Force' 15 Amp Power (Dummy-) Load. Easy to build!

Power-loads or Dummy-loads are expensive bits of equipment but if you need to test a powersupply or test a current protection circuit and need something that can draw a big current and is preferably regulated so you can dial in the current then this is the easiest and cheapest method.

The circuit below uses a BD182 transistor which I had lying around from an old audio amplifier. It is the equivalent to the well known 2N3055 and you can exchange the one for the other in this circuit without trouble.
The powerload needs no external power supply. It is a passive circuit but do make sure you use a big, and I mean BIG, heat-sink. The transistor will get hot very quickly.
Here is the schematic drawing of the circuit. I don't think you can make them any simpler than this.

I did away with the shunt resistor in the emitter to ground line because we don't need to measure the current. A shunt resistor is usually used for creating a feedback voltage to go into an opamp, In this case it only holds back the current so we leave it out.
I made my dummy load switchable. You can draw a big current and then by flipping the switch the current almost doubles. The lowest resistance I got from this circuit was 1.8 Ohms. That means with 12 volts connected you can draw a little under 7 Amps. and this transistor has no troubles with that (provided the heat-sink is up to it). Be sure to use a good heat sink and generous amounts of heat conducting paste around it.

Below are some pictures of how I build my power load. I build mine in an old Droste Chocolate can I had lying around. Previously I had built it in an old wooden cigar case so this is big improvement :)
Beware that the current-rise is exponential so when you turn the potmeter, the first few amps will go nice and smooth but at the end the current rises very quickly and even jumps to maximum just before the potmeter if fully turned. And yes, the potmeter I used is a linear type not a logarithmic one :)
The current fluctuates a little if you put an amp-meter in series with the load. The milliAmp digits will go up and down a bit as you read the amperes. But that's what you get when you keep it simple. The little 50nF capacitor is meant to suppress transients should they occur.
The maximum voltage from Collector to Emitter for the BD182 is 60 Volts so don't go higher then that and the maximum continuous Collector Emitter current is 15 Amps. That's a lot more then most of the circuits for powerloads you'll find online. They mostly go up to just 5 Amps. I try not to exceed 10 Amps with this design just to be on the safe side. I used a CPU heatsink on the 2N3055 which is not ideal but I mounted the transistor on a little copper sheet so that acts as a heat sink too and then I put the CPU heatsink on top of the transistor with heat conducting paste in between so the transistor can dissipate heat from the top of the TO-3 housing aswell. It's a very effective solution and works very well. If you need a power load that can draw even more current you can just put more transistors in parallel over the first one. Just connect base to base, collector to collector and emitter to emitter. (I haven't tested that btw, but it should work.)

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