Hi fellows,
Since I’m planning to build two bench power supplies, I needed to build a current load to test them. So, I’m going to show you my version of the dummy load.
Please note that this project was built ONLY using existing parts in my personal stock.
I had to be silly to put this project on a wood box, but it was the only box I found around. I don’t recommend you to do it, instead use plastic or metal cases.
I wanted to build something simple but with a minimum protection circuit. Thus I added a control circuit to limit the maximum dissipation power to protect the main transistor.
The main specifications are as follows:
- Current regulation: 0 to 10A
- Maximum input voltage: 40V
- Maximum dissipation power: 100W (limiting circuit with warning LED)
- Analog control: 10 turn potentiometer
- Analog display: 10A ammeter
- External power: 12V/0.3A
- Enable/disable switch
- Fuse protected: 15A
If anyone has suggestions or simple improvement ideas, I’ll be glad to hear them. Probably I’m not going to change my circuit because it’s almost finished, but it could be useful to someone that could use this circuit as a start point to build their own.
Let me talk a little bit about the circuit. First I wanted to build a self-powered circuit, but since it is necessary to cool the heat sink with a fan, that was out of question. Thus the circuit is powered by an external 12V supply. A TC962 generates the negative supply (-12V) needed by the opamps. I included a RC filter for this supply to reduce the ripple noise. The voltage reference of +4.096V is generated by MCP1541.
The main transistor (BU941) is a Darlington NPN capable to conduct 15A dissipating up to 155W. The transistor tab is connected directly to the heat sink without electric isolation. This ensures a better heat transfer to the heat sink.
The sense resistor is the one included in the ammeter (~5.2mOhm). The exact value is not important because the circuit can be calibrated later with R28 (see schematic below).
The most important of all opamps is U3 because it must have a low offset. I have chosen the MAX430, which is a chopper-stabilized opamp. The maximum input offset voltage is 10µV.
The load current is set by a 10 turn potentiometer (R23). The voltage applied to R23 is controlled by the enable/disable switch and by the power limiting circuit.
The power limiting circuit is based on a simple analog multiplier (circuit around U8). This circuit has 3 inputs: Vx (Vin), Vy (Iin) and Vw (constant divisor). The output signal is defined by Vpwr = Vx*Vy/Vw. R9 adjusts Vw in order to get Vpwr = 4V @ 100W. R18 is used to set the maximum power. When the power is higher than this value, U4B will decrease its output voltage and the voltage at Iref node will decrease too. Consequently it decreases the load current. Opamp U6 is used as a comparator to detect this condition and to turn on the LED. The value of R14 was chosen in order to decrease the output voltage of U4B but also to protect the inverting input of U6 in case the voltage gets negative.
I need to run some tests in order to check if 100W is not too much for the pass transistor. Probably I will need to reduce this value down to 90W or 80W. In any case I have a new and more powerful fan to replace the existing one.