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A DC Load is useful for testing lots of different things. If you have a power supply and want to make sure it’s able to deliver its fully designed potential, then you can use a DC Load to test that power supply. If you have batteries and you want to know if they’ll provide their rated capacity, then a DC Load can tell you that. I’ve also used a DC Load for testing USB cables to see which cables perform the best. There’s a lot of possibilities, and it’s a useful piece of test equipment.

I’ve built a few DC Loads before, and I even own a commercial unit, a Kunken KP184. So why build my own? Well there are a couple reasons:

I like raspberry pi based projects, because the pi is a great platform to roll out a web server that can be used to operate the project from a desktop PC or a mobile device. There’s the opportunity to build in data logging or graphic, or to set up custom ramp features for testing devices.
Building the load yourself allows you to select the MOSFETs and pick a voltage/amperage range that works for you. Sometimes I work on high voltage projects and having a range that goes into the hundreds of volts may be useful.
It’s a challenge and a learning experience. Even when you think something is simple, sometimes unforeseen complications arise and need to be worked through.

The basis of this circuit is ohm’s law. A 1 ohm current sense resistor is used to sense the amount of current that is passing from the +V to -V terminals. A 1 ohm resistor with 1 amp of current will drop exactly 1 volt. If you measure the voltage on each side of the resistor, you can easily compute the amount of current.

On the left side of the schematic we have a potentiometer and a unity gain op-amp. The unit gain stage isn’t strictly necessary, but we have a spare gate in the op-amp, so why not use it. This potentiometer gives us a variable voltage that ranges from 0V to 9V. That will be our setpoint.

The setpoint and the measurement from the current sense resistor are fed into the second gate of the op-amp. Now, when you feed an op-amp two inputs, it’s going to try to make them equal. If the current sense input is less than the setpoint, the op-amp will turn its output on. If the current sense input is greater than the setpoint, the op-amp will turn its output off. So what we have is a circuit that’s constantly evaluating it’s input and producing and output that attempts to equalize them.

The remaining component is a large power MOSFET. When you supply a gate voltage to the MOSFET, it’ll turn on and start conducting. The MOSFET conducts, completing the circuit through the current sense resistor. The more you turn the MOSFET on, the more current will flow through the resistor and the higher the resulting sense voltage. The less you turn it on, the lower the current and the lower the sense voltage.

This is a simple circuit that can often be built from parts sitting in your junk bin.

Note that C3 and R3 are something I haven’t included in my previous designs. They serve to dampen the oscillations of the circuit. Without these two components, the circuit will start to oscillate wildly about the setpoint at certain amperages.

Adding more MOSFETS
We’re going to take a little detour and talk about adding additional MOSFETs to the circuit. Why do this? Because a single MOSFET can easily be overheated. These components need to shed a lot of heat. Adding a few in parallel will be safer than a single MOSFET. In theory as a MOSFET heats up, it conducts a little less, and its load will shift to a cooler MOSFET. In theory this causes some equalization between the MOSFETs. I say “in theory” because there often seems to be some temperature differential between multiple MOSFETs in parallel.”

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