Electronic Load

I made a DIY electronic load, copying from GreatScott! video: DIY Adjustable Constant Load (Current & Power). The instructions are in his Instructable page. In my blog post I’ll just show you what I have made, and what will I do next.

Enclosure

The black box of utter nothingness.

I haven’t drilled the box because I haven’t figured out where to put the LCD, the banana jack, the rotary encoder switch, the power jack, etc. At least the box contains the whole electronics:

The electronics fit into the black box (with caveats).

The MOSFET will have to be put up outside the box, because it will get hot. The puny heatsink will also need to be replaced. I have already bought a cheap CPU heatsink and fan that I will use.

The Electronics

Here’s how the electronics look when it is powered:

In this picture, I powered it using a 12V wall wart.

Heatsink and Fan

Hopefully the heatsink is enough to cool the MOSFET under full load.

This CPU heatsink and fan will be used to cool the MOSFET, though it is too big to be put inside the enclosure black box. I haven’t figured out how to mount them outside though. This is a 12V fan 4-pin variety, the one with PWM control and integrated tachometer. However, I think I may have broken the PWM control circuitry while testing it, as now it constantly give 80Hz rotation constantly (4800rpm) regardless of PWM duty cycle I sent to the PWM pin.

Preliminary Testing

Because the MOSFET is not yet mounted to the CPU heatsink and fan I showed above, I only dare to test low voltage and low current draw: 1.5V at 0.5A. The result is that it works! The MOSFET’s puny heatsink only slightly warmed. In fact, the 7805 regulator is warmer, because I used 12V wall wart to power the electronics.

As DUT, I used my DIY adjustable power supply.

I set the power supply to 1.5V with 0.5A current limit. The electronic load is set to draw constant current of 0.5A. As you can see, my power supply’s current limiting isn’t very good. Heck, the voltage dropped to 1.45V.

After several seconds, the current draw increase 4mA, so it is looking pretty stable at this low voltage low current setting.
After about a minute, it increases another 8mA. Still good, I think. The MOSFET’s puny heatsink is also only a bit warm.
After one more minute, the current draw increased by another 19mA. Not that bad.

It is quite possible that the increasing current draw happened because the ACS712 20A module drifts, just like when I previously tested it.

To Do

  1. Mount the MOSFET to the CPU heatsink and fan.
  2. Put a header on the perfboard to power the fan.
  3. Test again with the #1 and #2 to do above.
  4. Lay out the LCD, rotary encoder switch, banana jack, power jack, and the big heatsink to make holes on the enclosure.
  5. Test again after #4, result shouldn’t change too much from #3.

There are many discussions on the internet that says linear MOSFET is better as electronic load compared to regular power MOSFET such as IRFZ44N used in this project. This short thread on EEVblog forum describes it in very beginner friendly way. For this project however, I will not change the MOSFET, as I just want to focus on finishing this.

Laters!

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