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Zarmin
Zarmin

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Electronic load repair

I wanted my first post to be interesting, but it was a facepalm instead.

While using my aliexpress cheap electronic load to test a 12V lead-acid battery, I smelled some burnt smell magic smoke and it showed 0A of current regardless of the position of the pot meters.

Original Aliexpress photo about the electronic load

Original Aliexpress photo about the electronic load

I removed the big heatsink and before measuring / inspecting anything, I quickly replaced the IRFP260N power mosfet, because I had one in the drawer.

Electronic load without heatsink

Electronic load without heatsink (and with the replaced MOSFET)

After that it showed the same symptoms, 0A of current.

I went through some systematic analysis steps:

  1. measuring the old and new power MOSFET (old was bad, new was good), so the replacement was neccessary, but not sufficient.

  2. inspecting the whole board under a microscope checking for burnt parts: found nothing.

  3. measuring a few components for damage, such as shorted caps and shorted/open resistors, but found nothing

  4. taking photos of both sides of the PCB, merging them as semi-transparent images and try to follow the traces to reconstruct the schematics

Reconstructing the traces

Reconstructing the traces

During the reconstruction it looked like a standard electronic load design using a power MOSFET and a comparator op-amp, but I had issues drawing the schematics, because the MOSFET pins were weird...

Part of the schematics

Simple Electronic load reference circuit

My wrong schematics reconstruction vs a good simple electronic load reference circuit... Check for the MOSFET pins

It took me half a minute to understand I could fix the whole issue under 2 minutes, if I was aware of the orientation of the power MOSFET...

After flipping it, everything was fine...

Facepalm

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