Hi to all!
I am new to the forum and thought I start by showing you one of my resent projects. It is an electronic load which can handle up to 60 watt without getting hotter than 80°C. A long time ago I did some dumpster diving and got a really nice and compact 5V GPU heatsink. I know right away that I must make a small and compact load with that ^^. Quite some time later the PCB was ordered and I assembled everything. It worked right out of the box!
….And then it did not work anymore. For some reason the Load created always a short and not the current value I wished for
not knowing if it’s the software or hardware I tried to debug the software until I got depressed and tossed it in the corner. Nostalgic feelings made me come back to it. “It wasn’t that bad right?” and it worked! I did not like that I did not understand the fault, but on the other hand was happy that it worked again. I added some new code and played around until it broke again. The same constant short and no control over the current, just like last time. I tried to debug but tossed it in the corner AGAIN, while remembering what a **** piece of**** that design was.
Hehe… a couple months later I came back to it, again forgotten the pain of the past and tried my best to figure out the problem. It worked… and then It didn’t anymore (short version).I took me hours and only after starting to do a reasonable zeroing down of the problem. As it turned out it was the hardware. The current was measured by using a resistor and sometimes I could not measure the right or no voltage over it. I had three devices connected to the heatsink and some screws. The screws did make connection with the ground of the PCB and the MOSFET tab was pressed against the metal heatsink. That did lead to the fact that the sense resistor was bridged and therefore no current could be detected.
I didn’t want to isolate the MOSFET from the heatsink, because that would have worsened the thermal conductivity. I isolated the screws instead and that worked fine. YAY happy end! Or was it?
The fault came again and it checked the screws like 100 times, they were not the problem. Just before getting crazy I noticed the temperature sensor was also metal, pressed against the heatsink and surly was connected to ground… A thin plastic foil fix the problem and I have been happy ever after ^^
This was the more or less short version, It was really heavy duty troubleshooting over a long time and I skipped some things ^^. In the end it was only the heatsink of the MOSFET and absolutely everything else worked fine.
I attached some pictures of it with a printed case that became a little bigger as I intended. I think that I could make it more or less easily 1/3 of the thickness.
But now some specs:
-Input Voltage up to 60V
-Input Current settable from 0.5mA to >40A
-constant 60W at a temp of 80°C Higher power is no problem, but not at a constant rate
-Temperature controlled quiet 5V Fan.
-Nokia 1.7inch color display I got from an “old” phone
-USB Serial interface
The software is not done jet. There is no menu and I want to implement later on some things like:
Battery capacity tester, constant power/(resistance?) mode, prettier graphics, PC control
I hope you like it!
Cheers,
Florian
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