LAPTOP BENCH POWER SUPPLY WITH FAN CONTROLLER

Many years ago (September 2009) I made a simple bench power supply using an LT1083 7.5A linear regulator with adjustable output voltage designed to be used with a 19.5V 3.42A laptop power supply. It still works fine after years of abuse, but my choice of output power connectors was poor (3 pin headers, the sort used for PC fans), it’s made on strip board and cooling was sometimes a problem, so time for an upgrade!

Real bench power supplies usually have features like adjustable current limiting and displays showing voltage and current. I continued to keep things simple (kind of) so none of that stuff was added, I also didn’t want to spend too much time on this project. Displays and current limiters can be easily added later anyway, going between the output of the power supply and the load.

LT1083 regulator ICs are no longer manufactured. They’re still available from China, but are most likely fakes. I found that the MIC29752 is very similar, with a few extra features like an enable pin allowing for easier power control, which is good since I wanted to remove the original power switch and use some kind of solid-state design. Without the enable pin some MOSFETs would have had to be added.

I often needed a second supply voltage so an auxiliary output was added using an LM317 regulator, with adjustable output voltage via a small trim potentiometer. The first PCB revision had the pads wired all wrong (based on an L78M), and upon powering up the board it quickly popped leaving a small hole in it.

I also noticed that the SOT-223 package of the LM317 had a much lower junction-to-board thermal resistance and higher current limit than the larger SOT-252-3/DPAK package, which seemed unusual, so I brought both types and tried them on the board. I found that the SOT-223 could dissipate far more power than the larger SOT-252 package before it started to thermally limit itself. I thought it would have been the other way around? Not to worry, the SOT-223 package sits nicely into the SOT-252 footprint.

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