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Cheap DC Filament PS

I bought a few cheap switchers through eBay recently to see how they might work for filament supplies. The 12V models are a no-brainer; just turn the adjustment to 12.6 VDC output, and you're done. The 5V / 4A unit I bought didn't have enough adjustment, so I opened it up.

Looking at the parts, the adjustment pot is 200 ohms. The pot is in series with a 4.7K SMT chip resistor, 0603 size. I assume this is typical of most units. After some experimentation, I replaced the resistor with a leaded 3.3K 1/8W. I was able to solder it between the pot and another pad away from the original resistor, so it was a perfect fit down against the board. The supply now adjusts from roughly 5.6-6.4 VDC. As a final test, I connected the output to a 1.5 ohm resistor bank and let it run for an hour or so. The case wall used as a heatsink was only lukewarm after that time, so I think the units will be OK for this use.

I really dislike the cheap Chinese case these are in, so I'll make new ones if I put any of these into actual use. Will also replace the electrolytics with name-brand 105° caps from Mouser or DigiKey. That should improve reliability.

Now I just have to decide whether to use these in new builds, or to stick with the linear approach. A transformer/rectifier and LM338 costs more and dissipates more heat (and takes up more space), but it's hard to beat for long term use. I have LM317s working great in test gear more than 25 years old. It's not clear whether these little switchers can be expected to survive so long.

Buy Chinese. Bury freedom.

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  Kimber Kable  

Topic - Cheap DC Filament PS - Triode_Kingdom 18:38:55 06/03/17 (7)


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