this post was submitted on 19 Jan 2026
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🔌 testing an a/c adapter before use-- 12v PSU measures at 18+v, increasing. Is that a problem? (lemmy.dbzer0.com)
submitted 2 months ago* (last edited 1 month ago) by diyrebel@lemmy.dbzer0.com to c/askelectronics@discuss.tchncs.de
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I bought a 2nd-hand 12v 2A power supply without branding. I intend to use it on a DVD player. Coming from the street market makes it dicey because anything in that market could be from someone’s dumpster dive. To ensure it’s useable I used a DMM to measure the volts. It started at 18v but continued to gradually climb. When it passed the 20v scale on the DMM, it quit reading. So it would probably go even higher in the next scale.

I expect the voltage to be higher than rated because the 12v rating is expected to be a measurement under load. But my whole point is to check whether it is safe /before/ driving the appliance. Seems strange how the volt reading kept increasing. Is that expected? Is there another test I should do?

Update

It was a lousy DMM, apparently. I retested later (again with no load) and it started at 15v and climbed up from there. That was with a tiny pocket-sized DMM. Then I tried a “Rigid” true RMS meter which gave a steady 12v.

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[–] cmnybo@discuss.tchncs.de 3 points 2 months ago

A switching power supply should be regulated. The one you have is defective. Some do have a minimum load, but I've never seen that on an adapter that can be unplugged from the load.

A linear power supply has a linear regulator and will supply its rated voltage with very little ripple. The unregulated power supplies are just a transformer, rectifier & capacitor. They only provide their rated voltage when they are supplying their full rated current and there will be a lot of ripple.