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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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[–] jet@hackertalks.com 15 points 2 months ago (1 children)

Apply a dummy load and measure, but you have already proven the supply is out of spec, I wouldn't use it

[–] diyrebel@lemmy.dbzer0.com 2 points 2 months ago* (last edited 2 months ago) (1 children)

I have an Arduino kit with a variety of resistors. Would I just take any resistor to use as a dummy load, or does it matter which resistor I use?

The PSU is rated at 2A 12v, so would I take 2A÷12²v? Should I look for a resistor that’s 0.0139ꭥ? My EE class was decades ago, mostly forgotten.

I also have a 12v LED lamp that I pulled from a dumped refrigerator which I could sacrifice.

I’m often in a situation of needed to repurpose dodgy PSUs with dodgy appliances, so I’d like to generally know how to do this.

[–] jet@hackertalks.com 2 points 2 months ago (1 children)

You would want big thermal resisters, or something that doesn't care about the correct voltage like a big beefy motor

[–] diyrebel@lemmy.dbzer0.com 2 points 2 months ago

In that case, it sounds like I should try using the PSU to hotwire the universal motor in my washing machine.

[–] cmnybo@discuss.tchncs.de 7 points 2 months ago (1 children)

Is that adapter heavy? It sounds like an unregulated transformer type power supply. It's not unusual for those to output twice their rated voltage with no load. They will damage equipment that requires a regulated supply.

[–] diyrebel@lemmy.dbzer0.com 1 points 2 months ago (1 children)

It’s a lightweight switching power supply.

But I also wonder why you say it sounds like the linear transformer variety. My understanding is the linear transformers give a more clean and stable output. I think transformers are more robust and longer lasting. OTOH, it’s not good that they warm up and waste energy (“vampire bricks”), hog space.. and not good that they are hard-wired to only handle 110v or 220v, not either/or.

Overall I prefer the small light switching PSUs, despite the fact they are often badly built and have short lifetimes.

[–] 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.