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Re-Born AC adapters

M

Malc

Veteran Member
Joined
Jul 16, 2009
Messages
2,394
Location
UK
Ever wondered where our old unwanted AC adapters go to die, They go to China and like idiots we buy them back.
The Horror?
 
And built using quality components no doubt, They don't make em like they use to, I've got a box full of old AC adapters that's been in my shed for years, I was going to dump them but will open them up instead for any usefull parts and for the fun of it.
 
I find that a rawhide hammer is useful in opening "sealed" wall warts--just tap around the seam where the top and bottom plastic overlap and join. Eventually, the parts separate and you can replace dead capacitors. or repurpose the innards. Use a bit of MEK to fuse the seam back together again.

I bother to do this with transformer-type wall warts--the little switcher supplies are junk and probably best thrown away when they go bad.
 
Chuck(G) said:
I bother to do this with transformer-type wall warts--the little switcher supplies are junk and probably best thrown away when they go bad.
Click to expand...

If you find a linear wall wart (the kind with a heavy transformer) that has popped capacitors, they're usually popped for a reason. Unlike switching power supplies, linear power supplies drift upward in their voltage output over time, and can sometimes be alarmingly out of spec after just a year. These linear type wall warts will drift upward in voltage whether they're being used or not. If you've had one sitting around awhile, you should check it with a volt meter before plugging it into anything.

The reason the voltage drifts upwards is because the secondary windings get shorts in them. I have a 25 year old linear wall wart that was originally 9v, and is now well over 20v. Even some of my younger linear wall warts less than 5 years of age are at least several volts off.
 
I've seen similar teardowns on cheapie 5V USB supplies meant for charging phones and such. I finally gutted an old rackmount power supply and built a replacement for the 12V wall warts and power bricks for the Mini-ITX gear in the server rack:

http://www.glitchwrks.com/2016/01/03/rack-dc-supply

Even when I had a single device on the oversized Mean-Well switcher in there, the Kill-a-Watt said it was drawing measurably less power than the 5A power brick that came with the 1U case I'd used for the router. I haven't opened it up (it still works, or did when it was replaced), I'm sure it's another Chinese recycled special.
 
GiGaBiTe said:
If you find a linear wall wart (the kind with a heavy transformer) that has popped capacitors, they're usually popped for a reason. Unlike switching power supplies, linear power supplies drift upward in their voltage output over time, and can sometimes be alarmingly out of spec after just a year. These linear type wall warts will drift upward in voltage whether they're being used or not. If you've had one sitting around awhile, you should check it with a volt meter before plugging it into anything.
Click to expand...

I check anyway, because you can never be certain that the polarity marking on the wart reflects reality. I've got a couple of them where what's measured on the end isn't what's shown on the case. But I've never run into what you describe. Certainly, on simple rectifier-with-capacitor warts, no- or low-load voltage will rise to the peak AC voltage, but that's not due to any malfunction. Caps usually go bad in those things because they get "cooked" by having no way to dissipate the transformer heat.

That being said, my favorites are the "desk wart" PSUs used for modems and such. For years, my network hub was powered by the "brick" from a Grid computer. Heavy as heck, with a metal enclosure, the thing just would not give up. I run an LED music stand light from an old Elpac 12v "desk wart".
 
Chuck(G) said:
I check anyway, because you can never be certain that the polarity marking on the wart reflects reality. I've got a couple of them where what's measured on the end isn't what's shown on the case. But I've never run into what you describe.
Click to expand...

If the voltage is higher, that's upwards voltage drift caused by shorted windings. If it's lower then it means that the wires coming out of the wall wart are too thin.


Chuck(G) said:
Certainly, on simple rectifier-with-capacitor warts, no- or low-load voltage will rise to the peak AC voltage, but that's not due to any malfunction.
Click to expand...

The output on a linear wall wart should never rise to mains voltage under any circumstance. This means there's a short between the primary and secondary windings and the unit has failed and should no longer be used.


Chuck(G) said:
Caps usually go bad in those things because they get "cooked" by having no way to dissipate the transformer heat.
Click to expand...

This is really only a problem if the transformer is run at near or exceeded its amperage output, or if it's using Wofat brand caps.
 
GiGaBiTe said:
The output on a linear wall wart should never rise to mains voltage under any circumstance. This means there's a short between the primary and secondary windings and the unit has failed and should no longer be used.
Click to expand...

I think Chuck means peak secondary voltage, i.e. stepped down, not primary/mains voltage.
 
GiGaBiTe said:
If the voltage is higher, that's upwards voltage drift caused by shorted windings. If it's lower then it means that the wires coming out of the wall wart are too thin.
Click to expand...

I might be convinced if you could show me some real-world cases.

The output on a linear wall wart should never rise to mains voltage under any circumstance. This means there's a short between the primary and secondary windings and the unit has failed and should no longer be used.
Click to expand...

That's not what I said. First off, we're talking about DC-output warts, not AC ones. Usually, these are scarcely more than a bridge rectifier and a capacitor-input filter; few have voltage regulation. They rely mostly on the fact that loaded, the output voltage will approximate the RMS voltage of the transformer. Unloaded, the output voltage will rise and approximate the peak-to-peak voltage. This has nothing to do with shorted windings or the primary of the transformer; it's Electronics 101.

This is really only a problem if the transformer is run at near or exceeded its amperage output, or if it's using Wofat brand caps.
Click to expand...

No--most transformer-type wall warts are closed boxes, with no ventilation whatsoever. Because of eddy-current losses, all transformers will heat. I2R losses generally are due to loading, but even unloaded, there are I2R losses in the primary. Plastic (usually something like ABS) is a lousy heat sink. So you've got a transformer and rectifier and capacitor all in the same small hermetically-sealed enclosure. Heat is the enemy of electrolytic caps--given time, that filter cap in the wart will dry out and the ripple in the output will just keep increasing.
 
Chuck(G) said:
I might be convinced if you could show me some real-world cases.
Click to expand...

That makes two of us. I think we'll be waiting a long time. Interestingly, GiGaBiTe is the guy who made the highly dubious claim that Asus have used fake Nichicon and fake Rubycon capacitors on their motherboards, here. He never provided any proof of that.
 
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