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I'm experimenting with PIC microcontrollers and various related circuits both on plugboards and simple homemade PCBS.

I'd like to use various old AC power adaptors I have lying around to power my circuits instead of relying on batteries all the time. I have several old mobile phone chargers for example that give out 5v with 500-1000mA of available power. ( I realise they are likely unregulated and I'd need a regulator circuit or similar).

My worry is that that they are very cheaply made so what happens if I accidentally short circuit one or try to draw 1000mA from a 500mA supply.

Are they likely protected against such abuse in any way even if only by an internal fuse? Or are they likely to be severely damaged or worse overheat and catch fire or something? Or will they simply deliver their maximum current and continue working?

None of this is likely to happen, but I want to be safe?

I doubt this affects the answer but this is the UK with a 240v mains supply.

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If it's not safe to rely on that I imagine I could build a small protection circuit and feed my real board from that. Any suggestions on what i need to do this? – John Burton Jun 10 '10 at 15:44
you could use a zener in reverse across the supply and an inline fuse. Choose a zener slightly greater than typical operating voltage and one rated for about 3W-5W dissipation. When it conducts either the voltage on the output drops or the fuse blows. – Thomas O Dec 9 '10 at 13:31

7 Answers

up vote 8 down vote accepted

You could try using a resettable fuse.

http://en.wikipedia.org/wiki/Resettable_fuse

http://tinyurl.com/37reex3

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I am glad it gives me a notice that you posted this, because I was typing almost the same thing. – Kortuk♦ Jun 10 '10 at 15:57
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That sounds good. I might make a little board with such a fuse and a 3.3 and 5v regulator on that I can plus my power supply into one end and my experiment into the other just in case... – John Burton Jun 10 '10 at 16:24
That is a good choice john. – Kortuk♦ Jun 10 '10 at 17:14
Mind you, I believe PTC's are very very slow. It might work for protecting from overload but not spikes like from shorts. – XTL Jul 22 '10 at 20:35
up vote 7 down vote

Wall warts are constructed as disposable commodity items, and they're usually 'potted', which means that the case is entirely filled with a solidified epoxy or epoxy-like substance. I think they just count on a fine-gauge wire burning up somewhere in that block of stuff. There's no oxygen available, and as soon as the fault goes open circuit, the show's over. Anyway, I've never seen one of these recover, though they can sometimes provide a bit more current than they're marked for. (..and more often than not, put out more voltage than they're marked for as well)

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Ok, if I accidentally destroy one then that's no problem. I only thought of using them because I have a cupboard full of old ones. – John Burton Jun 14 '10 at 10:16
I've never found a potted wall wart. Where are you from JustJeff? – tyblu Dec 8 '10 at 23:41
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A word of warning. I shorted out a 6Vac transformer/wall wart, obviously shorted something in the transformer windings, and it's now outputting ~40Vac. So don't rely on the transformer burning out. – Thomas O Dec 9 '10 at 0:12
@Thomas O: I +1'd your comment b/c though it may seem blindingly obvious, "don't rely on the transformer burning out" will be of inestimable value to some. – JustJeff Dec 9 '10 at 22:12
@tyblu: the few times i tried it, i encountered potting material, so i lost interest in getting them open - but that was (ugh) 30 years ago. Northeast US btw. – JustJeff Dec 9 '10 at 22:44
up vote 6 down vote

Most of the newer ones have switchers in them, with a stabilised output. They are short-circuit protected and almost indestructible. 5V ones are available from most suppliers, and are very useful.

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Yeah, I don't remember a wall wart dying on me, ever. They just seem to brown out when overloaded. – romkyns Oct 11 '11 at 19:33
up vote 4 down vote

Here in the Netherlands you can get used adapters for very cheap in charity shops. From telephone chargers (3V) to printer adapters (18V). Just clean them up and if they work your good to go.

Overloaded adapters can stink a lot.

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up vote 4 down vote

The transformers are probably made to open circuit internally when they overheat. It's probably required by UL regulations, etc.

When we do overheat tests on big power amps, the transformers will go open circuit and then you have to let them cool for an hour or more and they'll self-reset. I don't know what the actual component is since it's presumably buried inside the coils of wire.

Cheap wall wart transformers probably don't self-reset, but they are "likely" to at least have some form of internal protection so they don't start on fire.

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Thanks, makes sense. After some thought, and the other answer to my question I think my best option is not to assume anything but build a small protected board that I plug my adaptor into that I can test completely and then plug that into my more experimental circuits. – John Burton Jun 10 '10 at 17:48
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Remember, though, that not everything is UL listed. – Flyguy Jun 12 '10 at 3:09
Yeah, they are probably made to open-circuit is the problem. – Kortuk♦ Jun 12 '10 at 16:39
up vote 2 down vote

I think endolith and JustJeff are right for most adapters.

However I have seen that some drop in voltage when you rise the current to much, they seem to have a fix energy that they can deliver. Don't how those where designed thou.

If you find one of those it will not overheat and burn, but you design will not get the specified voltage and may malfunction.

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up vote 2 down vote

Almost all wall-wart type power supplies have a non-resettable thermal fuse built into the transformer these days.

Overloading them for a short period of time will not do any damage (the voltage output will drop, though). Significant overloading for a period of time will cause the adapter to fail open-circuit, in a manner that is effectively unrepairable.

It shouldn't be dangerous, though.

Note - Some chinese dc-dc bricks are so cheap that they don't even have modicum of protection, though. If they are 60hz transformer-based, you should be fine.

(And that's not even mentioning the issues with fraudulently-rated power supplies coming out of china)

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