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Shortwiring AC/DC power adapter

Posted: Sun Apr 13, 2003 5:43 pm
by samsmiles
Had mess on breadboard and it seems I shortwired ground and vcc. Adapters led is still on but its not functioning. Have I burned it and is there fuse in regular adapters I could replace or is it throw-away mat now?<p>Thanks for reply to my dumb-dumb question...Sam

Re: Shortwiring AC/DC power adapter

Posted: Sun Apr 13, 2003 6:52 pm
by russlk
What do you mean by "adapter", is this a wal-wart? Those things are not intended to be repaired, but you can't do any harm by taking it apart.

Re: Shortwiring AC/DC power adapter

Posted: Sun Apr 13, 2003 7:08 pm
by samsmiles
wall wart..yep..:-(<p>its still getting warm when in wall socket but when in circut with resistor i have 0 current output. Led on wallwart is on however.<p>:-( hm hm<p>Sam

Re: Shortwiring AC/DC power adapter

Posted: Sun Apr 13, 2003 7:24 pm
by Dimbulb
you are saying you smoked a wall transformer.
You are asking: is there a fuse
answer no seldom
should you throw it away
answer yes if there is no voltage
could it be a diode
yes you could unsolder the diode and replace it.<p>[ April 13, 2003: Message edited by: 1206DX ]</p>

Re: Shortwiring AC/DC power adapter

Posted: Sun Apr 13, 2003 8:07 pm
by Chris Foley
Throw out suspicious wall warts, just on principle. Seriously, wall warts are generally made "just good enough". If the beast has been stressed, it may have heated the wiring on the transformer, possibly causing a partial short in the primary or secondary wiring, or melted wire insulation, causing a problem with line isolation. The cap could also have been stressed as well as the diode(s).<p>If you keep an eye on things, you can intercept good wall warts going into the trash bin everywhere, or pick them up surplus for a buck or two each. Keep a box handy in your attic, and throw them in as you find/scrounge them. Try to invest in a current-limited variable power supply, too. They're available at surprising prices used at hamfests and such, and they make working with breadboards much easier. If you must, though, use an old technician's trick, and find an incandescent bulb with about the same voltage as the wall wart, and a current rating a little less than that of the wall wart. Hook it in series with your wall wart power supply. As long as your breadboard current drain is a lot less than the current rating of the bulb, there will be almost no voltage drop across the bulb during use. (The bulb resistance increases dramatically as voltage/current increases.) However, if there's a short, the bulb will light up, saving the power supply/wall wart, and letting you know about a wiring error. I like using 12VAC/1A wall warts (we've got crates of 'em where I work), and when I needed this trick for a customer test fixture, I used an automotive 10W bulb with wires tack-soldered onto the bulb sleeve. He got a good indicator for DUT short, and I was still able to use his specified AC power supply voltage, without the headache of putting together an AC current sensing/limiting circuit.<p>Try to find or make that bench power supply, and use it for development, only throwing the wall wart in when everything else has been proven OK. If you decide you really need a tool, you should just get it. You can always sell it on ebay when you're done with it, if it's a one-shot. Happy hunting.<p>[ April 13, 2003: Message edited by: Chris Foley ]</p>

Re: Shortwiring AC/DC power adapter

Posted: Mon Apr 14, 2003 2:16 pm
by frhrwa
I just happen to have... a Condor Inc Power supply, in fact, about 4 of them... model TD-60W input 115/230VAC 47 - 440Hz.. output +5VDC at 6.0A.OVP at 6.2+- .4v
+12VDC at 1.0A or + 15V at 0.8A
-12VDC at 1.0A or -15VDC at 0.8A<p>these are great power supplies if one of them will help you out?<p>[email protected]

Re: Shortwiring AC/DC power adapter

Posted: Mon Apr 14, 2003 6:33 pm
by samsmiles
Thx to everyone. Great help :-)<p>Wayne too bad :-( I ordered this one: http://www.web-tronics.com/30serdigrobe.html
earlier today. We´ll see how it goes.
Btw does anyone have experience with this power supply? <p>I don´t dare to wire ps on my own yet Chris. I´ll try the bulb trick, neat really. <p>Im still sweating on my avr starter kit. Can´t get pc to recognize my mic.<p>
Have a great day everyone.<p>Sam

Re: Shortwiring AC/DC power adapter

Posted: Tue Apr 15, 2003 5:26 pm
by jollyrgr
Just my two cents:<p>When I was a little kid, I repaired wall warts because I did not have the resources to find/buy them. Now I save every one I get. I will replace the coaxial end to make them work with a specific device (or if the wire breaks but the supply still works, or if the polarity is different). Once they die electrically I toss them. Many I get for next to nothing. (Bought a box of ten various warts at a ham fest for a buck.) Check places like ALLTRONICS. Suggestion, buy the higher amp units. These will work with the smaller devices as well as the bigger ones. You don't want to be too underpowered.<p>But as to the repair. Yes, you CAN repair them IF they have screws to open them up. Most modern ones do not. This requires that you cut them open. And, yes, I have done this. Carefully cut along the seam and split open. More than likely you have fried the diodes. There is three major ways I have seen the DC ones wired:<p>Single diode
Two diodes with Center Tap (most common)
Full-wave bridge<p>(There are regulated and other types but I won't go there.)<p>
If one or both diodes short on a two diode system, it produces a short and causes the wart to get warm. Usually the failure of the first diode will eventually kill the second. Pretty much the same thing in the full wave bridge circuit as well. <p>You might be surprised when you open the case to find the circuit potted in epoxy. TRUST ME, these are not worth fixing. Replace the diodes and you might as well replace the cap as well. If you have not melted the windings of the transformer you will repair the thing. Before changing the diodes and cap, check the coils for shorts. If fully shorted, toss it.<p>So let's say you have gone this far and still need to fix the thing. Replace the diodes and the cap. Even if only one diode is blown, replace both. Replace the diodes with higher amp ones if you can. In other words if you have a 500mA wall wart, replace the diodes with 1 amp or better units. Check the wire for shorts at the place where it exits the wart. (It is not worth fixing the thing if the wire blows your work out again.) Carefully test the wall wart. I suggest that you plug the wall wart into an extension cord then plug the extension cord into the wall if you plan on testing it while open. Once you are sure it is going to work, reassemble the unit and glue it shut. Use model or plastic cement. Try to avoid super glues; I've seen these glues fail. As an added safety, tape the wall wart shut with eletrical tape.

Re: Shortwiring AC/DC power adapter

Posted: Thu Apr 17, 2003 8:54 am
by samsmiles
Thank you Jolly. Very informative. I opened it....made it learning experience. I will get fresh diodes in a bit and we´ll see what happens.<p>Im using the lamp trick now too to be on safe side.<p>
Have a great day everyone...Sam

Re: Shortwiring AC/DC power adapter

Posted: Sat Apr 19, 2003 12:49 am
by gadgeteer
You don't hafta use a saw; put one blank edge (between plug & wire) into a bench-vice, and slowly clamp until the seam pops. then pop the other side, then pop the plug side. You should now be able to insert a slotted-screwdriver and pop the rest of the seams. Superglue, or better plastic solvent (go to a plastic-sheet-store) will reassemble excellently.<p>You all DO have personal vices, don't you???<p>HINT --- WEAR SAFETY GOGGLES! YOU ONLY GET ONE SET OF EYES IN THIS LIFE...<p>If the LED is still running, odds are that there IS a fuse or fusible resistor. LEDs require DC, so your rectifiers and capacitors sound like they're still in-circuit.<p>I bought a Marklin "Z" guage train set recently; but it was european version, 220v/50hz supply. No way was I gonna run a step-up transformer. Opened the case (if anyone tries this, REMOVE THE TWO DOTS ON EACH SIDE---THEY'RE PINS THAT HOLD THE CASE TOGETHER), found the transformer mildly potted. In freeing it from the case I broke the bobbin. Exposed 3-4 wires, tinned them, plugged it into 120 volts, measured for the wire that sat at midway. Used the one that measured 65 volts, better a little over than under. Nail-polish for the other tinned-wires...<p>Train works fine. MAN is "z" scale small!

Re: Shortwiring AC/DC power adapter

Posted: Sat Apr 19, 2003 12:56 am
by gadgeteer
BTW, I've used the light-bulb trick in a professional test fixture. Sensitive amplifier, accidentally drop a power lead on the input and amp is fried. Stuck zener diodes to limit voltage, and bulbs to limit current. At normal conditions bulbs are shorts, and zeners are non-existent. But screw up and drop the volts, zener clamps the amp to a couple volts, bulbs absorb the extra volts...<p>You can also use an LM317 and ONE RESISTOR to make a current limiter; it's in the databook as "TWO-TERMINAL-CURRENT-SOURCE"...

Re: Shortwiring AC/DC power adapter

Posted: Sat Apr 19, 2003 5:12 pm
by bodgy
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by gadgeteer:

You all DO have personal vices, don't you???
<hr></blockquote><p>
Yes I do, but I'm not going to bandy them about on this board. :D
<p>Colin

Re: Shortwiring AC/DC power adapter

Posted: Tue Apr 22, 2003 4:45 am
by samsmiles
Totally cool :-) I will try 317 as well.
Thx folks learning a bunch here. :-))<p>Sam

Re: Shortwiring AC/DC power adapter

Posted: Tue Apr 22, 2003 10:32 pm
by gadgeteer
I put a surface-mount 317 (soic) and chip resistor in an "N" guage 2-6-0 switcher engine (steam), to power an angel-hair-bulb (that's smaller than a grain-of-rice, it's the size used in wrist watches). Bulb is mounted in the tiny headlamp, comes on at very slow speeds, locks at a yellow color (drastically extending its life) over the full speed range.<p>Had to also use a blocking diode --- seems if you reverse-biase a 317 in its current-source configuration with the train-direction-switch, it PASSES EVERYTHING! :( <p>The 317 needs the predicted 2-volt or so drop to function; but it drops no more voltage until you exceed the designed current; then the voltage falls off as it current limits...

Re: Shortwiring AC/DC power adapter

Posted: Tue Apr 22, 2003 10:36 pm
by gadgeteer
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Yes I do, but I'm not going to bandy them about on this board.<hr></blockquote> ;)