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Power Supply mystery with current rating when it is unplugged

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0seeker0

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I'm fairly new to electronics and have an odd problem I don't understand. I just built a voltage regulator based on the lm317t. I had an old Hewlett-Packard printer power supply (AC 120V to DC 30V @ 400mA) that's about ten years old that I was going to use as the input. It has not been plugged in for at least 8 years. When I cut the cord on the DC side (still hasn't been plugged in yet), there was a large spark when the cutters connected through both wires. I assumed that it must've been a capacitor that never discharged. Anyway, when I hook it up to my multimeter, I get roughly the right voltage (~37 volts), but the current always shows up as around 12 amps. When I unplug the power supply, it continues to read the same voltage and current, even hours later. The only thing that resets this to zero is shorting the two wires on the DC side, again causing a large spark. I've tried another multimeter and get roughly the same voltage, but a 8 amp current reading. Likewise, I have to short the wires to zero things.

I want to use this as the input on my voltage regulator, but I'm afraid that much current will fry it. Why does it show so much current when the power supply clearly has "Input: 110/120 AC" and "Output: 30v DC, 400mA" stamped into it? And are the capacitors maintaining the charge/reading after I unplug it? If so, even after 8 years? Does this make it unsuitable for use?

To those of you for whom this is really easy and can help me, I offer many thanks. Hopefully I'll get there some day! :)
 

cameo_2007

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Power Supply Mystery

I didnt really understand the concept of current:-

see,how did you measure the current.Was it just like as you measure the voltage,by putting the meter in the current range and holding the probes across those two wires?
if so,thats not absolutely the current.As you said you are new to electronics,it can be pardoned

I can teach you some basics about the current:

you cant measure the current unless it is loaded.
it means,you need to connect some load,any other device,to the output of that said adaptor,to find some current is drawn or not.And that too,the meter should be in series in the ckt.Here as you said,the one you said can supply only 400mA.So never you will be able to read 8A in your meter.

Now how to go with it:-
just plug in that adaptor i/p to the specified 110/120 AC mains.Dont worry,Nothn will explode.If there was no blasting happnd,just take th emeter,see the voltage across those two o/p cables.If it shows 30,ii is fine.
Or else,the stuff is not working.
 

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