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> Classic Electromagnet Using Atx Pwr Supply ?, How To fix short circuit issue ??
Areal Person
Posted: March 20, 2017 03:02 pm
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Hey all,

Nice to post again to everyone, I've not been well, but am a little better now.

QUESTION:
How would you setup a circuit using a ATX power supply to drive a classic single coil magnet.

NOTES: * The power supply functions and is not the issue, the short is the problem to solve.
* Coil is wound and has a nail inside it.

This is a short circuit !!

How would you fix this short and implement the coil magnet correctly using the ATX supply ?

I should just add a small load, like a resistor and an LED ? Or, what ?

Thanks,

-Areal
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kellys_eye
Posted: March 20, 2017 07:55 pm
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If it is a 'single coil magnet' of any repute it will simply emulate a standard solenoid and, far from being a short circuit, will draw the appropriate current and do the appropriate thing!

Have you measured the coil resistance? You can then calculate the current and thus the HEAT that will be dissipated - the resultant heat being the limiting factor (as with any wire) to its correct operation and give you minimum requirements for the copper wire (csa).

There are formula/tables (which I don't have rolleyes.gif ) for the de-rating of the copper wire current carrying capacity due to it being layer wound and for the maximum recommended heat before you get insulation breakdown and its resistance etc. These will be the parameters you need to get a solenoid that will work without breaking down (or damaging the supply).

Example - 30 feet of 32awg wire is about 5 ohms. This will mean a current draw of 1A at 5V. BUT 32awg is only rated at 530mA (without taking de-rating into account) so this configuration won't work.

You'd need at least 60 feet of 32awg (total of around 10 ohms) for the current to be limited to 500mA(ish) but again this doesn't take into account de-rating BUT if the solenoid was only to be used intermittently then that's less of an issue.

You can, of course, use external resistance to limit the current tongue.gif


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dmg
Posted: March 20, 2017 08:34 pm
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i think with that kind of inductive loading the atx psu will instantly lift the hirts and throw the tits up.
but.. who knows, i only attempted capacitic loads, 20,000 uf and they reported the inrush current as a short and sortha.. did not turn on at all.

so i reckon you gonna need something to introduce the load gently.
probably a rudimentry resistor ladder with sorting switches can do the stuff, if the coil gets energies the psu will most likely not care. hmm but turning the psu OFF might introduce a new set of problems..
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deth502
Posted: March 22, 2017 12:15 pm
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have you actually tried this? ive powered many things, including large dc motors, with pc psu's and ive never had one shut down on me. plus these things are usually rated for SEVERAL amps, so i cant see the draw of a solenoid being a problem. as kellys said, the coil would likely burn up before you overdrew the psu.
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dmg
Posted: March 22, 2017 11:59 pm
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amplifiers. i did run a lot of amplifiers on ATX psus.
and if you add too lagre tank capacitors it won't turn on.
it has to do with the inrush current.

not sure if they dislike inductive loads ,
http://www.formfactors.org/developer%5Cspe...X12V_PS_1_1.pdf
this here tells standard ATX only manages 1000 uF capacitive loads, and atxv12 can hadle up to 20,000 uf capacitive loads. i found even chieftec psus won't switch on over 15,000 uF capacitive loads.
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