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Author Topic: electromagnet and PSU  (Read 840 times)
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Hello,
I'm trying to use some elctromagnets with an arduino...building them exactly as shown here:
http://education.jlab.org/workbench/magnetstand/coil.html

that project calls for a PSU at 3.3 amps, 12V...I have a 12v psu at 5 amps.

When I put the electromagnet in my circuit...just using it with some 12V, 5amp relays...if I have the polarity of the electromagnet one way...it kills the PSU and I get no power...when I reverse the polarity of the wires...everything works fine in the circuit. I'm just curious why this would be...I haven't messed around much with making electromagnets before. They're very curious!

thanks for any info!
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if I have the polarity of the electromagnet one way...it kills the PSU and I get no power...when I reverse the polarity of the wires...everything works fine in the circuit.
No I am sure you are wrong. That doesn't make any sense at all. That is not how coils work, reverse the power and you just reverse the field. You must be doing something else.
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Yeah...that's what I figured. But basically...I have the relays hooked up all fine. they are working. they are 12V, controlled through TIP120 transistors with a 1k resistor to the arduino...I'm using diodes and everything. I've used these for other projects with no problems. When I put the two wires coming out of the electromagnet one way into the relay coil...everything works fine...when I put them the other way...the PSU stops giving me power and the little green light on the PSU box dims and starts flickering....

I'll try to type out the schematic:
 
     DPDT relay (I know I don't need this big of a relay but It's all I have lying around at the moment and I'm just using it as a SPST)
    1              2 (pin 1 to TIP120, to GND, pin 2 to 12V)

     3             4 (pin 3 in 12V, pin 4 to GND)
    5---coil----6 (electromagnet plugs in here....this is where when I reverse it everything gets weird)
    7             8 (nothing in these pins)
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Does your electromagnet has a diode across it as a flyback? so when you reverse it you essentially short out the psu
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I'm using diodes and everything
@winner
Yes spot on.  smiley

The diode needs to be in the same direction only the coil needs to be swapped over.
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i don't have diode on the electromagnet...just on the tip120...should i use a diode on the electromagnet also?
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just on the tip120
Where?

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should i use a diode on the electromagnet also?
Yes.

OK so the diode isn't doing it, there must be something that you are doing wrong, can you post a picture of your setup with the coil one way and then a photo of it the other way.
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treebykooba,
Shouldn't the TP120 ground the  power to the coil to energize it?
Then the selenoid thru the common and a NO contact to energize when the coil is activated?
Of course with flyback diodes on both.
TomJ
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If you've got a part number for that relay see if you can find a datasheet for it.

I wouldn't mind betting that it has built-in flyback diode across the coil.

Some relays do.
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I don't think you connected the grounds, Dave.
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A multimeter would settle the question.
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I note that electromagnet has a centre-tap - how are you connecting it when you "reverse" it - we assume a 2-wire electromagnet I think!

If you are "reversing" by keeping the centre-tap as one electrode and swapping from one end to the other for the second electrode then it clear you have a short-circuit somewhere in the coil.

The instructions say wrap directly onto the bolt - but did you remove all rough-edges first? (steel will cut through copper really easily).

Have you measure the winding(s) resistance with a meter?
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