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I have an electromagnet and a 12V power supply. I would like to apply +12V to the electromagnet at some times and -12V at other times.

My current thought is to use four transistors (two NPN, two PNP) in a matrix (see diagram). I have two concerns:

  1. When all four transistors are open, the "inside" leads will be floating. Is this a problem? If so, would a pulldown resistor on the "inside" sides mitigate it?
  2. Using two transistors (one each side) would be cheaper and take up less space, but I can't figure out a way to do it (even with diodes) that doesn't short out the power supply. Is there an easy way to do this (besides adding a second -12V supply)?

Thanks!

Four-transistor matrix

Voltage Spike
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randomhead
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    The name for what you're trying to build is an 'H-Bridge'. – brhans Oct 23 '17 at 20:49
  • Re: 1. you must never allow that to happen (shorted), and the answer to 2. is, no, I'm pretty sure you need four transistors. – Marcus Müller Oct 23 '17 at 21:03
  • If you don't need a lot of power at the output, a simpler and smaller solution could be to use a mosfet gate driver as the H bridge. See https://electronics.stackexchange.com/questions/278645/how-to-generate-a-bipolar-waveform-using-mosfet/278812#278812. – dim Oct 23 '17 at 21:18
  • @brhans thank you, that is what I'm looking for! I feel silly now. – randomhead Oct 23 '17 at 23:02
  • @MarcusMüller (out of curiosity) why would putting all four transistors in the "open" state cause a short? – randomhead Oct 23 '17 at 23:02

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