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7130 Views 5 Replies Latest reply: May 29, 2014 12:56 PM by AJ2X RSS
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Apr 23, 2014 3:22 PM

0-5VDC swing changed to 0-60VDC swing

I want to drive an electromagnet with my PIC, but I know that 1) I won't be able to provide the current necessary and 2) I won't be able to vary the voltage as much as I want.  The PIC outputs 0-5V and I'd like to be able to swing between 0 and 60VDC.  Any thoughts on where I should start for this?

 

Thanks!

  • papalyle Novice 55 posts since
    Apr 6, 2014
    Currently Being Moderated
    Apr 23, 2014 8:44 PM (in response to Riley)
    0-5VDC swing changed to 0-60VDC swing

    Dear Sir:

     

    I would start with a 60 volt D.C. power supply. Then a NPN darlington arrangement with the opto isolated base connected to the darlington, driven by the controller.

     

    The load is connected between the darlington collector and the 60 VDC.  I'm here most of the time. If you want to send me your e-mail address, I will send you a schematic.

     

    papalyle

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    dear Sir

    • NITMOI Novice 26 posts since
      Mar 14, 2014
      Currently Being Moderated
      Apr 25, 2014 12:05 AM (in response to papalyle)
      0-5VDC swing changed to 0-60VDC swing

      You'll need to know how much current will flow on the 60VDC side and whether it's resistive or inductive.  If you're driving a relay (which drives something else) then you'll need some additional protection, e.g. a diode.  Then you can choose an adequate Opto-NPN or know how much current limiting to design in (e.g. resistor) so as to not fry the Opto-NPN.  Darlingtons are useful when you need a lot of gain (but they also typically handle more current) and given the opto-isolation, you may be fine with just an opto-transistor (NPN or PNP depending on your setup).  Make sure it's good for 60VDC too.  Your other electronic switching (i.e. NPN) options are FETs.  Usually provide much higher isolation but some are highly susceptible if the wrong voltage (or out of range) is applied (e.g. MOSFETs).  JFETs are less sensitive.

  • MetMan Apprentice 93 posts since
    Oct 24, 2011
    Currently Being Moderated
    May 1, 2014 9:06 AM (in response to Riley)
    0-5VDC swing changed to 0-60VDC swing

    A coworker of mine suggested you use the PIC 0-5V output to drive a switching power supply (feedback input).  Switchers can give you a 0-60V with pretty high current output capability.  Downside is it'd be a little noisier output.

     

    Good luck!

    • AJ2X Novice 2 posts since
      May 29, 2014
      Currently Being Moderated
      May 29, 2014 12:56 PM (in response to Riley)
      0-5VDC swing changed to 0-60VDC swing

      Since you're driving an electromagnet with the 0-5 volt output, are you just turning it off and on, or are you planning to vary the output voltage (DAC output)  to control the current through the magnet?  The suggestions above are all for just on-off control. 

      In any case, the current needed to run the magnet is as important as the voltage for the transistor or FET doing the controlling. When the device is Off, it must withstand not only the DC supply, but also any back-emf generated from the inductance in the electromagnet (which is why one person rightly suggested a diode across the magnet).  And when it's On, the device must withstand not only the current drawn by the magnet, but also the heat created by the current times the on-voltage (Vce or Vds), which can be non-trivial.  Power FETs nowadays have very low Rds(on) (often just a few milliohms), while bipolar transistors rarely have on-voltages of less than a few tenths of a volt.  Watch out for that heat dissipation! 

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