mixing 12v and low-volt switching

Discussion in 'General Electronics Chat' started by halley, Dec 28, 2008.

  1. halley

    Thread Starter Member

    Dec 27, 2008
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    I'm sure this is very simple, but any time it comes to transistors, I seem to hit a wall of not understanding anything.

    If I have a major circuit, say a few motors or solenoids, that work on 12v and have large current draw, what's the basic circuit trick that I need to do, in order to switch these devices on and off with a low-voltage minor circuit?

    For example, I have a 12vdc power supply that can deliver the current to the major circuit device(s). I have a minor circuit with its own little power regulator to feed its own 5v or 3.3v requirements (like an Arduino or Basic Stamp). The logic circuit has wimpy little outputs that can source or sink a little HIGH/LOW signal, but not the current or 12v levels of the major circuit.

    I imagine some form of NPN would do the trick, but (1) what package would you select, and (2) what are the necessary resistors, diodes and chokes required to manage the voltage differences, and protect the minor circuit? Let's say it's a single solenoid, which draws 1A, and there's a backlash current when a solenoid goes unpowered.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Look at logic-level MOSFETs.
    Most of International Rectifier's IRLx series are logic-level compatible. Many of them have built-in diodes to take care of reverse EMF.

    All of the IRL series are N-channel.
     
  3. halley

    Thread Starter Member

    Dec 27, 2008
    11
    0
    I know nothing about MOSFETs, so this might be dumb, but is N-channel the same as NPN?
     
  4. halley

    Thread Starter Member

    Dec 27, 2008
    11
    0
  5. halley

    Thread Starter Member

    Dec 27, 2008
    11
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    Hope I'm not spamming by jotting notes and additional questions on my own thread.

    Found http://homepages.which.net/~paul.hills/SpeedControl/MosfetBody.html which helps me understand the different specs to select a mosfet. All I really know at this point would be the Id(max) =~ 1A is fine, and Vgs(th) needs to be far enough below 3.3V if I want 3.3V "high" to turn it "fully on." I guess the diode helps protect it from damage, so the Vdss is probably pretty high?
     
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