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Will the LM393 drive a small relay?

Discussion in 'Electronic Basics' started by Bob, Apr 24, 2007.

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  1. Bob

    Bob Guest

    I need to switch on a relay with the output of the LM393. The relay
    is a 5vdc unit from RadioShack. Does the LM393 have what it takes to
    trigger this relay? If not what should I put between the relay and
    the LM393 to permit the 'output' of the relay to properly 'close' the
    relay?
     
  2. Unless the relay is very small (low power), probably not.
    The LM393 saturates fully on only when the pull up current
    is a few milliamperes. Even a relay with a .1 watt coil
    draws 20 milliamps.

    I would add a driver transistor with a clamp diode across
    the coil.

    If you can reverse the sense of the comparator (reverse its
    inputs, so that the relay is on when the comparator is off)
    all you need is to connect an NPN transistor like a 2N3904
    or 2N4401 emitter to ground, base to comparator output,
    collector to coil, other end of coil to +5, 1k resistor base
    to +5.

    The diode connects cathode to +5, anode to collector.

    If you cannot reverse the sense of the comparator, then it
    is easier to use a PNP transistor, like a 2N3906 or 2N4403,
    and connect emitter to +5, collector to coil, other end of
    coil to ground, 1k resistor base to output, 1k resistor base
    to +5.

    In this version, the diode connects cathode to collector and
    anode to ground.

    Note that if the relay includes a diode (many 5 volt DC
    units do) you have to connect its coil so that the internal
    diode is right way round, in place of the external diode
    described above.
     
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