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converting circuit from relay to something else ?

Discussion in 'Misc Electronics' started by exxos, Apr 22, 2005.

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  1. exxos

    exxos Guest

    Hi all,

    A simple switch uploaded here

    Bascially 100VDC unregulated supply rail. Drops 72V via a zener to power a
    24V relay. Capacitor just to add some small smoothing to prevent the relay
    from buzzing.

    While the relay is turned ON, it connected the output to the 24V supply via
    a 100K resistor.

    When the relay turns OFF it shorts the output down to ground.

    Sounds really simple, though I have been trying for the past 2 days to
    convert this to a simple transistor circuit. To which after several attempts
    given up so decided to ask here before I use the relay verson to see if
    anyone can suggest a simple circuit which would do the same.

  2. Dave

    Dave Guest

    I would suggest an OPTOISOLATOR chip.
  3. exxos

    exxos Guest

    Been looking for a PNP version, will have another look, its the inverting
    thing which is the problem...

  4. CWatters

    CWatters Guest

    No the tricky bit is getting a low impedence output when there is no power
    on the circuit to turn on a regular transistor. Try a depletion mode FET.
  5. exxos

    exxos Guest

    Yeah low output impedence with no power is the fundimental problem, not
    heard of a fet like that, just googled for them and doing a bit of reading
    now :)

  6. exxos

    exxos Guest

    several pages latter I quote..

    "A depletion-mode MOSFET conducts current without a gate bias. In an
    n-channel device, a thin n-type region exists under the oxide in the absence
    of an applied bias. It connects the source and drain allowing current flow.
    In fact, a negative voltage is required to drive the electrons out of
    (deplete) the region to increase channel resistance and reduce current

    so basically its normally on though I found they actually have a on
    resistance of 100ohms going by the ones ive found so far. All sounds good
    until you read that it needs a negative to switch it, this reminds me of
    thermal valves also, maybe if I fake the ground level by a volt or 2 then in
    effect the gate voltage will be 2 volts under the source voltage... even so
    the actual supply rail goes high so not sure how I can get negative switch
    from a possitive rail.. oh the joy.....

  7. CWatters

    CWatters Guest

    I'm a bit rusty but I think you are referring to an N-Type device. Perhaps
    look at at P-Type instead.
  8. CWatters

    CWatters Guest

    See also P-Type JFETs
  9. exxos

    exxos Guest

    will have a look for some p devices,

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