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Solid state relay replacement

Discussion in 'Electronic Design' started by Signumx, Aug 17, 2005.

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  1. Signumx

    Signumx Guest


    I'm currently using a photovoltaic solid state relay (Fairchild
    HSR312L) to open or close the path of a square wave signal. It works
    fine but at over a 1$ per unit it's rather hard on my circuit budget.
    I tried to replicate the internal circuitry of the chip (see datasheet: by using two MOSFETs
    with their sources connected together but I can't seem to turn off the
    switch (open the path). Anyhow, would anyone have a suggestion of a
    low cost, low current consuming solution to replace the HSR312L?

  2. Robert Scott

    Robert Scott Guest

    It is exceedingly rare that you can build something with discrete
    components for less money than a large-scale manufacturer has already
    built as an integrated circuit.

    -Robert Scott
    Ypsilanti, Michigan
  3. Mook Johnson

    Mook Johnson Guest

    I don''t think you'll get isolation for less than $1 each. If the isolation
    requirement was removed, it might be possible but still kinda tight.

    Maybe if you shared more about what the circuit is attempting to do we could
    be of more help.
  4. Signumx

    Signumx Guest

    I forgot to mention that the isolation is not required. Here is a
    summary of my project. We buy a unit that controls DC motor. Right
    now we also buy an off-shelf wired remote control which has simple SPST
    switches to open or close what seems to be current loops (I measure
    40mA when a loop is closed). We want to add a module between the
    remote and the control unit to allow or prevent some of the remote keys
    to function. This needs to be managed electronically (or digitally).
    A 0-5V signal would take care this. Also, I've been told that the the
    return path of the control signals is not connected to the unit's GND.
    Therefore, I can't put my module to the same ground reference as the

    In brief, a HI signal would allow the loop to close and current to flow
    and a LOW signal would open (or put in high impedance) the loop.

  5. Mook Johnson

    Mook Johnson Guest

    If you need to get a singal that is referenced to one ground to appear on
    another non-connected ground, you need isolation.

    I think you're at about as cheap as you can get for this part.
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