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Using momentary switch to activate/deactivate 12v Relay

Discussion in 'Hobby Electronics' started by Jimmy T, Jul 26, 2005.

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  1. Jimmy T

    Jimmy T Guest

    Can anyone please help to provide circuit to activate/deactivate 12v Relay
    using a momentary switch (push on / push off function). I've searched on the
    net found circuit at , I’ve
    built it & doesn’t work all it does is
    turn on relay when switch is pressed & doesn’t turn it off again when switch
    is pressed again.
  2. Geoff C

    Geoff C Guest

    That is a neat trick, that circuit... Did you try increasing tha value of
    the cap significantly? What are your circuit values? The circuit will need
    to be somewhat optimised for the relay characteristics wrt it's dropout
    voltage and coil resistance.
  3. Jimmy T

    Jimmy T Guest

    Thanks Geoff , I'll try increasing the CAP & see what happens. other wise
    i'll try using the links that John Crighton suggests,
    thanks again for your Info.
  4. Jimmy T

    Jimmy T Guest

    Thanks for the info John & I'll give these sites a look tonight when I get
    Thanks again
  5. Terry Given

    Terry Given Guest

    relay latching current is obviously OK, and R1 can clearly pass the
    holding current, so C1 kinda has to be the culprit. Like Geoff says,
    crank up C some more. At a guess I'd pick 10-100uF. Ideally the cap
    energy needs to be much greater than that stored in the relay coil - not
    that you necessarily know the coil inductance (which varies with contact

    So I built one. The 12V relay sucks 40ma at 12Vdc. From 24Vdc, I drop
    12V at 40mA across R1 = 300R. I chose R1 = R2 = 317 Ohms (I have
    thousands of leaded 475R resistors, and 4 leaded 330R resistors).

    With C = 100uF, the relay turns on but not off. ditto for 570uF. 1040uF
    and it turns off again. The relay holding current is about 10mA. With
    the pushbutton shorted, it oscillates with a period of a few Hz.

    Of course during normal operation R1 dissipates 12V^2/317R = 454mW.
    quite a bit....I used 475//(475 + 475) so the single 475R dumps 300mW,
    which its OK with :). With the relay OFF, both 317R resistors dissipate
    454mW. not so good for battery gear then eh?

    OK, what about a circuit running from 12V with re-scaled R1, R2? At a
    guess, drop 2V across R1 giving R1 = 2V/40mA = 50R. R2 has 10V across
    it, so must be 5 times bigger than R1 IOW 250R. Using these exact values
    (thousands of 100R's): with 10mF of capacitance (5 1800uF 450Vdc caps, 2
    470uF 25V caps and a 100uF 50V cap) it didnt turn off terribly well,
    depending on how I drove the "switch" (aka clip lead). It looks like its
    to do with the RC time constant...... I dont have any large
    electrolytics, so cant increase C above 10mF, but I suspect 20mF or so
    will sort it out. Not that thats a useful value in practice....

    so R1 = R2 is a pretty good idea, and C wants to be a few mF.

    but the circuit does work, and is about 100 years old - this is how
    vibrator power supplies were build, using an oscillating relay as both
    the oscillator and switching element.

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