# Using momentary switch to activate/deactivate 12v Relay

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

1. ### Jimmy TGuest

using a momentary switch (push on / push off function). I've searched on the
net found circuit at www.oldradio.com/current/pushon-pushoff.htm , 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 CGuest

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 TGuest

Thanks Geoff , I'll try increasing the CAP & see what happens. other wise
i'll try using the links that John Crighton suggests,

4. ### Jimmy TGuest

Thanks for the info John & I'll give these sites a look tonight when I get
home,
Thanks again

5. ### Terry GivenGuest

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
position).

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

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.

Cheers
Terry