Relay question.

[TheBigMan]

I just bought a component batch for some old diodes and trannies.  Got some 6V DPDT relays in there as well.

Question is though, can I use a 9V supply with a resistor in series and then another resistor in series (- coil resistance) to power an LED?  Seems to me it should be fine, but I always like to know for sure without blowing things up...  

[TheBigMan]

Surely somebody knows?  :cry:

[R.G.]

You can do one of a couple of things.

First and simplest, you can use a 6V relay coil in series with a resistor to get the coil voltage lower than the 9V you'll be using. To pick the right resistor, just measure the resistance of the coil wire and then figure out the current at rated voltage. That's Icoil = 6Vdc/Rcoil. Then you need a resistor of (9Vdc-6Vdc)/Icoil in series. You may want to make the resistor a little low, as battery voltage drops as the battery wears.

You can then hook a resistor/LED in parallel with the coil. The LED voltage is between 1.2V and 2.4V depending on color (if you didn't use blue or white) and you figre the resistor the same way: Rled= (9V-Vled)/Iled.

Now the LED comes on when the relay is energized.

If the coil current in the LED is under the 20ma that most LEDs will eat without dying, you can just stick the LED in series with the relay coil. The relay coil resistance limits the LED current. If the coil current is more than 20ma (likely with a 6vdc relay) then you can't do this without burning out the LED. The first version is simpler and cleaner.

[TheBigMan]

Thanks.  

I was lucky in that I've got data on the relay.  The coil resistance is 100R so that gives a relay current of 6mA and a series resistor of 50R.  That would make it OK for the LED to be in series, but if it's safer (and it's hardly any more hassle) to do it in parallel then I think that's what I'll do.

[R.G.]

The coil resistance is 100R so that gives a relay current of 6mA and a series resistor of 50R. That would make it OK for the LED to be in series,

ACK!! Six volts and 100 ohms is 60 ma! You'll cook any LED I know of if it's hooked in series. The 50 ohms sounds right though.

I'd go with parallel.

[TheBigMan]

:oops:

Parallel it is!

How I managed to convince myself that there's only 100mA in an amp is beyond me.  :?

And now that I think about it a pedal that uses more than a digital delay just for it's bypass switching is of very little use.  Still, at least I know what to do if I find some cheap low current relays.

[R.G.]

And now that I think about it a pedal that uses more than a digital delay just for it's bypass switching is of very little use. Still, at least I know what to do if I find some cheap low current relays.

Take a look at latching relays. They only use a pulse of current when they switch. There's an article on how you might do this at GEO.