12V relays and 9V supply

[Dimitree]

hello, I have some 12V relays that I need to use inside a pedal that got 9V supply.
The relay datasheets says that the "must operate voltage" is 8.5V DC. I was wondering if there was some issue using 9V instead of the nominal voltage that those relays require. Or it would be better to place a small charge pump and supply them at 12V?

[Seljer]

Just try them? if they switch then they're good

[Dimitree]

ok I will try soon but I was wondering if there was any downside (that maybe I cannot notice on a simple test..)

[R.G.]

If your "9V" supply is always guaranteed to be above 8.5V, you're golden. Note that batteries start at 9.4V or so for new, fresh alkaline types and drip to about 7V before the manufacturer thinks they're exhausted.

If you are always running from an adapter that's always over 8.5V, this isn't a problem. If you may run from batteries some times, then you'll find that (a) batteries don't last long because of the relay current drain unless you use latching type relays and some kind of pulse set/reset circuit and (b) the relay will quit operating before the circuit would otherwise think the batteries are too far down.

[Dimitree]

many thanks, no I won't never operate on battery on this device, not only for this relay thing.. so it will be fine

[Dimitree]

now another question came up to my mind: how do I find the max voltage that the relay coils can handle? It is supposed to be on the datasheets? (I only find "must operate" and "must release" voltages, and of course nominal voltage)

[DiscoVlad]

Calculate it from whichever is given of the coil resistance, current rating and, power rating. <- these will probably all work out to around the nominal values anyway.

Remember that the coil is an inductor made up of many turns of fine wire. Increasing the voltage across it will increase the amount of current through the coil in accordance with Ohm's law. As long as the coil current is below the current rating of the coil you should be fine... If the coil current is too high, the wire has a disturbing tendency to melt.

The max voltage the coil can handle would be at least as large as the back EMF when the coil current is switched off. This voltage spike will be as large as it needs to be to maintain the current through the coil. For this reason we use a reverse biased diode (on DC coil relays) as a snubber to limit this voltage spike to levels that aren't going to damage the coil driver.

Hope this helps!

[R.G.]

It should be on the datasheet, perhaps as a graph or as a tolerance on the nominal coil voltage.