Negative trigger relay backfeed

[ZeroAviation]

Hi all,

Is there a simple way to prevent backfeed on a traditional relay triggered by GND? Entire circuit is 12V.

I have tried a 1N4004 tied over the coil (85,86), that didn’t work. I also tried the same diode (in both polarities) on negative wire only with no success.

The only thing that dropped the voltage down was a 1K resistor tied from GND to the negative of the coil. At this point though, it is drawing (small) unnecessary current.

Any thoughts on that issue?

Also, would the solution be able to be used whether the relay was positive or negative triggered?  I want my project to be versatile to be triggered either way, without the voltage backfeed.

Thanks in advance

[Zero999]

What do you mean by back-feed?

A diode in reverse parallel will get rid of most of the back-EMF. If you're worried about the last 0.6V, then add another diode in series.

[ZeroAviation]

When I connect one side of the relay coil to positive 12V, and then measure the negative side of the coil to GND, I get 12V. The 12V is getting fed through the relay, and causes an issue with the circuit.

A diode in series prevents the back voltage, but doesn't allow powering of the coil

[HB9EVI]

hmm, I'm trying to get the clue, but...

what about a schematic? of course the positive rail voltage appears on the other end of the coil; it's not so high resistive, that it wouldn't

[Zero999]

When I connect one side of the relay coil to positive 12V, and then measure the negative side of the coil to GND, I get 12V.

That's what you should expect to happen. The multimeter has an impedance of around 10 million Ohms and the relay coil an impedance of no more than a few thousand Ohms, so if they're connected in series, nearly all of the voltage will be dropped across the meter.

The 12V is getting fed through the relay, and causes an issue with the circuit.

It sounds like you're connecting it up incorrectly. If the circuit controlling the relay, is also switching other loads, then they should be in parallel with the relay, not series.

More information is required to give a definitive answer. Please post a schematic.