12V Relay - Polarity On Activating Relay

[Lexus-IS]

I have a 12v DC relay that I am working with, the relay is activated by a power source connecting to the positive and negative of the relay. however, the source that i am trying to use to activate the relay changes polarity (positive becomes negative and vice versa). I only want the relay to activate if polarity is certain way. can anyone tell me if the polarity is reversed on the relay activation, will the relay still work or will it end up blowing the relay? or will the relay only work with the polarity correctly matched otherwise it just doesnt activate? if so then that is exactly what i'm looking for. But dont want to risk blowing the relay without some advice first

[IHgs300]

if you want the relay to only activate when feed is positive just earth relay to ground point and use your supply to relay positive. Will only activate when feed is positive,wont blow relay as when supply changes to negative just wont do anything.

[Lexus-IS]

if you want the relay to only activate when feed is positive just earth relay to ground point and use your supply to relay positive. Will only activate when feed is positive,wont blow relay as when supply changes to negative just wont do anything.

thanks for your reply, but does will it not effect the relay because the ground on that circuit doesnt seem to be shared with the ground of the car? i did a continuity test to see if the ground is linked to the earth of the body but doesnt seem to be

[IHgs300]

if you want the relay to only activate when feed is positive just earth relay to ground point and use your supply to relay positive. Will only activate when feed is positive,wont blow relay as when supply changes to negative just wont do anything.

thanks for your reply, but does will it not effect the relay because the ground on that circuit doesnt seem to be shared with the ground of the car? i did a continuity test to see if the ground is linked to the earth of the body but doesnt seem to be

ok, just connect relay earth to body earth and supply to + as normal, as ground should always stay the same just supply alternates.to be safe just add an inline fuse to both wires.

[Geoffers]

ok, just connect relay earth to body earth and supply to + as normal, as ground should always stay the same just supply alternates.to be safe just add an inline fuse to both wires.

The problem you have with doing that is that when the circuit side of the relay becomes powered by the circuit, the relay will act as a resistor and there will be a potential difference to the ecu. This is because the ECU's negative is "floating" and not in respect to the body earth (which is why you don't get continuity to earth when you tested it) and could cause an issue. You wouldn't like to let the magic smoke out of your ecu in 3 weeks time.

You would be better wiring straight across the device but have a diode in series with the relay. This will make sure all the points stay at the correct levels, and only allow current to pass one way. You can actually buy polarised relays, but they are a bit more expensive and not as well stocked.

Absolutely put a fuse inline with all equipment you are adding.

Here the diode is inline, and I've drawn it with pos/neg flowing in that direction, so the relay will actuate. Putting the voltage the opposite way, nothing will happen and no current will flow across the relay.

[Lexus-IS]

Thanks for your reply Geoff, very much appreciated. So by putting a diode on the positive line of the relay, when the power source switches polarity and positive is being sent through the negative of the relay, i am assuming because of the diode on the actual positive of the relay, the current will not be able to come backwards and complete the circuit. is that correct?