Leaving a Relay latched?

[vette427-sbc]

Designing the wiring harness for my car and was wondering... can you leave a relay powered for extended periods of time?
I am working out the whole push-start thing for my car and was planning on using my alarm to trigger a relay to cut power to the ignition when it is armed. Im using a SPDT relay where the ignition circuit will be connected through pins 30 and 87a. 87 will be left open. I prefer this just incase something were to fail, the ignition would not cut out when driving. On the other hand, Im not sure how reliable it would be to be powering a relay (when the car is locked/armed/off) for possibly over a week at a time.
Any input?

Also, any good places to buy relay plugs (not pigtails)?

[BonzoHansen]

I buy all the relay parts from waytek and make my own harnesses

[dontlifttoshift]

You need a latching relay. A momentary ground clicks the relay on or off and it is mechanically locked in position, so no constant draw. Watsons Streetworks sells a kit and we use them as a battery disconnect. If you can get a momentary output from your alarm you can disconnect the battery with your remote.....I think that is what you are trying to do.

Here is there website, it does not get along with my computer for some reason.

watsons-streetworks.com

[vette427-sbc]

I would like to keep this as simple as possible for reliability reasons. The alarm only outputs a constant low mA (-) when armed. No outputs when it disarms.
Im really not worried about the current draw as it will most likely be on a battery tender if it is left alone for a while. Im more worried about killing the relay with its coil constantly being powered. Is that a valid concern?

[kawaden]

Try running the relay trigger wire off the alarms central locking output as that is usually momentary.

[dontlifttoshift]

Chris, it is simple, in fact simpler than what were planing. It's a relay that is designed to do exactly what you want it to do. I have installed 50 of them, at least, and haven't had a failure.

[vette427-sbc]

Chris, it is simple, in fact simpler than what were planing. It's a relay that is designed to do exactly what you want it to do. I have installed 50 of them, at least, and haven't had a failure.

Did some research on the latching relays... Seems like it is a better solution.
Got a link to the Watsons Streetworks part? Seems like the best latching 12v relay to use (thats readily available) is one from napa for a VW headlight switch or something.
Thanks for your help

[dontlifttoshift]

That VW latch was good. The early ones were in a metal case and well isolated, when they went to a plastic case they started picking up interference would latch or unlatch at will. Not good. That relay isn't heavy enough to kill the power anyways.

This is latching relay used as a battery disconnect. http://www.watsons-streetworks.com/battery.html It looks like a Ford starter solenoid, it is not.

[vette427-sbc]

I dont want to kill the main power... just the ignition circuit... is the VW relay reliable enough for that? It will be surrounded by other electronics. Is that going to cause interference? I definitely do not need this thing latching or unlatching on me when Im driving.

[Six_Shooter]

You don't need a latching relay to do this, in fact this circuit is included with every security system sold and uses a basic Bosch style relay.

What you don't want to do is leave a relay latched on in a vehicle with the engine not running, the 250mA (approximately) of constant draw will kill the battery very quickly.

The way to get around this is connect the negative trigger of the relay, lets choose pin 85, to the negative armed output, and the other trigger of the relay, pin 86, to an ignition source.

The way this works is that the relay will be at rest when the security system is armed, and the ignition is off, but if the ignition is turned on, the relay will then become active and cut power to the circuit you want to cut, which will be connected to pins 30 and 87a. This is most commonly used as a starter kill, but I have used this same circuit as an ignition kill in several customer's vehicles as well, as many (most) of my own. The only thing you need to keep in mind is that if you are using the same ignition circuit to power the replay as you want to cut is that the relay coil (pin 86 in my example) is connected towards the power source, which would be the ignition switch, so that the relay doesn't cycle, or "buzz". There is no draw when the ignition is off, armed or disarmed, and only draws any current when the security system is armed and the ignition is on.

You could use a latching relay for this, and I have in the past, for certain reasons used what was referred to as "Gord's piss off relay", as an ignition kill in a security system, that was created using a normal Bosch style relay, a diode and wired in a specific manner, but was really only used as a "just because I can" thing to do. The way the Gord's piss off relay worked is that if the system was triggered into alarming, the relay would latch and kill power completely to the ignition switch (could have been used for other circuits as well), that would stay latched even if the thief found the security system itself and removed it.

[vette427-sbc]

I just finished wiring everything up... Didnt use the latching relay and I did not think about the relay killing the battery. I used two relays for ignition. One is for the main ign switch, the other is inline of that one as the ign kill when armed.
I have the ign kill wired so that it is un-latched at all times when the system is armed. I like your idea of making it un-latch only if the ign switch is flipped.
Thanks!