How to find out at what voltage a 12v automotive relay will "trip"

[1hawaii50 10+ year member]

I need to install a relay on my 12v switched line as with the ignition off, I'm getting 5-13mv "bleed-through" on the line and it is causing a problem with my PAC SWI-PS. I want to install a relay and use the 12v switched line to "trip" it, but I need to make sure that the relay will "turn off" when I turn the key off (even though the line is still seeing 5-13mv). I checked Radio Snatch, but the idiots working there had no idea at all. Autozone is also clueless. I found on bcae1.com that a "typical" bosch relay closes at 8v and releases at 1-5v. This doesn't help me as I don't know if the relay at Autozone is the "typical" Bosch relay. Any help is appreciated.

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[Einzee 10+ year member]

You used the magic word, "typical". The term bosch refers to the configuration of the terminals on the relay. There are many manufacturers of bosch style relays.

I have a thought about your situation with the mv's still on the wire. Something in your car is "backfeeding" onto that circuit. If in fact that minimal amount is actually causing problems then it would only make sense to track the source of the voltage and remedy it rather than trying to work around it.

 

[AlterEgo99 5,000+ posts]

I have a PAC SWI-JACk for my steering controls in my '08 Silverado and per PAC I needed a SPST relay to ensure it would operate properly. I picked one up at Pep Boys, the only SPST relay they carried. Works!

 

[Stephen_c 10+ year member]

mines 5.7v to trip the relay. i used an adjustable power supply and ran 86 pin to + 85 to - and turned it up while i watched the meter waiting for the click. 5.7 on all 3 that i tested

 

[1hawaii50 10+ year member]

You used the magic word, "typical". The term bosch refers to the configuration of the terminals on the relay. There are many manufacturers of bosch style relays.
I have a thought about your situation with the mv's still on the wire. Something in your car is "backfeeding" onto that circuit. If in fact that minimal amount is actually causing problems then it would only make sense to track the source of the voltage and remedy it rather than trying to work around it.

I traced the 12v switched wire all of the back to the stock radio harness, and the small 5-13mv is still present. I thought it could be the Metra NIRD-01 causing the problem, but the problem is present before the NIRD, it is in the stock harness. I haven't added anything else to this car other than the stereo system and amps, if there is something backfeeding I would guess it came like this from the factory.

 

[1hawaii50 10+ year member]

mines 5.7v to trip the relay. i used an adjustable power supply and ran 86 pin to + 85 to - and turned it up while i watched the meter waiting for the click. 5.7 on all 3 that i tested

This is good to know. I'll go to Radio Snatch tomorrow and pick one up and we'll see what happens.

 

[smoka 10+ year member]

mines 5.7v to trip the relay. i used an adjustable power supply and ran 86 pin to + 85 to - and turned it up while i watched the meter waiting for the click. 5.7 on all 3 that i tested

Need to check current too. The relay coil has a resistance to it.

I want to install a relay and use the 12v switched line to "trip" it, but I need to make sure that the relay will "turn off" when I turn the key off (even though the line is still seeing 5-13mv)

From my experience, I've always assumed that the relay's rated power consumption is close to the switching level. I usually need to take this in consideration when adding transistors to low level electronic boards to switch higher power circuits etc. I need to know how much power I need to switch a relay and that helps me choose the transistor to use.

Check out the relay highlighted in the middle of this page:

http://www.mcmaster.com/#9672k32/=28edky

Since the power consumption is rated at 158mA @ 12vdc you can calculate the power to be (P=IxE) or 1.9W

If you see 13mV you would need in the range of 145A to switch the relay which is not going to happen.

I'm not sure at what levels the relay releases at....

So you should be fine. But like stated don't put a bandaid on a problem or you'll end up regretting it one day.

 

[smoka 10+ year member]

I traced the 12v switched wire all of the back to the stock radio harness, and the small 5-13mv is still present. I thought it could be the Metra NIRD-01 causing the problem, but the problem is present before the NIRD, it is in the stock harness. I haven't added anything else to this car other than the stereo system and amps, if there is something backfeeding I would guess it came like this from the factory.

Is your deck grounded to chasis?

 

[1hawaii50 10+ year member]

Is your deck grounded to chasis?

I grounded my deck harness, deck chassis, my RCA's, the NIRD-01 interface, the PAC SWI-PS, the GEX-P10HD, and the CD-100USB all to the same point. They are grounded to bare metal bracketry behind the radio. Just about everything behind the dash is plastic, but there is a large metal brace running left to right across the back of the radio...that's what I used for a grounding point. As stated, I traced with my DMM back to the factory harness... I'm not really sure how to trace any farther to find the problem.