Is the direction the Current flows, Important in a SPDT Relay ?

[aktundra]

I am wiring in a Matrix Switch & Inverter into the dash of my 80 series.

I would like the inverter output (the plug socket) to be "hot" if the motor is running (will be wired in to the cig circuits and operate under the same conditions as the aux power at the cigarette lighter.

I would like to use the Matrix's switch, if I make the conscious decision to press the switch, make the plug socket hot, and run off the battery when the motor is shut off.

Will this work for SPDT relay wiring:?

85 coil unput from Matrix switch
86 GND

30 output to the inverter
87A Cig 12v (switched)
87 +12v (unswitched, fused)

 

[george_tlc]

That'll work fine from a switching/control perspective - just make sure the cig 12V circuit can handle the current requirements of your inverter.

Obviously if you are drawing a bunch of current through that circuit with the inverter, then the cig lighter outlet can't have an additional large load applied or the stock wiring/fuse will be compromised...

A better solution would be to run a single fused +12V through the relay (30/87) and add 2 diodes (1A diodes would be fine) to "or" +12V to relay 85 so either the vehicle running (cig +12V active) or the matrix switch on will provide 12V to the coil.

That way, ALL +12V inverter power goes through the same path - the new heavy/fused wire from the battery, versus 'stealing' from the cig lighter power.

cheers,
george.

 

[sdnative]

To answer what I think was the original question: for a DC relay coil direction does matter. For a Bosch style relay, pin 85 is (+) and pin 86 is (-) or ground.

What George proposed is a great idea, but it does add complexity. Just depends on your objectives. Personally I would just wire the relay coil through the switch, that way you have control over the inverter operation regardless if the vehicle is running or not. Of course if you forget to turn off the switch and have a load attached you could drain your battery.

 

[george_tlc]

For "most" DC type relays polarity does NOT matter. There are "some" relays that have a built in diode across the coil to absorb back emf when driven by a semiconductor device and must be wired correctly such that the diode is reverse biased in normal use. So, check the type of relay you are using, i.e. whether it has a built in diode AND which pins the anode/cathode are wired. Often a diode will be shown in the graphic on the relay or check the datasheet for the specific relay you are using.

Some relays use a resistor to absorb the back emf, so those have no coil polarity requirement.

Anyhow, I'd question the blanket statement that coil polarity is important with a DC relay. It is IF it has a built in diode...

Bit of web searching found this nice tutorial:

http://www.bcae1.com/relays.htm

cheers,
george.

 

[sdnative]

Thanks for clarifying and I apologize for the misinformation. The blanket statement was not appropriate.

The relays I use all have the suppression diode so direction does matter. Without the diode, direction wouldn't matter electrically, although I still think the convention would be to wire the (+) to pin 85, maybe not.