Questions and Answers
Headlight Relays
Q. I'm an Avionics instructor in the Air Force and I use a car headlight relay as an example of how a basic relay works. The question I have is: Why is there a resistor wired in parallel with the coil. The "schematic" I'm referring to is the one drawn on the plastic cover for the relay, so it's the only info I have on this particular system.
It's a basic 4 wire relay with the "control" wires energizing the relay and the "High Current" flowing through the other 2 wires. The confusion is the "control" wires have a resistor in parallel with it. I'm familiar with impedance matching and other electronic theories, but that doesn't seem to fit this situation.
The only theory I could come up with is that the resistor acts as a threshold that only the proper voltage/current could overcome and would eliminate any intermittent lighting problems. But that's just a guess. Any help would be greatly appreciated.
Thanks,
JeffA. I know the resistor you are talking about and I included a picture of one from a Ford here so others will know what you are referring to.
As to the purpose of the resistor in the relay, I have to be honest and say I don't know. Since you have more experience and education in electronics than I do, your guess would be more informed than mine. Perhaps someone who lnows more than both of us will chime in with an explanation for us.
Additional Information provided courtesy of ALLDATA
