Relay wiring question

[Piwoslaw]

So I've installed my DRL's. They're wired to the cig lighter socket, so they turn off when I take the key out of the ignition. But the law here states that DRL's must automatically turn off when the headlights switch on, so I need to hook up some kind of relay. The manual off switch isn't enough, since my headlights automatically turn on when it gets dark, and I might not notice and remember to off the DRL's. In theory I could (and should) just go through the relay that already turns the headlights on, but that is buried somewhere in the bowels of the BSI and fuse box, all neatly sealed so that there is no chance of getting my slimy tentacles in there. I'm thinking that I'll tap into the wire going into one of the headlights (if the bulb burns out and the other light comes on it won't turn the DRL's off, but then I can explain that it's a safety function for burned out bulbs) and put the relay between that signal and mass. The relay and headlight will be wired in parallel, not in series. The DRL's would be wired to the contacts of the relay.

My question is: Should I put a resistor and/or diode in series with the relay coil? If so, what kind? Would the current be too big without a resistor? Is there any special kind of relay I should use, or is a generic 12V OK?

I'd put up a diagram of what I'm planning, but I can't reboot at the moment and don't have any usable drawing tools under this system.

Thanks for any help,
Adam

[JacobAziza]

Originally Posted by Piwoslaw

My question is: Should I put a resistor and/or diode in series with the relay coil? If so, what kind? Would the current be too big without a resistor?

Depends on the rating of the relay you get.

I have a "generic 12v auto relay" still in the package, its only 30A, which should be more than enough for a 55w headlight (30A x 12v = 360W) without a resistor. (unless standard headlight and relay ratings are different in...um...where ever you are)

Although I am not sure that it'll work in parallel, as it won't get full current and might not be enough to close the relay. On the other hand, in series it might dim the headlight, so...

[Piwoslaw]

Thanks for the reply, Jacob. Yes, in my neck of the woods headlight bulbs are also 55W. Here is what I was thinking about:



I am concerned about a power drain to the headlight, so maybe the series version is better?



I read recently that the diode is needed to suppress any bad things that may happen to the ciruit after switching the power off, but I've no idea what kind of diode to use.

I understand that a relay labeled as 30A/12V means that that is the current and voltage for the coil, not the contacts, right? Since 55W is about 4.5A @ 12V, then wouldn't a 5A relay be enough? Maybe 10A to be safe? Will the relay in series dim the bulb noticeably?

So many questions...

[JacobAziza]

disclaimer: I don't have any electrical training at all. Like everything I do, its all self taught and largely trial and error.

it looks like the diode is in parallel to the relay, in which case I'm not sure what good it would do. But then again, I can't imagine why it would be necessary anyway. Where did you read that, and what sort of "bad things" did they suggest?

With the resistor in line in the parallel version I suspect it would affect the headlight less than if it was in series (though, again, I don't really know much electrical theory)

As far as the relay power, I'm not sure if it is contact (switch) or coil or both, but considering they are usually used to use a low current switch to control a higher current circuit, I would expect the rating to refer to the switch side.
It certainly won't hurt to have one rated for higher than you need in any event.

What might be slightly more challenging is finding a normally closed relay.

[roflwaffle]

It's there to help w/ relay lifespan.

Quenching Diodes:
Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. The diagram below will show the connection of the diode. Initially, you may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach 200 volts. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts and may cause the failure of power switching transistors. You can use virtually any type of rectifier or switching diode

[JacobAziza]

thanks! I just learned something new

[roflwaffle]

No problemo!

[wagonman76]

Originally Posted by JacobAziza

What might be slightly more challenging is finding a normally closed relay.

A typical automotive 30A relay (at least all the ones I have used) can be used normally open or normally closed. There are 5 pins. 2 for the coil. 1 for the common terminal. 1 for normally open to the common terminal. 1 for normally closed to the common terminal.

I would just tap into the a headlight wire and use it to power the relay coil. Run your DRLs through the relay, normally closed, so when the headlights are on, it will break the circuit to the DRLs. I've never used a diode but it certainly couldn't hurt.