Headlight relay wiring

So my LR4 LED lights are on the way and I'm getting setup to complete the installation. I plan on using a common automotive relay to power the lights and I've prewired the FJR's headlight relay with a tap from the high beam wire to use as a trigger for the LR4 power relay.

My question is this: Should I install a low amperage in-line fuse between the LR4 relay trigger and the FJR headlight relay? Something like 3amps. My thinking is that if the automotive relay malfunctions, could it damage the FJR headlight relay, and is the 3amp fuse weak enough to protect it?

Some of you sparky types, fell free to chime in. Bike is 2014ES.

Thanks.

FJReady said:

Should I install a low amperage in-line fuse between the LR4 relay trigger and the FJR headlight relay? Something like 3amps. My thinking is that if the automotive relay malfunctions, could it damage the FJR headlight relay, and is the 3amp fuse weak enough to protect it?

Click to expand...

No, not between a trigger wire and the relay. It's exactly that...a trigger wire. The trigger wire isn't going to damage anything and doesn't need a fuse. The kind of failure that would occur is that it just doesn't work...not that you burn anything up. Focus on that fuse from the 30 terminal of the relay to the battery. (substitute "light" for "horn" in the schematic below.

So, how paranoid are ya feeling? There is no harm and perhaps some small benefit to adding a fuse to the circuit. Headlight relay coils are typically 180 ohms so the current @ 14.0 volts is 77 mA. If there is a failure the current most likely won't be an issue that it's drawing an extra amp or two, it will almost certainly be like a short issue so 3 amps is sufficient and not pop from in-rush current.

For belt & suspenders you could put a snubbing diode across the headlight relay coil to protect the LED driver.

It is highly unlikely that anything short of but mechanical damage would hurt the relay and there is nothing electrical in the FJR headlight circuit that would electrically cause a relay issue. Only a LED driver that fails in a way that shorts the high beam trigger circuit to ground would be a problem.

Edit: I see Matt's post. Yes, protect the +12 volt circuit. I think Russ is concerned about what happens if the LED driver has a failure and it does Something Bad to the headlight high beam circuit, like short to ground.

Edit II: Russ, if you need a hand, I sorta owe ya... LMK.

ionbeam said:

I think Russ is concerned about what happens if the LED driver has a failure and it does Something Bad to the headlight high beam circuit, like short to ground.

Click to expand...

That would have been in the Electrons 401 or Smoke Check 398 course I didn't get to. I've been so enamored with sourcing an actual Hella brand relay and pigtail that I didn't get to that fancy-schmancy stuff. Ionbeam is the smart fella here.

Uncle Rusty:

I put a fuse in there.

YMMV

Thanks guys, I am a little paranoid about damaging the bikes wiring with my Autozone grade $7.00 relay, hence the question.

Iggy, thanks for the diagram, that's how I did by horns on my 2005 many years ago. And I will have that 10 amp fuse on the hot side of my new set up as shown. My question was about the wire I'm installing between the two relays(FJR headlight hi-beam output and auxillary relay trigger) Again my concern was protecting the wiring harness against cheap relay malfunction. Maybe total unnecessary but still its cheap and easy to do. I think that's what perfessor 'beam is implying..

Thanks again, fellas

FJReady said:

...Maybe total unnecessary but still its cheap and easy to do. I think that's what...'beam is implying..

Click to expand...

Yup.

A fuse between the LED driver high beam trigger wire and the high beam circuit wouldn't hurt and will provide paranoid, doomsday protection but there is nothing that a malfunctioning relay can reasonably do to hurt the circuit.

Actually, on my install I put the fuse between the High Beam hot wire and the trigger into the LEDrider Dimmer module. The dimmer module switches between dimmed and full on with the high beam trigger. Of course I also fused the power to the cheapo 12V DC power relay that supplies the power to be dimmed.

Having that dimmer is great. I can turn the brightness in the low beam mode way down, or off, and still have full brights on high beam. Or if you go to the click stop off neither high or low beam come on. I wired it so the dimmer module is inside the glove box; easily accessible, but out of sight and won't get wet.

The coil wire on the trigger coil is basically a fuse in and of itself. The wire is pretty teeny, and if there's an issue, it would probably cook itself before your fuse popped.

audiowize said:

The coil wire on the trigger coil is basically a fuse in and of itself. The wire is pretty teeny, and if there's an issue, it would probably cook itself before your fuse popped.

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You don't seem to understand the purpose of a fuse, which is to protect the wiring of the bike in the case of a malfunction such as a short circuit.

usually the $500 module burns out to protect the 10 cent fuse.....

mcatrophy said:

audiowize said:

The coil wire on the trigger coil is basically a fuse in and of itself. The wire is pretty teeny, and if there's an issue, it would probably cook itself before your fuse popped.

Click to expand...

You don't seem to understand the purpose of a fuse, which is to protect the wiring of the bike in the case of a malfunction such as a short circuit.

Click to expand...

There's very little receiving protection from a 3A fuse that just feeds the trigger coil on the relay, especially if the length of wire between the two is short and well protected. If the relay craps out and the trigger coil shorts, my point was only that a 3A fuse would probably sit there and let the coil cook until it opened up.

Ummm... nope.

A 3A fuse will blow in a heart beat if you short the coil. 3 amps is not very big, and it certainly wouldn't let the wiring get hot, which is why the fuse is there, not to protect the relay.

But the more important point is, if you install a relay (or any other device) wired directly to the High Beam headlight power without an inline 3 Amp fuse (as suggested), if that relay coil shorts out it will, in the best case, blow the 25A Headlight fuse , leaving you with no high or low beam headlights at all. It will heat up and may burn up the small gauge wire that you probably ran from the headlight high beam to trigger the relay first (because 25A is a considerable amount of current) and light your bike on fire.

How hot could that wire get? Well some smart old guy named Ohm said that 12V X 25A = 300 watts. So in the case of a direct short circuit, the wiring will be heated by 300 watts of power before the headlight fuse burns out.

That is the reason why you would want to put a small fuse in line with the relay appropriately sized for the smallest wires in the circuit.

audiowize said:

mcatrophy said:

audiowize said:

The coil wire on the trigger coil is basically a fuse in and of itself. The wire is pretty teeny, and if there's an issue, it would probably cook itself before your fuse popped.

Click to expand...

You don't seem to understand the purpose of a fuse, which is to protect the wiring of the bike in the case of a malfunction such as a short circuit.

Click to expand...

There's very little receiving protection from a 3A fuse that just feeds the trigger coil on the relay, especially if the length of wire between the two is short and well protected. If the relay craps out and the trigger coil shorts, my point was only that a 3A fuse would probably sit there and let the coil cook until it opened up.

Click to expand...

As I said, it's the bike's (or your additional) wiring that is being protected by the fuse, not any load that maybe presented on that wire. If your relay coil fails in a way that makes it draw more current than it should, as a functional relay it's probably already failed. If it catches fire, that's tough (your 3 amp fuse can deliver 42 watts without flinching, enough to start a serious fire).
However, if the wire's insulation frets through and shorts to the bike's frame, the fuse will blow safely. (I nearly wrote "fail", but of course if it blows, it has succeeded .) Without that your short circuited wire would ask for many tens (hundreds?) of amps, hundreds (thousands?) of watts, an explosive amount of energy.

[edit] fred w posted as I was typing, saying much the same thing, but much more eloquently .[/edit]

I think I'll just put about a half dozen fuses in there, in parallel so they'll all get a chance to work!