Fusing/Relay on a 2A Third Brake Light?

[opus10583]

Hi,

I’m going to wire up a Hella Red Rear Fog Light into my brake light circuit as a Low Left Mounted Stop Light (LLMSL) and, once my head stops spinning long enough to draw the circuits (don’t know much about electrics), a selectably full-on rear fog light.

The light takes a 26watt bulb, that’s a nominal 2 amps:

A relay seems a bit of over-engineering for the brake light add-on, a fuse for the separate full-on circuit necessary all the same.

Have I got it wrong?

Thanks,
Mark

[baltobernie]

I'd use a double-pole relay, DPDT with 12VDC coil. Provide fused 12V source and output, one trigger is the stop lamp feed, the other trigger is the same 12V supply thru a toggle switch for fog function. The relay energizes whenever either or both pole receives voltage. I suppose you could also do it without a relay, using a toggle switch and diodes, but any time the light is On, the other diode is dissipating 26W, which gets pretty warm pretty fast.

[opus10583]

I'd use a double-pole relay, DPDT with 12VDC coil. Provide fused 12V source and output, one trigger is the stop lamp feed, the other trigger is the same 12V supply thru a toggle switch for fog function. The relay energizes whenever either or both pole receives voltage. I suppose you could also do it without a relay, using a toggle switch and diodes, but any time the light is On, the other diode is dissipating 26W, which gets pretty warm pretty fast.

Thanks,

Now I have even more homework!

Mark

[opus10583]

I'd use a double-pole relay, DPDT with 12VDC coil. Provide fused 12V source and output, one trigger is the stop lamp feed, the other trigger is the same 12V supply thru a toggle switch for fog function. The relay energizes whenever either or both pole receives voltage. I suppose you could also do it without a relay, using a toggle switch and diodes, but any time the light is On, the other diode is dissipating 26W, which gets pretty warm pretty fast.

Hi Bernie,

I've been looking at relays for a while now, and I just don't see it; will you please send me a link to what you're referring to?

Thanks,
Mark

[baltobernie]

Something like this:
http://www.te.com/catalog/bin/TE.Connec ... -1419124-6 Available from Mouser Electronics online.

Inexpensive, but PC board mount. You'll have to jerry-rig some kind of mounting arrangement, and solder wires to the pins. Two independent triggers; one each to your rear fog switch and brake lamp feed.

[opus10583]

Something like this:
http://www.te.com/catalog/bin/TE.Connec ... -1419124-6 Available from Mouser Electronics online.

Inexpensive, but PC board mount. You'll have to jerry-rig some kind of mounting arrangement, and solder wires to the pins. Two independent triggers; one each to your rear fog switch and brake lamp feed.

Hi Bernie,

Please go slowly, everything I learned about electricity I learned at the end of a paperclip in an outlet:

That's DPST, with one circuit for the coil switching two circuits independently, isn't it?

http://www.te.com/_TEincludes/ver/595/T ... Size=false

I'm thinking of using this, or something like it, to feed a SPST relay for the light, with one circuit being the brake light feed and the other simply 12v, with a totally OFF option:

http://www.radioshack.com/product/index ... Id=2062537

?

Thanks,
Mark

[baltobernie]

This switch will work, but you'll need a diode to prevent "back feed" to the circuit when you have the fog light On and you touch the brakes. I will draw you something next week. In the meantime, if anybody else wants to "enlighten" Mark, go right ahead.

[opus10583]

Here's what's going to happen:

I picked up a center-off SPDT switch and some 18ga wired from Radio Shack.

I have an 20A rated relay around here somewhere... Two, I think.

I'm going to run fusebox current to one side of the switch (A), tap into the brake light current downstream of the brake switch for the other side (B) and run the common connector to the switching current input of the relay with a fuse on the battery feed to the relay power input thence onto the rear light.

Switch position A: Red rear fog light.
Switch position C: OFF
Switch position B: Low-mounted third brake light.

The relay is overkill for the amperage, but I wanted to isolate the third light from the brake light circuit as a fail safe.

Any thoughts?

Thanks,
Mark

[majicwrench]

What you have described above, with the three pos switch sounds functional, but...

A relay for a 2 amp draw is indeed serious overkill.

Keith