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Old 12-31-2008, 03:33 AM   #1
N3v
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Default headlight wiring

On my moss motors lowpro setup, the projectors work amazing, they totally illuminate the road, but when you switch to the highbeams and it turns the lows off, there is light everywhere, but not as much on the road. When I hold the switch just right to turn them both on, its amazing, total virtual daylight. So, I'm trying to wire it up where when you switch it to highbeams, both bulbs stay on. I think some kits some stock that way. Anyway, I figure I could do it with a relay pretty well, as illustrated in this awful, awful mspaint drawing:
Name:  headlightsetup-1.jpg
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The only con to this is the couple of bucks i'll have to spend at radioshack, and that i'll be adding a little more hardware in each headlight housing. Is there a simpler way to do this? I tried tinkering with the switch behind the steering wheel, and immediately i realized it would be much less of a pain and more reliable to add the relays. Is there a better solution than that that comes to mind for anyone?
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Old 12-31-2008, 10:09 AM   #2
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Your first idea (with the diode) was on the right track, but the problem there is finding diodes big enough to handle the load of a headlight bulb. Diodes also create some voltage drop (0.7v in the case of a normal silicon type) which would affect brightness.

Your relay circuit won't work as drawn. You've got the power for the hi-beam trying to go through the relay coil, which in this case will act like a resistor and prevent the lamp from getting anywhere near enough current to light up. Here's a modified version of your drawing:



Also, be aware that all four filaments are powered from a single fuse- the 30A headlight fuse, and you're now drawing double the current across it. You should be ok so long as your total headlight load is 300w or below, but carry some spare 30A fuses with you just in case.

Last edited by Joe Perez; 01-01-2009 at 11:07 AM. Reason: Deleted incorrect schematic
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Old 12-31-2008, 02:47 PM   #3
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Quote:
Originally Posted by Joe Perez View Post
Your first idea (with the diode) was on the right track, but the problem there is finding diodes big enough to handle the load of a headlight bulb. Diodes also create some voltage drop (0.7v in the case of a normal silicon type) which would affect brightness.

Your relay circuit won't work as drawn. You've got the power for the hi-beam trying to go through the relay coil, which in this case will act like a resistor and prevent the lamp from getting anywhere near enough current to light up. Here's a modified version of your drawing:
http://img37.picoodle.com/img/img37/3/12/31/thraddax/f_Hibeamm_f1fff6e.gif[/IMG]
Notice that here, the relay coil is fed in parallel with the hi-beam lamp. That's the only difference.

Also, be aware that all four filaments are powered from a single fuse- the 30A headlight fuse, and you're now drawing double the current across it. You should be ok so long as your total headlight load is 300w or below, but carry some spare 30A fuses with you just in case.
awesome thanks! If it blows I've got a pack of 40's, I'll pop one of those in.
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Old 12-31-2008, 07:24 PM   #4
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i installed a relay just like the diagram, and it works, except for a couple of things:
1. one relay on one headlight makes them both do it. Heh, guess they're on the same circuit, which means I need one beefy relay instead of two

2. the only relays they had at radioshack (yeah i know, radioshit) were SPDT, which means it was the same as joe's diagram, except there's one more pin for when the circuit is 'open'. I just left that pin with nothing on it.

3. now, they all four turn on, but when i flip back to low beams, the highs stay on. I have to turn the headlights off totally then flip the lows back on. That's not really too big of an issue I guess, but joe, if you have any idea why that happens, that'd be awesome. Thanks!
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Old 01-01-2009, 09:46 AM   #5
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I'm an idiot.

In the diagram I posted, the relay will hold itself on once it's switched as power is back-feeding into the coil through the points.

Gimme a minute to this about this...
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Old 01-01-2009, 11:05 AM   #6
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Ok, we need to isolate the control signals from the loads. We could use diodes, but that would require high-current, low-drop schottkeys which are expensive and not available at Radio Shack. It would also decrease the brightness of the headlights, as even the best diodes in this current range have a forward drop of about 0.5v.

The proper way to do this is with two high-current relays, and a low-power steering diode on the control side. The diode can be anything in the 1N400x family.

You can wire both hi beams to the "hi" relay, and both low beams to the "low" relay. Power (marked BATT) will need to be taken from a point before the headlight switch. I'd suggest you either run a new 14ga wire (with an inline fuse) direct from the battery, or if you want, you can tap the Red/Blue wire coming off the Headlight fuse. The headlights can ground to wherever. You'll probably improve the brightness of them slightly if you ground 'em to the chassis right at the point of use, rather than re-using the long-*** stock ground wire.
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Old 01-01-2009, 04:30 PM   #7
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hmm, well having already spent $4 on one relay, I can calculate that that setup would cost me another $12. It would be simpler to just wire the high beam circuit to a flip switch in the cabin. Isn't the headlight system already controlled by a relay? I'm gonna check that out today. Maybe if I can just modify the wiring on the stock relay I can get it to do what I want.
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