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Old 02-03-2009, 11:54 PM   #1
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Default Halogen lighting wiring

So I finally got around to putting my driving lights in. Hella FF75's mounted in the bumper. Now I'd like to upgrade my bulbs. I know I'll need to rewire, I'm going for 80/130w in the E-codes and 100w in the driving lights for a total of 460w on high beam.

My concern is the current load at startup. At 12v I'll be pulling 38 amps continuous and I'm sure a little higher inrush. Should I add a capacitor to the system to cover this, and what size? Or should I not worry about it?
--Alex
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Old 02-05-2009, 01:45 PM   #2
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With cold filaments, it'll be a lot higher inrush. However the alternator is capable of dealing with brief transients without killing itself. The capacity of the field coil to maintain voltage is less than the ability of the rectifiers to pass current, so when the lights are first switched on, the voltage will sag briefly and then recover.

I am assuming that the driving lights will be on a separate switch from the main headlights, yes? This will somewhat mitigate the startup load, as you won't have four cold filaments all hitting the supply at once.

Another thing that you could do to reduce the effect of the inrush load on the system somewhat would be to source contact power for all the relays (and I assume that with 80/130 lamps, even the headlights are getting new relays) directly from the battery, rather than from the alternator. The battery will happily supply the startup requirements of the lamps and insulate the alternator somewhat from their inrush, though of course the maximum theoretical power available to the lights will be slightly diminished during operation as power will have to travel from the alternator back to the battery and then up to the lights again. Just use nice heavy wire between the battery and the lights.

As to capacitors. I've never really calculated the effect of one on lights, so this will be interesting and possible full of errors.

A 1 farad cap, assuming a nominal 14.5v system voltage, will store 105 joules of energy. A watt is 1 joule/sec. Let's say that the startup load of the lights would be 1,000 watts if they were capable of being sustained at 14.5v during startup. That would require 1,000 joules/sec to satisfy, however this is a very brief period of time. I honestly have no idea what the startup characteristics of an incandescent filament are, but let's just say that within 250msec of initial turn-on they have stabilized at nominal power. So, if a capacitor were to supply that load all by itself it would have to supply 200 joules of power (total) which is just about double what the 1 farad cap is storing. (Likewise, the cap will never actually deliver all of its stored energy, as the surrounding system voltage will never reach zero.)

So, this is actually kind of ambiguous. It's certainly within the realm of a 1 farad capacitor to assist in the startup process, but I'm not entirely sure that it passes the reasonableness test. I've certainly never heard of anyone doing it on a lighting system before- headlights aren't nearly as intolerant of voltage sags as amplifiers.

I'd say go ahead and install the lights without a capacitor (but with new, individually-switched relays) and see how that works for you. If the engine stalls when you turn 'em on, it might be worth exploring the capacitor idea. Maybe we'll learn something new.

Also, remind me never to drive in Binghamton at night. I really don't want to be going towards you on a dark road with that Baja-1000 lighting array shining at me.
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Old 02-05-2009, 02:23 PM   #3
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Thanks for the input Joe.

You're right, as far as realys go. I will be using three relays one for high, one for low, and one for the driving lights. The driving lights are wired to come one with the high beams. So on high beam I will be seeing 38 amps. I am adding an auto-off-manual switch, but more for alignment purposes. Standard use will have high beams and driving lights together.

Wiring to the battery seems like a good happy medium solution. I've got a 1/2 farad worth of caps if need be. I'll just need to refigure my wire gauge on the new length. Maybe this will let me use car stereo style fusing and distribution blocks. Would you believe no one carries high amperage lighting stuff?

And not to worry.... I am very diligent about dipping my lights. I've run 100w high beams for a while so it's become habit. Now I work 2nd shift and share the back roads with deer. It's time to upgrade from adequate to overkill status.

Thanks again.
--Alex
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Old 02-05-2009, 02:30 PM   #4
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Quote:
Originally Posted by AlexO35 View Post
So I finally got around to putting my driving lights in. Hella FF75's mounted in the bumper. Alex
Hey Alex, could we get a pic of this setup?

Thanks
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Old 02-05-2009, 05:05 PM   #5
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I've got 100w high beams and 60w driving lights, and it's great. Next time I'm going bigger. Overkill is impossible.
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Old 02-07-2009, 02:51 PM   #6
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Here't he pic....
--Alex
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Old 02-07-2009, 02:52 PM   #7
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Sorry for all the salt and crap.... Gotta llove NY winters...
--Alex
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Old 02-07-2009, 03:56 PM   #8
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Thank you Alex
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