Quad low beams / Which fuse to use?

[DrWin]

Hi guys,

I've decided to change the headlights to 4 "outer", so they all have both low and high beams.
I think the conversion will be easy enough, it's just adding a piece of wire from where the power supply goes into the outer headlight (blue w/red tracer) and connecting that to the contact on the inner headlights.

I'm having trouble figuring out which fuse to use, though. The stock setup is 20amps for both low and high beams, and the deloreango recommandation is 25 amps low beam / 30 amps high beam, which is what I'm running atm.
I sat down trying to figure out what low beam fuse I should use after converting to 4 lowbeams, but I'm getting a reading of 1ohm across the low beam circuit in the headlamp, which would mean for 4 low beams in parallel connection that I would get 48 amps total, 12 amps for each low beam thats on. Propably not the right answer. :) Is there significant resistance from the power supply to the headlights or something else I'm missing?

Any input appriciated. :)

[Bitsyncmaster]

With incandescent bulbs you can not use the cold resistance to compute current. That is the peak current you will get for an instant when first turned on but quickly as the filaments heat the resistance goes up.

Use the power rating of the bulb to compute current. I think the EU bulbs are rated at 50 watts or more.

I= P/V where I is the current in amps. P is the power in watts. V is the voltage in volts.

Hence, I = 50/13.5 computes to 3.7 amps for each bulb. Hence 4 bulbs would be 14.8 amps.

Now fuses run hot the closer you run them to the rated current. So using a 20 amp fuse would run pretty hot.

Now if you run both the high and low beam filaments in each bulb then your power for each would be around 80 watts.

[DrWin]

With incandescent bulbs you can not use the cold resistance to compute current. That is the peak current you will get for an instant when first turned on but quickly as the filaments heat the resistance goes up.

Use the power rating of the bulb to compute current. I think the EU bulbs are rated at 50 watts or more.

I= P/V where I is the current in amps. P is the power in watts. V is the voltage in volts.

Hence, I = 50/13.5 computes to 3.7 amps for each bulb. Hence 4 bulbs would be 14.8 amps.

Now fuses run hot the closer you run them to the rated current. So using a 20 amp fuse would run pretty hot.

Now if you run both the high and low beam filaments in each bulb then your power for each would be around 80 watts.

Thanks a lot Dave, I'll try sticking a 30amps in there instead of the 25amps I got now for the two lowbeams. :)

[David T]

Before you do this you should check with your local DMV about the rules for the headlights. You may find yourself getting pulled over for using your high beams, with all four lit up it will look like you are driving with the high beams on all the time. Some States are very particular about this stuff and even if they aren't it is a great excuse to pull you over and check you out. Once the Police pull you over for some minor thing, they now have an excuse to check your paperwork and examine your vehicle to see what else they can find. In many jurisdictions it is all about the revenue (fines) that they can generate for the town.

[DrWin]

Before you do this you should check with your local DMV about the rules for the headlights. You may find yourself getting pulled over for using your high beams, with all four lit up it will look like you are driving with the high beams on all the time. Some States are very particular about this stuff and even if they aren't it is a great excuse to pull you over and check you out. Once the Police pull you over for some minor thing, they now have an excuse to check your paperwork and examine your vehicle to see what else they can find. In many jurisdictions it is all about the revenue (fines) that they can generate for the town.

Not in rural Denmark it isn't. I can go 6 months w/o seeing a squad car, and when I meet the police they are usually some of my own patients. :) At any rate, it's a done deal and it looks totally awesome - not to mention it improves visibility at night, since I'm now packing four flickering candles in stead of just two.