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Posted by: Krieger Aug 26 2012, 08:39 PM

As some of you know my car has different "headlights" I no longer have the pop up style. The lights are in the turn buckets. I bought a 15 amp toggle switch instead of the stock unit. This is what I would like to run on my new switch, but I do not know how many amps these items will take: front and rear running lights, license plate lights, dash lights and the stock relay to trigger the fog lights (this is how I chose to power my new "headlights". I was using the original hot wire that was on the oem headlight switch. It comes directly from the ignition switch to the headlight switch. Maybe I should put a 15 amp fuse in line on that hot wire? Other minor issue is the toggle switch has two posts and I have 5 wires to put on one end...

Posted by: GeorgeRud Aug 26 2012, 08:54 PM

Don't run the lights through the toggle switch alone. Use the switch to energize a simple relay and you'll be in good shape.

Posted by: Krieger Aug 26 2012, 09:12 PM

Why not use a toggle switch?

Posted by: kg6dxn Aug 26 2012, 10:04 PM

Toggle switch will arc internally, just like a set of points and eventually stop working. Use a relay with the toggle switch. You should never exceed 85% of the switch rating.

Additionally, most switches are made in China now and not really rated for the stated amount.

Ohms Law:
E=IxR
P=IxE

E=Voltage
I=Amperage
R=Resistance
P=Wattage

Posted by: Krieger Aug 26 2012, 11:03 PM

Thanks for the added nudge Mike. I think I will use a relay. So do I add up the wattage of all the goodies and divide by 12 and that gives the Amps? Still not sure how to get some of them watt values. Oh hell, I have a Bosch relay that says 20/30 Amps. Think thats good enough? I'll even put a 20 amp fuse before it.

Posted by: GeorgeRud Aug 27 2012, 06:38 AM

The relay should work fine.

FYI, these square relays and associated wiring are available at Parts Express at very reasonable prices if anyone is adding circuits to their car.

Posted by: Krieger Aug 27 2012, 08:04 AM

Thanks George.

Posted by: Dave_Darling Aug 27 2012, 09:12 AM

All of your bulbs should have a rating in watts. Add all of those together.

--DD

Posted by: Krieger Aug 27 2012, 09:18 AM

What value do I assign to the lead that goes to the fog light relay?

Posted by: Tom Aug 27 2012, 01:47 PM

Use an ohmmeter and measure the coil resistance of the relay. Divide that number into your running voltage. I=V/R That will give you the rated ( approximate) current in amps. Probably something like 150-250 mili-amps.
Tom
Edit: a good safety tip when working on old car electrical systems, when you hook the battery back up, leave the bolt loose in case something is shorted. A better way is take a reading of the resistance of the positive batt cable to ground before doing the work, then again after your mods are complete. If there has been a significant change, especially lower resistance to ground,recheck your work.

Posted by: Krieger Aug 27 2012, 09:25 PM

Well, there might be an easier way. I wired it up with just an inline fuse. I guessed at 15 amps. I turned it all on and.....nothing but light. I left it on for maybe 5 minutes. No blown fuse. So I think its less than 15 amps. Should I try a 12 or a 10?

Posted by: kg6dxn Aug 27 2012, 10:04 PM

The size of your fuse is determined by the wire size. Not the load. If you have too much load, you shed some to another circuit. Too large a fuse on too small a wire = the wire becoming the fuse.

Here is some good reading for you.

http://www.rowand.net/Shop/Tech/WireCapacityChart.htm

Posted by: Krieger Aug 27 2012, 11:02 PM

Thanks Mike. I did keep feeling the wires every couple of minutes to see if they were getting hot and they never did. I should have said that before. The wire for the fuse is 12 gauge and the assembly is rated for 30 amp. I will check out the reading later.