The Stovebolt Site Forums The Shop Area The Electrical Bay 6V headlight switch in a 12V truck

Will it work and not cause problems?

Yup.🛠

Succinct. Thanks Martin.

...one more thing, 12V will fry the rheostat in the switch, not that I've been able to find a used 6V rheostat that still works!

I for one don't believe 12 volt will fry anything in your light switch. When you increase supply voltage you decrease the amperage if the load remains the same. As most of us know, amps kill components (motors are different) in trucks/cars not voltage. At least in the low D.C. Voltages we are addressing here.

12 volts will fry an original six volt rheostat. So will 6 volts if its 60 years old and corroded.

52Carl, Would you enlightened us as to why 12 volts will fry a six volt rheostat for a light circuit? Some of us are still in the 'learning mode'. Thanks

I don't think it fried my original light switches (6v 1954 3100s) - they were cleaned-up (about 20 or 14 years ago) with contact/rheostat spray (like DeoxIT). Those switches have been used with 6v systems and 12v systems for many years after using contact/rheostat spray.

Ran 12 volts through the 6 volt switch for years on my old 4100 with a fair amount of night driving. No problems whatsoever.
🛠

I will vote for 12 volts in the 6 volt switch works just fine.

My point was that 60 years of corrosion will doom a rheostat regardless of how many volts you put through it.
An original, non-corroded rheostat will do just fine on 12 volts.
(Sometimes I talk inside-out, due to my filthy German heritage.)

Thanks for clearing that up. Corrosion is the enemy that us old truck fans must always be vigilant of.
Hey, people that make beer that taste that good can't be all bad. The ladies ain't bad either. Please don't ask my wife why I know this.

I guess it could be worse. At least I am not Irish, with their small hands, and smelling of cabbage...

Resurrecting an old thread to ask the opposite question: Will a 12V headlight switch work on 6 V?

The rheostat for the dash lights on my old 6V headlight switch is toast (mainly because if fell apart). I have a 12V repop switch and wondering if there's a reason it wouldn't work on 6V, which I plan on retaining. I haven't checked the resistance across the rheostat between the two, but if it's different, may cause issues with the dash light brightness. Also, there's the circuit breaker for the auxiliary loads that come off the headlight switch that may cause an issue, Although I could work around that by installing a separate fuse on the aux loads. Haven't done any math on resistance and amp ratings.

Haven't checked if 6V repop switches are available, which may be the answer. I got the 12V switch for cheap, so would use it if it'll work.

If anyone had experience or insight, please chime in.

Thanks

https://www.classicparts.com/Switches/products/45/2/0

My guess is that the internal breakers in the 12v switch will cause problems when used in a 6v system?

It will work, the difference will be unnoticeable. Any fuses or breakers will operate the same. A 10 amp fuse will still blow at 10 amps.

That was my thought as well. A breaker rated at 10A at 12V (or 120VA power draw), would pop at twice the amps with a 6 V system, not providing the protection needed. Or maybe pop earlier, I'm confusing myself now. Been too long since I took an electrical engineering course in school.

@LeeP, I don't think that's correct. Fuses are rated for a certain voltage (not just amps) and blow when the power (volts x amps) thru them exceeds the rating.

Simple solution - Get the right switch.

t seems that many people here have installed relays to make their 6v headlamps brighter. If you have done this, then I expect that the 12v switch will be OK as all the current to the headlamps is going through the relay.

I can plainly see that you are an engineer. I think you are over thinking fuses in a dc system.
As far as I know every fuse I have put in a car has a rating of amperage and no voltage marking except sometimes a rating of 120v. max So what is it that you are saying? If I put a 120v 10amp glass fuse in my car it will blow at aprox 1 amp? I don't believe it works like that. A fuse melts because the excessive amperage heats up the narrow strip and causes it to melt (blow). 10 amps at 12v is the same 10 amps at 6v, and the same 10 amps at 120v. But I'm not an engineer.

Now getting back to the 6v verses 12v light switch. you have to add up the total wattage of your bulbs and divide by 6 and see what your amperage through the switch will be. If it exceeds the max amperage on the 12v switch it will cause the breaker (if it has one) to cycle. If it does you need a relay. If it has a fuse, replace it with the applicable sized one for the amperage found above.

@LeeP, your latest post is correct, it's the total wattage (VxA =watts) or power is the critical thing for fuses or circuit breakers, so voltage is just as important as amps.

Anyway, the mystery of the circuit breaker on the light switch is solved. They're removable, so I can switch them out. The picture is the two side-by-side with the rating markings on them, so clearly the manufacturers understood the importance of power capability for them. The shiny one on the left is from the 12V switch and the one on the right is from the original 6V switch (took a little cleanup to reveal the rating). 20A at 12V versus 30A at 6 V. Not identical power, but definitely different amp rating.

The only outstanding issue is if the dimmer will work correctly. I'm going to check them and if the rheostat on the 12V switch is close in resistance range, it should work OK.
[On edit] The rheostat full resistance on the 6 V switch is ~5 ohms and on the 12V switch, ~10 ohms, so the only thing that will happen is the instrument bulbs will be able to dial down to a dull glow.

FYI, the fuse out of the feed wire to the turn signal switch on my truck is marked 25A 32V (which is strange).

And yes, I'm an Engineer, but Mechanical, not Electrical. I know enough electrical stuff to be dangerous. Which is why I ask questions.

OK, I have to fess up to being wrong (sort of ). I was having a hard time wrapping my head around the power issue.

It IS the power across the fuse that causes it to blow, but I was assuming that the full voltage of the circuit (6V or 12V) factored into that equation. But the power across a fuse is developed by the voltage drop ACROSS THE FUSE which is a fraction of the circuit voltage. The power across a fuse can be expressed as P = I squared x R where R is the fuse resistance and I is the current running thru the fuse. Resistance is fixed, and so the power (and amps) to blow the fuse is also fixed for a particular fuse.

So the bottom line is don't use a 20A rated circuit breaker in a 6V circuit that had a 30A circuit breaker in it, as it would be under-protected and might cause a fire.

You might want to reverse the numbers on your bottom line. if you replace your 30a breaker with a 20a breaker its probably going to trip a lot. Do not replace a fuse or breaker with a higher rated one, you risk melting the wire its there to protect. You could ruin your harness, or have a fire.

@LeeP - you misread that last part. My plan is to use the original 30A breaker, not the 20A.

@Dads50 - those were responses to the original question which was the reverse of mine. I'm using a 12V switch in a 6V system (but with the original 6V, 30A breaker), and other than being able to dim the instrument panel lights WAY down, I've convinced myself that it'll work fine.

If you replace a 30a breaker with a 20a breaker, all that is going to happen is that you may trip the breaker a lot. it will not cause a fire. It may be so over protected that it can not function because the breaker keeps opening.
Sorry, but I grow tired of all the math formulas, and equations. I try not to confuse other people with my prattle. Thought I could help, but you will figure it out.

I started this thread so I could learn something. I've learned some things but I'm still learning. I'm guessing the 12V switch will work fine with 6V power as long as the circuit doesn't see more than 30Amps. My understanding is that the wire size (gauge) determines the amount of amperage allowed and as long as the truck has the original wiring that is in good shape there shouldn't be a problem.