12v bulb in a 6v socket?

Hey I just bought a new tail light bulb, but they sold me a 12 Volt. Am I gonna wreck anything if I use it? Will it work at all?

Re: 12v bulb in a 6v socket?

Steamboat Aka J. R. Stevens /

It won't hrt your bike but it will only be half as bright if it has the same watt rating. Jim

12v bulb in a 6v socket?

Steamboat Aka J. R. Stevens /

Some older bikes use the tail lights as a ground for the coil. What type and year of bike do you have?? Jim.

Re: 12v bulb in a 6v socket?

It's a 1980 puch - I think it's a newport but not really sure. Runs fine without a bulb at all, though the local police disagree.

12v bulb in a 6v socket?

Steamboat Aka J. R. Stevens /

6v to 12v Same brightness

old bulb new bulb

6v 12v

watts watts

---- X 2 = -----

As long as the base is the same

the 12v bulb should slip right in.

Just enter the 6v watts, multiply

by 2 giving the 12v watts.

.Some say a picture is worth

a thousand words. Jim

Re: 12v bulb in a 6v socket?

Not quite true, Jim. Power is I

Re: 12v bulb in a 6v socket?

are you sure about the math?

Power is I

nevermind..algebra aint my forte.

nevermind.. i see where the P=V

Re: 12v bulb in a 6v socket?

Joe, I agree that you'd want a 12V bulb of double the wattage if you were going to put the same amount of amps through it. The problem is, you can't force an arbitrary current through a bulb at a fixed voltage, because of the bulb's fixed resistance. A bulb that draws 2 Amps at 12V, as in your example, would only draw 1 Amp at 6V.

A 12W, 6V bulb draws 2 Amps.

A 48W, 12V bulb draws 4 Amps @ 12V (48W=4Ax12V)

It has a resistance of 3 Ohms (12V=4Ax3Ohm)

Put 6V across it, it will draw 2 Amps (6V=2Ax3Ohm)

Its power output is 12W (12W=2Ax6V)

Re: 12v bulb in a 6v socket?

lemme see if i follow this.. seems that something aint right here..

_A 48W, 12V bulb draws 4 Amps @ 12V (48W=4Ax12V)_

_It has a resistance of 3 Ohms (12V=4Ax3Ohm)_

I think you mistakenly calculated 3 ohms _assuming_ 4 amps, but your 4 amps figure assumed a 12 volt supply .. However, the magneto only delivers 6 volts (theoretically).


A 12V, 48 watt bulb has a resistance of :

(ohms = V

damnit.. scrwed up again. Can't do math either.

did it again.. 144/48 = 3 ..

jeeze ..

Re: 12v bulb in a 6v socket?

A bulbs resistance is not fixed, especially in an AC system. Bulbs are heavily inductive, and as they heat up their imedance increases.

Re: 12v bulb in a 6v socket?

ok... well.. i'll leave the technicalities alone.

Alls i wanna know is why a 12V bulb of about twice the wattage of a 6V works just fine on my bikes.. seems like they provide the same amount of illumination and they never seem to pop.

BTW.. these mopeds have no voltage regulation (a rectifier off the lighting coil does drop voltage by one or two volts), and if i put a voltmeter on a lamp socket, this 6 V magneto delivers close to 30 V at top RPM.. maybe this has something to do with it all.

Re: 12v bulb in a 6v socket?

Brianf, for all intents and purposes, we can model the bulb as having a fixed resistance. Yes, it has some inductance, and yes, its resistance increases by some percentage with temperature. That's not the point... I was trying to illustrate that you can't expect a bulb that draws a certain current at 12V to draw that same current at 6V. I'm trying only to get as technical as I need to teach something; I don't think getting into inductance will help anyone understand any better.

Re: 12v bulb in a 6v socket?

I feel very stupid now...

Interesting point

Joew, you're probably right about the wildly varying output of the magneto. If your bike doesn't have a well-regulated lighting circuit (and I guess few do), then all of this is kind of academic. In practice, if you've been using a 12V bulb rated for twice the power of the 6V bulb you're replacing, it will draw only half as much power as the old bulb (as I've shown, hopefully). Since the magneto has a reduced load, the peak voltage probably climbs higher-- maybe not to 12V, but somewhere between 6V and 12V. Also, I think human perception of the intensity of light is logarithmic, not linear, and it varies with ambient light levels... all in all, it's pretty hard to say one light is as bright as another when they're not both running side by side.

The reason you've never popped a 12V bulb in a 6V system is that it's operating at only 25% of its rated output, a huge margin of safety. That means the filament is a lot cooler than it was designed to be.

Don't feel bad

Rick, I didn't know anything about circuits when I was your age. If you read this discussion and thought any part of it was interesting, you're probably ahead of the curve already.

Re: 12v bulb in a 6v socket?

It should not wreak anything, but the light output has good chance of being less than when it is in a 12V circuit. There are some 12V bulbs do to construction, i.e. filament material and size will not work in a 6V system, but those are very few. What you are going to have to watch how much Current is being drawn, you only have a FININT supply. If you are already close to max and you put in your new bulb and it draws more than the old one, it will DIM all the lights. Since most Moped are using TWO pickups, one for lights and one for Spark Plug, it should not effect running of moped.

As for the you others learn your basic math.

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