VCCA Home Forums TECHNICAL - GENERAL Electrical - 12 volt Horn Voltage

I could not determine if I should post this is the 6 volt or 12 volt or Personal Modifications section.

Based on the searching I have done, you can operate a 6 volt horn on 12 volts with a few points to note. It will be much louder, you might need to adjust the points gap slightly to get the right tone, and there should not be a problem with burning it out if you only operate it intermittently.

We are working with the original 6 volt horn on a ”˜49 3100 pickup. It operates great on 6 volts. There is no relay in the original wiring.

My searching did see some mix of recommendations about adding a relay.

My thinking is that amperage draw should not be an issue. In fact the horn should draw about half the amps at 12 volts. So I see no need for a relay especially if there was not one in the original circuit.

Your ideas and inputs are welcome.

Hi Rusty
Having a set of vibrating points opening and closing complicates things but...
Doubling the voltage to any unit without changing the resistance of that unit should roughly double the amperage.

I think you are thinking of a unit designed for 12v versus 6v (a starter for example) where the design provides more resistance in the 12v unit thereby reducing the amperage but keeping the wattage about the same.

In this case, the horn has the same resistance as before, so the amperage must increase.
That may necessitate a relay. Especially if you use the horn for more than very brief toots.

Yep, Stovblt is correct on the current...a relay and maybe a kill switch might be an idea.
I have a bunch of 6v horns myself that I want to put on modern cars eventually since they are so loud....

Experiment with a pair in series - the volume likely won't be balanced but you might get lucky. You could measure the OHM's of each and compare them. If they are close to even, the volume may be close. If one has significantly lower resistance, that one should be louder.

Thank you!

I knew there was something wrong with my thinking! I was confusing Ohm’s law and Watt’s law.

I know that we often talk about 12 volt systems using smaller size wiring because the higher voltage means less current.

The catch is that electrical devices such as bulbs, motors, and horns that are designed for 12 volt systems have lower resistance. They still do the same amount of work (watts) but with less current.

At a minimum we will probably need to replace the fuse in that circuit with a larger one.

Yes, except something designed for 12 volts will have four times the resistance of an equivalent 6 volt device, not half.

This is a bit of an oversimplification when something is buzzing like a horn, and isn't quite pure DC anymore, but it will be close.

In DC circuits (as used in cars), wattage is simply volts multiplied by amps. Wattage is a unit like horsepower, and one can be converted to the other using math. So, if we want to do a given job, no matter what voltage we use, we need the same amount of Watts.

To design a 6 volt device that uses 12 watts we need 2 amps of current. Note: a horn draws way more than 2 amps and uses way more than 12 watts, but lets keep the math simple. Likewise, to design the same device to run on 12 volts, we need 1 amp of current.

Our 6 volt device will need to be designed with 3 Ohms of resistance. Why? Ohms law. Voltage = Amps x Resistance(Ohms). That is the only way to get 2 amps of current, and 2 amps of current is what we need to get 12 Watts.

Why? Any other resistance will result in a different amount of current, and a different amount of current multiplied by 6 volts will give us something other than 12 Watts.

If we design an equivalent 12 volt device to use 12 watts, we will need only 1 Amp because Watts = Volts x Amps.

Since our new design runs on 12 volts and draws 1 amp, the resistance has to be 12 ohms, four times what the 6 volt device had. Why? Ohms law again. Volts = Amps x Resistance(Ohms).

Also, since we have 1 amp instead of 2, we can use smaller wire because Amps represent current flow, and determine the wire size needed.

Now, if we take our 6 volt device and connect it directly to 12 volts, something quite different happens. You will remember our 6 volt device had 3 Ohms resistance. Using ohms law again (Volts = Amps x Resistance(Ohms)), our device will now try to draw 4 amps of current rather than the 1 amp it was designed for. You can't fool mother nature.

Since our 6 volt device, running on 12 volts, draws 4 amps, it will attempt to do four times the work and use 36 watts rather than 12 (Watts =Volts x Amps).

Because of Induction and losses and a bunch of other things that are beyond a simple forum post, it will never get to 4 times the wattage, but it will get quite a bit hotter than it should. People often get away with it on horns because it is a short blast.

Thanks bloo!

I had rotated my mental VIR triangle! Need to go back to remedial high school physics.

This also explains why a 6 volt horn is so loud on 12 volts. It is producing 4 times the energy to move air.

Am I correct in the assumption that you can run both horns in series, isolating the first from ground, and grounding the second, and getting the same results with 12 volts as with 6 volts that are not in series? I know, it’s not easy to isolate the first horn and attach it to the metal of the car. One must use plastic and/or rubber for the mounts. It seems that I have read that some where. You should also get the same sound as original. Anyone care to chime in on this one?

I doubt it but try it and see. A horn works by turning on and off at a high rate of speed, and while one is off the other wouldn't be getting any current. My suspicion is that it is going to be wimpy and weak if it works at all, but I just don't know. If you try it let us know.

Good inputs to consider Bloo. I had not thought about the “phasing” issue with the dual vibrating coils.

My 1951 GMC already had a horn relay. When I converted to 12 volts I changed the relay to a 12 volt one. All the classic truck suppliers have them. My truck horns weren’t very loud on 6 volts but they are good on 12 volts. My horns on the 38, running on 6 volts are extremely loud.

I don’t intend to try, I’m content with the way they sound with 12 volts. And as stated earlier, they’ll be fine as long as you don’t lay on the horn to let some idiot know what you think of him/her!

The later 6V horns have a 1.6 ohm resistance across the contacts to reduce arcing. The earlier ones have a condenser. The resistance horns could probably be used in series for 12V with the resistor providing some elecrical stability. Worth a try if the horns are solidly mounted and one horn is insulated from the chassis.