I'm wanting to run a vintage Klaxon horn on my truck, but they were made when vehicles were 6v. I know they make new 12v versions, but I think vintage horns sound better. I know there is a way to do it, but need a little guidance.

 

See if you can figure out how many amps it draws. Use a battery charger set on 6v and a meter, or some other method.

If you know the amp draw, you could make something like this.
http://www.vintage-planet.com/dropping-resistor.html

 

That's kinda what I figured, but it's been a while since my DC analysis classes. Thanks for the link!

 

I had a 6v horn on a 12v system years ago and it worked as long as I had the car(maybe 3 years), but I didn't abuse it either, just when needed.

 

You can also just wire a ?AMP resister (depends on current draw of the horn in question) in series with the power wire to drop the voltage down to 6-7 volts.

 

If you've got some long pieces of 18 gauge wire, put a voltmeter across the horn, and through trial and error find a length that has enough resistance to drop it to 6V. Or, grab some nichrome wire from an old dryer element, and find the right length. Or, try a number of ignition ballast resistors in parallel.

Or, run the horn from 6 or 7 volts and measure the current. Then calculate the required resistance. If current at 6V is I, then the resistor should be R=6V/I
The power dissipated in the resistor would be P(in watts)=6V*I
Since the horn will only be use intermittently, a resistor with a much smaller power rating should be OK, providing it isn't so low that it blows like a fuse.

 

Got the horn in the mail today. Measured horn resistance is 1.1ohm. Angus, here's my math:
I=V/R I=6v/1.1ohm I=5.45A
P=I*E P=5.45A*6v P=32.7w
So according to this I need a 1.1ohm resistor rated >32.7w.

 

See if you can find a big wirewound resistor that has the windings exposed. You can use a hose clamp as a tap to vary the resistance a little bit. The reason you figuring might be off a little bit is the horn is a inductive load, so the straight calculations might be off a little bit.

Did you buy this off ebay or something? I thought this was something you had laying around. A very simple solution would be to buy 2 horns and hook them in series.

 

use a ballast resistor

Ballast Resistors | StreetPerformance.com

 

or this

Heater blower motor dropping resistor

 

Or a 6v light bulb in series.

 

A 6v lightbulb in series would kill the horn, since the horn draws around 5 amps. You couldn't get 5 amps to go through the lightbulb, unless you used a very large bulb, possibly a 6v headlamp.

 

My bad, maybe I should try something before making comment.

 

A second horn never even crossed my mind, but that would do it simply. I also thought about the blower motor resistors. It seems as though there are quite a few ways to skin this cat. I'll have to do some experimenting and post my results. Luckily I have two multimeters so I can monitor voltage drop across both horn and whatever I'm testing. Thanks to everyone for the input!