The Ford Barn

Planojc · 05-03-2022, 10:21 AM

Anyone know the dropping resistor value used on a 6v Model A horn in a 12v system?
Thanks, JC

MAG · 05-03-2022, 11:58 AM

about 2 ohms. One of the white resistors found on the firewall on 50/60's chevy ignition systems will work fine.

nkaminar · 05-03-2022, 04:00 PM

Wattage is just or more important than resistance. The small ones that mount in the horn housing can burn out easily. The larger ones that mount on the frame work fine. You can also rewind the horn for 12 volts. That is what I did.

Power is I squared R. So at 10 amps (probably high) the power dissipated is 200 Watts. 5 amps would be 50 Watts. The common 1/4 Watt resistors clearly will not work and would make a fuse with a smoke signal to let you know it burned out.

katy · 05-04-2022, 10:25 AM

Quote:

Originally Posted by Planojc

Anyone know the dropping resistor value used on a 6v Model A horn in a 12v system? Thanks, JC

If you know how many amps it draws on 6 volts you can figure it out.

nkaminar · 05-04-2022, 10:57 AM

Katy, That is mostly correct, but the horn has induction which is part of the reason why it has the distinctive ah-og-ga sound. So the current starts out higher then goes down as the reverse EMF builds at higher rpm. So the dropping resistor works but the sound is slightly different.

To do the math, first measure the current on a correctly adjusted horn with 6 volts. Say you get 5 amps. R=V/I so the equivalent resistance would be 6/5 or 1.2 ohms. Power is W=I X V so the wattage would be 5 X 6 or 30 Watts. To be on the safe side use a resistor with a higher power rating.

It would seem that the 2 ohm resistor is assuming the horn draws 3 amps with 6 volts. It probably takes some experimentation to get the right sound.

Tom Endy · 05-04-2022, 11:56 AM

Sponsored Links (Register now to hide all advertisements)

As stated above, the Ahooguh horn is an inductive animal, and a dropping resister is not the best answer when converting a Model A to 12-volts. Most dropping resisters sold by the suppliers cannot handle the power requirement and they will burn out with repeated use.

The best solution is to rewire the field coils, it is not difficult, anyone can do it. See attached article.

Tom Endy

http://www.santaanitaas.org/wp-conte...orn-to-12v.pdf

Ivan in southeast va. · 05-04-2022, 03:47 PM

My 6-volt horn burned out. I bought a new 12-volt horn from A&L before the pandemic. You can still get them from the major suppliers.

Herb Concord Ca · 05-04-2022, 03:54 PM

X2 what Tom Endy said.

nkaminar · 05-04-2022, 08:48 PM

I re wired mine. It was not hard to do.

05-03-2022, 10:21 AM	#1
Planojc Senior Member Join Date: May 2010 Location: Plano, Texas Posts: 1,062	Model A horn resistor Anyone know the dropping resistor value used on a 6v Model A horn in a 12v system? Thanks, JC

05-03-2022, 11:58 AM	#2
MAG Senior Member Join Date: May 2010 Location: Austin, Texas Posts: 926	Re: Model A horn resistor about 2 ohms. One of the white resistors found on the firewall on 50/60's chevy ignition systems will work fine. __________________ I noticed the harder I work the luckier I get!

05-03-2022, 04:00 PM	#3
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,860	Re: Model A horn resistor Wattage is just or more important than resistance. The small ones that mount in the horn housing can burn out easily. The larger ones that mount on the frame work fine. You can also rewind the horn for 12 volts. That is what I did. Power is I squared R. So at 10 amps (probably high) the power dissipated is 200 Watts. 5 amps would be 50 Watts. The common 1/4 Watt resistors clearly will not work and would make a fuse with a smoke signal to let you know it burned out. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.

05-04-2022, 10:57 AM	#5
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,860	Re: Model A horn resistor Katy, That is mostly correct, but the horn has induction which is part of the reason why it has the distinctive ah-og-ga sound. So the current starts out higher then goes down as the reverse EMF builds at higher rpm. So the dropping resistor works but the sound is slightly different. To do the math, first measure the current on a correctly adjusted horn with 6 volts. Say you get 5 amps. R=V/I so the equivalent resistance would be 6/5 or 1.2 ohms. Power is W=I X V so the wattage would be 5 X 6 or 30 Watts. To be on the safe side use a resistor with a higher power rating. It would seem that the 2 ohm resistor is assuming the horn draws 3 amps with 6 volts. It probably takes some experimentation to get the right sound. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. Last edited by nkaminar; 05-04-2022 at 11:19 AM.

05-04-2022, 11:56 AM	#6
Tom Endy Senior Member Join Date: May 2010 Location: Southern California Posts: 3,168	Re: Model A horn resistor Sponsored Links (Register now to hide all advertisements) As stated above, the Ahooguh horn is an inductive animal, and a dropping resister is not the best answer when converting a Model A to 12-volts. Most dropping resisters sold by the suppliers cannot handle the power requirement and they will burn out with repeated use. The best solution is to rewire the field coils, it is not difficult, anyone can do it. See attached article. Tom Endy http://www.santaanitaas.org/wp-conte...orn-to-12v.pdf

05-04-2022, 03:47 PM	#7
Ivan in southeast va. Senior Member Join Date: Oct 2012 Location: Portsmouth, Virginia Posts: 307	Re: Model A horn resistor My 6-volt horn burned out. I bought a new 12-volt horn from A&L before the pandemic. You can still get them from the major suppliers.

05-04-2022, 03:54 PM	#8
Herb Concord Ca Senior Member Join Date: May 2010 Location: Concord CA Posts: 755	Re: Model A horn resistor X2 what Tom Endy said.

05-04-2022, 08:48 PM	#9
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,860	Re: Model A horn resistor I re wired mine. It was not hard to do. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.