HELP with horn

m.ralph

m.ralph

Senior Member
Does anyone know what values of resistors to use for converting a 6 volt to work on a 12 volt system. I bought a resistor from Speedway , but it wouldn't work. The horn works great on 6 volts. I need to know the watts and the ohms. I've used resistors before and they worked good, but can't remember what values that I used before. Thanks for any help. m.ralph
 
John Elliott, who posts to Ford Barn from time to time, was kind enough to provide a list of 6-volt to 12-volt conversion parts for our club newsletter. Although his list did not provide the resistance value, he did state that an Ohmite RKS2RO resistor was the item needed when using a 6-volt horn in a 12-volt system.
 
Bratton's sells a resistor kit especially for this (6v horn on 12 volt system) for about $20.
Goes inside of the horn cover and can't be seen.
 
I find a good working horn draws 5 amps, so 5 amps x 6 volts = 30 watts, but allow some extra for safety. A 35 or 40 watt resistor should be fine.
 
Something is wrong here.

If a good horn draws 5 amps, and the correct resistor is 2 ohms, then the battery has to have 16 volts output so that the horn gets 6 volts.

5X2 =10volts (ohms law).

Marc
 
I have seen the Ohmite RKS2RO 6-volt-horn-in-a-12-volt-system resistor referenced in a couple of different places, the most recent being the "Standard bearing parts numbers?" thread posted yesterday. Perhaps someone who has successfully used a resistor from one of the Model A parts suppliers can measure the resistance and confirm or deny the adequacy (and wattage rating, inasmuch as this is a voltage-dropping application) of the 2-ohm resistor.
 
marc hildebrant said:
Something is wrong here.

If a good horn draws 5 amps, and the correct resistor is 2 ohms, then the battery has to have 16 volts output so that the horn gets 6 volts.

5X2 =10volts (ohms law).

Marc
Click to expand...

You need to remember that the horn has an internal resistance of approx. 1.2 ohms. So, 5A x 1.2ohm = 6volts
To take it up to 12 volts the formula would be 5A x ohms = 12volts.
ohms would equal 2.4 ohms and then you need to subtract out the original 1.2 ohms giving you a 1.2 ohms resistor needed. But, since 6 volts is really more like 6.5 volts and 12 volts is not exactly 12 volts and 5 amps is just what this one horn pulls, a 2 ohm resistor is close enough as long as you get one rated for the high watts. Clear as mud?
 
Sorry 30cabriolet , not clear.

If you are using an external 2 ohm resistor in the circuit, and the horn SHOULD draw 5 amps when it operates correctly, then the external resistor will have a voltage drop of 10 volts across it.

So, if 10 is lost across the resistor and 6 across the horn, the supply needs 16 volts.

Now, if the horn draws less than 5 amps, less is lost across the external resistor. So, I reduce sounding horn could be the result with a 12 volt (really 13.8V) battery.

Marc
 
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