The Ford Barn - View Single Post

COE Dan · 07-21-2016, 09:52 AM

This thread presents a couple potential misconceptions:

1) Running a 6V horn on 12V uses half the amperage
2) The heavier wiring will prevent damage/destruction of the horn

Both of these are not true. The first is only true when comparing a 6V horn against a horn designed for 12V. When you apply 12V to a 6V horn, the current is not halved, it is doubled.

Explanation: Let’s assume a 6V horn has a notional resistance of 12 ohms. Resistance remains essentially constant regardless of the voltage applied (there are some slight changes due to thermal/inductive properties that result from changing the voltage but they are negligible in our application).

Using Ohm’s Law, a 6V horn with 12 ohms resistance draws a current of 0.5A (A = V/R = 6/12 = 0.5A).

Taking the same 6V/12-ohm horn and applying 12V draws a current of 1A (A = V/R = 12V/12 ohms = 1A).

Therefore, you end up pushing about twice the current through your 6V horn if you hook it up to 12V. For short periods, you probably won’t overload the horn wiring and can get away with it. Eventually the horn contacts may burn (a parallel situation is removing the ignition resistor from your ignition circuit which increases voltage across the points – potentially burning them). It appears “rlhinit” may have encountered this. You’ll also observe the horn changes pitch as others have noted.

Solution: The only way to maintain proper operation of a 6V horn in a 12V circuit is to add a resistor in that drops the voltage to 6V across the horn.

Resistors have two important values in this case: resistance and wattage. To compute the resistance value needed, you need to know either: 1) the resistance of the horn or 2) current drawn during 6V operation.

If you know the resistance, compute current: Current = V/R
If you know the current draw, compute resistance: Resistance = V/Current

Besides resistance, the resistor must be rated with sufficient capacity to dissipate the power flowing through it otherwise it will burn up. Therefore, the resistor must have a wattage rating equal to or higher than the power consumed. To determine the wattage of the resistor, use the following Ohm’s Law relationship: Power = Current x Voltage.

To drop the 12V input to 6V across the horn, add a resistor equal to the resistance of the horn in series with the horn (Resistance is additive in a series circuit).

Thus, you now know the proper resistance and wattage values and can obtain the appropriate resistor. This maintains the correct operating conditions at the horn ensuring longer life and allows the horn to sound as it should.

One last note on dual horn cars: Some folks have taken their 2 horns wired in parallel (6V across each) and rewire them in series and apply the 12V across both. Seemingly, this is identical to adding a resistor in the circuit as outlined above. However, these Ford horns will likely not have identical resistances. Therefore, you are not guaranteed than the voltage drop across the two horns will be split evenly. One may see 3V, the other 9V – making the sound come out off-pitch. Best to keep the 2 horns in parallel and apply the same voltage to both for the best sound.

07-21-2016, 09:52 AM	#12
COE Dan Senior Member Join Date: May 2010 Location: Now in Wichita, KS Posts: 251	Re: '37 horn (12v) This thread presents a couple potential misconceptions: 1) Running a 6V horn on 12V uses half the amperage 2) The heavier wiring will prevent damage/destruction of the horn Both of these are not true. The first is only true when comparing a 6V horn against a horn designed for 12V. When you apply 12V to a 6V horn, the current is not halved, it is doubled. Explanation: Let’s assume a 6V horn has a notional resistance of 12 ohms. Resistance remains essentially constant regardless of the voltage applied (there are some slight changes due to thermal/inductive properties that result from changing the voltage but they are negligible in our application). Using Ohm’s Law, a 6V horn with 12 ohms resistance draws a current of 0.5A (A = V/R = 6/12 = 0.5A). Taking the same 6V/12-ohm horn and applying 12V draws a current of 1A (A = V/R = 12V/12 ohms = 1A). Therefore, you end up pushing about twice the current through your 6V horn if you hook it up to 12V. For short periods, you probably won’t overload the horn wiring and can get away with it. Eventually the horn contacts may burn (a parallel situation is removing the ignition resistor from your ignition circuit which increases voltage across the points – potentially burning them). It appears “rlhinit” may have encountered this. You’ll also observe the horn changes pitch as others have noted. Solution: The only way to maintain proper operation of a 6V horn in a 12V circuit is to add a resistor in that drops the voltage to 6V across the horn. Resistors have two important values in this case: resistance and wattage. To compute the resistance value needed, you need to know either: 1) the resistance of the horn or 2) current drawn during 6V operation. If you know the resistance, compute current: Current = V/R If you know the current draw, compute resistance: Resistance = V/Current Besides resistance, the resistor must be rated with sufficient capacity to dissipate the power flowing through it otherwise it will burn up. Therefore, the resistor must have a wattage rating equal to or higher than the power consumed. To determine the wattage of the resistor, use the following Ohm’s Law relationship: Power = Current x Voltage. To drop the 12V input to 6V across the horn, add a resistor equal to the resistance of the horn in series with the horn (Resistance is additive in a series circuit). Thus, you now know the proper resistance and wattage values and can obtain the appropriate resistor. This maintains the correct operating conditions at the horn ensuring longer life and allows the horn to sound as it should. One last note on dual horn cars: Some folks have taken their 2 horns wired in parallel (6V across each) and rewire them in series and apply the 12V across both. Seemingly, this is identical to adding a resistor in the circuit as outlined above. However, these Ford horns will likely not have identical resistances. Therefore, you are not guaranteed than the voltage drop across the two horns will be split evenly. One may see 3V, the other 9V – making the sound come out off-pitch. Best to keep the 2 horns in parallel and apply the same voltage to both for the best sound.