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Old 08-20-2013, 08:28 AM   #1
johnbuckley
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Default 12v to 6v drop down resistors

Why do you need to use a different ( and more expensive) resistor for the horn and wiper when converting to 12v. Why can't you use the same cheapo type that is used for the coil?
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Old 08-20-2013, 08:35 AM   #2
gweilbaker
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Default Re: 12v to 6v drop down resistors

Because the horn and wiper use more current. More current with the 6 volt drop is a larger watt resistor which dissipates more heat than the cheapo.
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Old 08-20-2013, 09:09 AM   #3
steve s
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Default Re: 12v to 6v drop down resistors

Using Ohm's law, V = I R or R = V/I, to get a 6V drop across the resistor if your horn uses 6 amps current, R = 6/6 = 1, so, you want a 1 ohm resistor (adjust accordingly for whatever current, I, your horn draws.

The power dissipated by the resistor would be I-squared x R, or, for this example, 36 x 1 = 36 watts. Probably could get by with less, unless you're planning on continuously laying on the horn or the resistor is going to be located with no ventilation.

The wiper will require a lot less.

Last edited by steve s; 08-20-2013 at 09:22 AM.
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Old 08-20-2013, 09:30 AM   #4
johnbuckley
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Default Re: 12v to 6v drop down resistors

So the 3 resistors are only different in their physical size and their ability not to fry up with too many amps through them?
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Old 08-20-2013, 10:57 AM   #5
Jim/TX/GA
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Default Re: 12v to 6v drop down resistors

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Originally Posted by johnbuckley View Post
So the 3 resistors are only different in their physical size and their ability not to fry up with too many amps through them?
Yes. It's all about the heat generated while the current is flowing through it. I have seen a horn resistor with aluminum fins all over it to shed the heat.

The value of the resistor you use (the ohms) and the wattage needs to be specific to the thing you are connecting to the resistor to get the 6 volts you desire.
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Old 08-20-2013, 11:08 AM   #6
Purdy Swoft
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Default Re: 12v to 6v drop down resistors

I haven't tried it, but i've heard of using an old ignition coil, mounted and hidden inside the frame rail.
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Old 08-20-2013, 03:27 PM   #7
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Default Re: 12v to 6v drop down resistors

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I haven't tried it, but i've heard of using an old ignition coil, mounted and hidden inside the frame rail.
Sure. Old 6 volt ignition coils and 12 volt coils that require external resistors are about 1.5 ohms.

Ceramic ballast resistors are about 1.5 ohms.

Either should work about the same, if what you need is 1.5 ohms. Hook one up and measure the voltage you get when you have the thing running that is connected to it (horn, wiper, etc.).

It is somewhat counter-intuitive, but the less current draw the device has (horn, wiper, etc.) the larger the resistor needs to be to get you the final 6 volts that you want when that thing is running.
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