difference in 6 volt and 12 volt ignition ?

Tom Bellfoy

Tom Bellfoy

Senior Member
Is there any electrical difference between a 6 volt and a 12 volt ignition coil ?
Also is it ok to use a 6 volt coil in a 12 volt system ? keeping in mid that I would be limiting the primary current to 3 or 4 amps by using an exterior resistor if needed.


Can anyone recommend a 12 volt coil that is available off the shelf at NAPA for my following specifics:
- motor is a 1937 V-8 flathead with helmet distributor
- motor is in a 31 Model A Ford with 12 volt alternator system
- not using the original coil, using the replacement cover on the helmet dist. that requires a ingition coil
- this is not a hot rod, it is simply a nostalgic conversion without high performance
 
I sounds like you are on the right track. The Bosch "blue coil" #00012 made in brazil is listed to have 3.4 ohms resistance. You could use any coil with 1.5 primary resistance with a 1.8 ohm ballast with the same results.
 
What is often called a 6v coil will work fine on 12v, positive or negative ground. As you posted, it is the current in the circuit that needs to be controlled in the 3 to 4 amp range. With a "6v" (really one in the 1.5 ohm range) you need a ballast resistor (also in the 1.5 ohm range) for use with 12v. So, you are looking for ~3.0 ohms of resistance for a 12v ignition circuit. Or, you can use what is referred to as a 12v coil, really a 3.0 ohm resistance coil, as it provides the 3.0 ohms of resistance without a ballast resistor. The polarity of the coil needs to match the polarity of the rest of the system, so a negative ground system has the negative terminal of the coil going to the distributor points & capacitor and the positive terminal of the coil connected to the ignition switch.
 
your reply is exactly what I wanted to know. I have always enjoyed your knowlegable input on the forum
Thanks much
Tom
 
OK, so I don't want to get into the weeds of electrical theory, but a resistor doesn't limit or reduce current, it limits or reduces voltage.

You cannot use 6volt coil for 12volt system or the reverse. Well actually you can and it might work poorly or fry the coil, but that's your choice.

Coils are rated in ohms, which is the measurement of resistance. Typically you want a coil around .5 ohms in a 6 volt system, and around 1.5 ohms for a 12 volt system.

Ballast resistors are used to lower the voltage to the coil, and most particularly to the ignition points, because points last longer with lower voltages. In theory, you could use a 6 volt coil in a 12 volt system if you used a big enough resistor to lower the voltage from 12 to 6, but again, that 's not the best way to build an ignition system.
 
Afraid your theories are not correct. Ford used what are often referred to as "6v" coils in 12 volt systems for many years after they switched to 12v systems. There are advantages to doing this. It allows the ballast resistor to be bypassed during startup making it easier to start an engine. It is the resistance of the coil that matters, "6v' coils are often in the 1.5 ohm range which works fine in a 6v system and coupled with a 1.5 ohm resistor works fine in a 12v system. Or, you can use a 3.0 ohm resistor without a ballast resistor in a 12v system.

Current, Voltage and Resistance are all related. What you are really wanting to do is limit the current in the ignition circuit to around 3 to 4 amps. 6 volts divided by 1.5 ohms equals 4 amps, so is on the high side, but works. For 12 volt systems you need around 3.0 ohms, so, a 3.0 coil or a 1.5 ohm coil coupled with a 1.5 ohm ballast resistor.

May be confusing something like a GM HEI ignition system which uses a .5 ohm coil.

A .5 ohm coil in a 6 volt circuit would give you around 12 amps, why to high and very short points life. Same goes for a 1.5 ohm coil in a 12 volt system. 12v / 1.5 ohms = 8 amps, still way to much current.
 
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JSeery said:
Afraid your theories are not correct. Ford used what are often referred to as "6v" coils in 12 volt systems for many years after they switched to 12v systems. There are advantages to doing this. It allows the ballast resistor to be bypassed during startup making it easier to start an engine. It is the resistance of the coil that matters, "6v' coils are often in the 1.5 ohm range which works fine in a 6v system and coupled with a 1.5 ohm resistor works fine in a 12v system. Or, you can use a 3.0 ohm resistor without a ballast resistor in a 12v system.

Current, Voltage and Resistance are all related. What you are really wanting to do is limit the current in the ignition circuit to around 3 to 4 amps. 6 volts divided by 1.5 ohms equals 4 amps, so is on the high side, but works. For 12 volt systems you need around 3.0 ohms, so, a 3.0 coil or a 1.5 ohm coil coupled with a 1.5 ohm ballast resistor.

May be confusing something like a GM HEI ignition system which uses a .5 ohm coil.

A .5 ohm coil in a 6 volt circuit would give you around 12 amps, why to high and very short points life. Same goes for a 1.5 ohm coil in a 12 volt system. 12v / 1.5 ohms = 8 amps, still way to much current.
Click to expand...

OK, just to clarify, in a non-ballasted system, a 3 ohm coil would be called a 12volt coil and a 1.5 ohm coil would be called a 6volt coil.

If the systems are ballasted, a 12 volt coil would have 1.5ohms and the 6 volt coil would have less resistance (I used .5 as an example, but that would vary depending on the value of the resistor that gets added to it).

Current draw (amps) will vary depending on the voltage, but it is really total wattage (Volts x Amps) that has to be limited.
 
Mike51Merc said:
OK, just to clarify, in a non-ballasted system, a 3 ohm coil would be called a 12volt coil and a 1.5 ohm coil would be called a 6volt coil.

If the systems are ballasted, a 12 volt coil would have 1.5ohms and the 6 volt coil would have less resistance (I used .5 as an example, but that would vary depending on the value of the resistor that gets added to it).

Current draw (amps) will vary depending on the voltage, but it is really total wattage (Volts x Amps) that has to be limited.
Click to expand...

Yep, if I am following your response correctly. It is the total resistance that has to add up. Most so called 6v coil will not measure right on 1.5 ohms and likewise with the ballast resistors. And all of these numbers are just at contain points in time anyway, a 6v system is not 6v, it is over 6v on a fully charged battery, under 6v when cranking the engine and over 7v when running at a cursing speed. The numbers are just for talking purposes, not detailed circuit analysis. What you are concerned about is mainly the current and keeping it around or below 4 amps. In a 6v system somewhere around 1.5 ohms will do that (there is also resistance in the wire, connections, etc.) and in a 12v system 3.0 ohms.

Another variable is the ballast resistor if one is used. The actual resistance changes with temperature. Some of the early car manufactures didn't use ballast resistor on the OEM style coils. All of the above relates primarily to "can" coils which do not require a ballast resistor (assuming it has the correct resistance). With the original Ford coils, they were a lower resistance value and coupled with a ballast resistor (I think the numbers were somewhere in the .8 ohm range). The ballast resistor had a lower resistance when cold which increased the amperage in the circuit to ad start up. As the current flowed through the resistor it would heat up and increase the resistance which lowered the current in the circuit to prolong point life.

So, again, there are variables all over the place in the ignition circuit and the resistance numbers used is just guidance as to what is needed.
 
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