Ignition Ballast Resistor

Bob Bidonde

Bob Bidonde

Well-known member
Does the replacement 6v coil sold by "A" parts suppliers have a ballast resistor? In my opinion, a 6V replacement coil should not have a ballast resistor because it drops the coils input voltage to 5V or less and that results in a lesser hi-voltage output.

In the case of using a 12V battery with 6V components, a dropping resistor is necessary to limit the input to the coil to 6V, and it will also act as ballast resistor limiting the current input to the coil. This is a good situation in my opinion.

In the case where a 12V battery with a 12V ignition coil is used, a coil with a ballast resistor reduces the coil's input to a continuous 9V which is not beneficial in a Model "A". So a coil with that normally uses an external resistor, by running it without the ballast resistor gives a continuous 12V input and a higher hi-voltage output.

So in the case of a 12V system including a 12V ignition in the Model "A", is a coil without an internal resistor is the right choice?
 
I run 12 volts, and bought a coil from NAPA that didn't require a resistor, most 12v coils do require them. Most cars that originally had a 12v system, ran on 8v but used 12v when starting the engine to insure easy starting.
 
Running a coil that is designed to use a ballast resistor on 12 volts may burn out the coil, or maybe not. You can buy "hot" coils that put out more voltage to the plugs but still have the internal ballast resistor. Search online but beware of knockoffs that are not the same quality.

If you decide to run an external ballast resistor, it can be bypassed for starting with a simple circuit incorporating a switch. The reason that this is done on more modern cars is because the battery voltage is often about 8 volts when starting the engine. Most Model A's don't need to have the ballast resistor bypassed for starting.
 
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For my 12 volt application what I did was simply buy a coil with and internal resistor from Amazon
Standard Ignition #UC15T .
I didn't want to look at an ugly ballast resistor on my firewall.
 
I have come to the conclusion that a coil with a ballast resistor is not necessary and in fact, it is deleterious to the a Model "A" & Model "B" ignition systems.

Although a ballasted coil will work in the Models "A" & "B", it reduces the input voltage to the coil all of time, so a 6V coil's input voltage would be 4.5 to 5 Volts. This reduces the hi-voltage output and weakens the spark at the plugs after the engine is running.

I understand that the battery's voltage drops a couple of volts while the starter motor is operating, so the coil's hi-voltage output also drops while starting the engine. Unfortunately, while starting is when a greater hi-voltage spark is beneficial. However after starting the Models "A" & "B", the battery's voltage recovers so the coil gets a 6V input and a greater hi-voltage output. In the case of a 12V system, the battery voltage drops to 8 or 9 Volts while starting and recovers to 12V when the battery recovers to a full charge.

The above is opposite of the way modern ignition systems work. More modern systems use a 6V or 12V hot shot to the coil for starting by bypassing a ballast resistor, but drop to a lower voltage after starting by connecting the ballast resistor.

Is my conclusion correct?
 
Bob, your post #5 is correct. No ballast resistor needed or advisable on a 6 volt coil. The "modern" (50's, 60's and 70's) 12 volt cars use a ballast resister and bypass it for starting through the ignition switch that has a start position. I don't know what is done on current production where each plug has its own coil. I don't think the ballast resistors are used on electric cars.
 
Basically coils are 6 volts. To drop 12V to 6V Chevy & Mopar ran ballast resistors and Ford used a resistance wire from the key to the coil. If you remember starter solenoids had a second wire that led to the coil. It supplied 12V to the coil for a hotter starting spark. Coils marked 12V don't need a resistor but won't supply much spark if used on a 6V system.
 
ModelA29 said:
Basically coils are 6 volts. To drop 12V to 6V Chevy & Mopar ran ballast resistors and Ford used a resistance wire from the key to the coil. If you remember starter solenoids had a second wire that led to the coil. It supplied 12V to the coil for a hotter starting spark. Coils marked 12V don't need a resistor but won't supply much spark if used on a 6V system.
Click to expand...

I thought ignition coils were measured by Ohms, -and not voltage. Am I wrong??
 
BRENT in 10-uh-C said:
I thought ignition coils were measured by Ohms, -and not voltage. Am I wrong??
Click to expand...

The ohms and other things determines the voltage. It all has to do with Ohms law.
There is so much misinformation and misunderstanding about electricity on the internet, it is amazing anything works any more...LOL
 
BRENT in 10-uh-C said:
I thought ignition coils were measured by Ohms, -and not voltage. Am I wrong??
Click to expand...
True - but volts is easier to understand. Tell the kid at the parts store you need a 1.5 ohm coil and all you'll get will be a blank stare.

You only need ohms when testing to see if one is bad.



6 volt coils (and 12v coils that use a resistor) usually have a primary resistance of around 1.2-1.5 ohms. It should be around that to draw enough current to magnetize the core between firings, but not so much that the points fail right away.


And with coils marked 12V you'll be looking at double the ohms.
They like 3ohms of total primary resistance, they will tolerate 2.9 to 3.5. Below 2.9 the system unit flows more current then it was designed for and reliability suffers. Above 3.5 too little current flows through the system box and the spark energy decreases and may not be sufficient to start the engine
 
Talking to the kid at the parts store, ask him to bag your stuff, he’ll start pecking on the keyboard trying to find bag ? LOL
 
Also impedance. If you really want a blank stare from the kid in the auto parts store, ask him what the coil's impedance is. The impedance of the coil and the capacitance of the capacitor work in conjunction to form a tuned L-C circuit. L is the symbol for impedance and C is for capacitance. Getting too far away from the designed L-C circuit, such when the capacitor fails, will cause the points to burn and the ignition system to not put out a hot spark.
 
nkaminar said:
Also impedance. If you really want a blank stare from the kid in the auto parts store, ask him what the coil's impedance is. The impedance of the coil and the capacitance of the capacitor work in conjunction to form a tuned L-C circuit. L is the symbol for impedance and C is for capacitance. Getting too far away from the designed L-C circuit, such when the capacitor fails, will cause the points to burn and the ignition system to not put out a hot spark.
Click to expand...

And L/C resonant circuits would take up WAY too much space to discuss here.
 
The 1.5 Ohm can type coil works fine on 6-volts. When Ford went to the distributor mounted coils after the 1932 V8 cars came out, the coils and core were a lot different than the can type. These Mallory type coils did use a ballast of around 0.8 Ohms to control the amperage for peek performance of the design. After they went back to the can type coils with the 8BA engines, they didn't use a ballast. When Ford went 12-volt in 1956 then they added a ballast resistor to use the 1.5 Ohm can type coils. The 3 Ohm epoxy filled coils came much later for 12-volt systems with no need for a ballast.
 
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