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Old 11-26-2019, 12:02 PM   #1
f balsiger
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Default 12 volt coil with resistor

will a 12v coil with resistor send 12v to the points? I don't understand if the resistor lowers voltage or ohms
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Old 11-26-2019, 12:06 PM   #2
rotorwrench
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Default Re: 12 volt coil with resistor

The ballast resistor is more for current control than dropping the voltage even though it does both (basic OHM's Law). The coil doesn't need to draw as many amps with the higher voltage. Some coils are designed with higher primary coil resistance so that they can be run direct off 12-volt but there are still plenty of 1.5 ohm primary coils out there. The 3.0 ohm primary coil is more for a 12-volt system.
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Old 11-26-2019, 12:36 PM   #3
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Default Re: 12 volt coil with resistor

When cold it will send 12 volts to the points, as the resistor warms up it creates more resistance dropping the voltage to points. That is why many cars have a bypass for starting, so even if the resistor is warm the system sends full voltage for hotter spark for starting.
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Old 11-26-2019, 12:56 PM   #4
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Default Re: 12 volt coil with resistor

Brand X (GM) actually had a bypass circuit to bypass the resistor or resistance wire so that it would have a full 12-volt while the starter was turning. Ford did that later to some extent but they had a different type solenoid than GM. The resistor will heat up as soon as the points are closed so they warm up almost instantaneously but it certainly controls the current by bleeding off what it doesn't need to function. The old Ford/Mallory coils only needed around 3 or 4 volts to function normally. Much more than that would burn them up. Can type coils can take a lot more heat so most never needed a ballast running on 6-volts. Modern insulating materials allow coils to take a lot more heat than they ever could before.
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Old 11-26-2019, 12:59 PM   #5
Purdy Swoft
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Default Re: 12 volt coil with resistor

I use the three OHM Pertronix flamethrower coil on my 12 volt converted 31 roadster . No resister is used or needed when a three OHM coil is used on a twelve volt system . No wires will need to be changed or reversed as long as it remains positive ground . . If a person feels that they need or must use a negative ground setup , reverse the wires at the coil and ammeter . The battery must be connected negative ground and you will be hooked up negative ground . I don't know what voltage is sent to the distributor but it works and I have had no problems with it .

If a 1.5 Ohm coil is used on 12 volts it will be necessary to use a resistor . Coils are rated in OHMs . In other words three Ohm for twelve volt will make a cleaner more ioriginal look without a resister .

There can be confusion between a six volt and twelve volt coil . When buying at a local parts house , I find it is best to ask for a coil in OHMs rather than volts Some counter people still won't know the difference and there will be NO telling what coil that you may end up with . Usually they will ask what year and model vehicle the coil is for . If you tell them the coil will be used on a model A Ford , they will usually tell you that they don't have one . This is why The OHM rating is sio important . I mostly order my coils from Pertronix and ask for three OHM for twelve volts or one point five OHM for six volts . .
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Old 11-26-2019, 02:40 PM   #6
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Default Re: 12 volt coil with resistor

I've read that some like running 6v coils with the external resistor on 12 V systems.
Power dissipation of teh coil is substantially reduced since the power in the circuit is shared by the resistor. The benefit is the coil runs cooler and extends the coils life.
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Old 11-26-2019, 03:23 PM   #7
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Default Re: 12 volt coil with resistor

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Originally Posted by f balsiger View Post
will a 12v coil with resistor send 12v to the points? I don't understand if the resistor lowers voltage or ohms



The easy and fast answer is no. Points don't like or live long on anything greater than 8v. The lower the voltage the better, to a point. [ pun intended]
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Old 11-26-2019, 03:48 PM   #8
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Default Re: 12 volt coil with resistor

On a 12v system if you measure across open points it measures 12V. When points are closed it drops to 0V since one side is ground. This is for all combinations. Since there is no current flow, voltage is not dropped across the components of the system.


You do not use an external resistor on 12 V coils in 12 volt systems, The 6 V coil has an internal resistor of 1.5 ohms., so when used in 12 systems an external 1.5 ohm resister is added. The 12v coil has a 3 ohm internal resistor.


The external 1.5 ohm resistor is only used if you have a 12v system with a 6v coil.


It is mostly the current that causes points issues if a resistor is not used with the 6 v coil.


Additionally if the resistor is not used in 12v systems/6 volt coil - the coil has to dissipate a lot more heat/power than it was designed for.
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Old 11-26-2019, 06:16 PM   #9
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Default Re: 12 volt coil with resistor

When the points are closed and the ignition switch is on then the coil is in effect an electromagnet so it does drop some current to maintain the electromagnetic field. This is why some folks end up with a burnt up coil if they forget to turn off the ignition and the points just happen to be closed. The primary will generate a lot of heat and some coils just can't take it.

When Ford changed over to 12-volt in 1956, they added a ballast resister to the system so they could use the same coil but keep the current in check. They did that for a long time. In the modern era, there are a lot better materials for manufacturing ignition coils than there were back when they changed over to 12-volts.

The breaker points can take whatever the system is designed for as long as they have a good condenser to make & break the circuit with. The condenser protects the points and slows the collapse of the magnetic field in the coil core just a bit. This makes for an efficient high tension induction force to the secondary winding and on to the spark plug. Bad condenser equals burnt points and poor or no spark.

Last edited by rotorwrench; 11-26-2019 at 06:23 PM.
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Old 11-26-2019, 10:45 PM   #10
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Default Re: 12 volt coil with resistor

Don't many 12 volt coils have a built in resistor?
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Old 11-27-2019, 07:44 AM   #11
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Default Re: 12 volt coil with resistor

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Originally Posted by redmodelt View Post
Don't many 12 volt coils have a built in resistor?



Many/most do, but, many don't. Thats why the ohm rating is important to know. Most with a resistor installed will state it on the case, but, then again, some don't.
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Old 11-27-2019, 07:46 AM   #12
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Default Re: 12 volt coil with resistor

I won't get into the voltage at the points discussion, only to say points will not stand 12v for very long.
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Old 11-27-2019, 08:36 AM   #13
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Default Re: 12 volt coil with resistor

In 12V electrical systems with breaker point ignitions, ignition coils start the vehicle on 12V through the coil, and the resistor drops the coil to 6V to 8V once the engine is running.
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Old 11-27-2019, 09:28 AM   #14
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Default Re: 12 volt coil with resistor

All ignition coils have a built in resistor. It's called a primary winding. More turns, more resistance.
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Old 11-27-2019, 10:56 AM   #15
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Default Re: 12 volt coil with resistor

My head hurts.
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Old 11-27-2019, 11:08 AM   #16
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Default Re: 12 volt coil with resistor

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Originally Posted by rotorwrench View Post
When the points are closed and the ignition switch is on then the coil is in effect an electromagnet so it does drop some current to maintain the electromagnetic field. This is why some folks end up with a burnt up coil if they forget to turn off the ignition and the points just happen to be closed. The primary will generate a lot of heat and some coils just can't take it.

When Ford changed over to 12-volt in 1956, they added a ballast resister to the system so they could use the same coil but keep the current in check. They did that for a long time. In the modern era, there are a lot better materials for manufacturing ignition coils than there were back when they changed over to 12-volts.

The breaker points can take whatever the system is designed for as long as they have a good condenser to make & break the circuit with. The condenser protects the points and slows the collapse of the magnetic field in the coil core just a bit. This makes for an efficient high tension induction force to the secondary winding and on to the spark plug. Bad condenser equals burnt points and poor or no spark.




This is one of the best/thorough explanations I've seen. You have to read/understand every word, every word is important. - Thanks
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Old 11-27-2019, 11:23 AM   #17
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Default Re: 12 volt coil with resistor

I'm not too technological on electronics and electrical. That being said I have done real well keeping trucks and cars running up till all of this new computer age. Here's one for you, I bought a 30 model A a few years back with a 12 volt modern Motorcraft coil on it. I noticed this as soon as I raised the hood but didn't get around to changing it and it ran so good I never have changed it because I am still running 6-volt. Never had a problem yet after 4 years driving.
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Old 11-27-2019, 01:38 PM   #18
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Default Re: 12 volt coil with resistor

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Originally Posted by 30 Closed Cab PU View Post
I've read that some like running 6v coils with the external resistor on 12 V systems.
Power dissipation of teh coil is substantially reduced since the power in the circuit is shared by the resistor. The benefit is the coil runs cooler and extends the coils life.
Some may want to run an original Ford script coil , Others may just not want to spend the money for a new coil . If running a six volt coil on twelve volts a resistor mounted at the coil connection will be needed or the coil will burn out . The three Ohm coil will give hotter spark than the six volt coil being used on twelve volts with a resister . I run a three Ohm coil on twelve volts and my coil doesn't get hot . I use the Pertronix three Ohm Flamethrower coil . The Flame thrower coil is a performance coil that will add power that can be felt . The spark comes from the coil . I'm not saying that a performance coil must be used . its just a matter of choice . The model A needs all the help it can get !!!
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Old 11-27-2019, 03:13 PM   #19
30 Closed Cab PU
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Default Re: 12 volt coil with resistor

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Originally Posted by Purdy Swoft View Post
Some may want to run an original Ford script coil , Others may just not want to spend the money for a new coil . If running a six volt coil on twelve volts a resistor mounted at the coil connection will be needed or the coil will burn out . The three Ohm coil will give hotter spark than the six volt coil being used on twelve volts with a resister . I run a three Ohm coil on twelve volts and my coil doesn't get hot . I use the Pertronix three Ohm Flamethrower coil . The Flame thrower coil is a performance coil that will add power that can be felt . The spark comes from the coil . I'm not saying that a performance coil must be used . its just a matter of choice . The model A needs all the help it can get !!!

Am just trying to understand.



Why does the 12 V coil give a hotter spark than a 6 Volt coil with resistor? Both draw the same current, and both use the same spark plug gap/points gap. What am I missing? Does the 12v Coil Store more energy for the discharge? If so wouldn't that be hard on the points?
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Old 11-27-2019, 05:44 PM   #20
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Default Re: 12 volt coil with resistor

The coil doesn't store any energy. It's just an inductor. Induction coils generally don't work on direct current (DC) power but they will work for alternating current (AC) power and are used a lot in electronics to step voltages up. They work for an automotive ignition due to the pulsating direct current provided by the breaker points since it is half wave alternating current.

A hotter spark is a product of more modern technology. The systems only need to create a spark hot enough to light off the flame front in the cylinder. If a designer wanted a hotter or higher voltage spark back in the day, it would cost them in lower reliability of the system components. After the advent of modern electronic ignitions, there was a lot less worry about reliability with breaker points and condensers since they no longer had them. Coils were designed with materials that can take a lot more heat so reliability was no longer a problem with them and they could add a lot more windings to increase voltage step up. They can generate a spark that can actually be heard snapping while the engines running on some designs. I could always hear the spark on the GM HEI set ups while the engine was idling. They had to have a lot wider gaps for the spark plugs as well. A DC points ignition would never be able to do this for very long.

The model T uses four trembler coils for its ignition (one for each cylinder). They put out a shower of sparks instead of just one spark per ignition event. That makes them hotter than any 6-volt coils but they were powered off a magneto set up on the flywheel.

The model A went to a can type coil that had an oil inside for an insulator so it could take some heat and not need a ballast to control current.

When the Ford V8 came out, they changed the coil to a different design that evolved with several different configurations but they had to have a ballast to control the current (heat) or they would lose reliability. They would put out just as hot a spark as the model A but they only used about half the voltage of a 6-volt system to do it.

When Ford went back to the can type coils for the 8BA in 1948, they did away with the ballast resistor and it didn't come back till they changed over to 12-volt systems. The ballast resistor is all about reliability in the old systems.

Last edited by rotorwrench; 11-27-2019 at 05:51 PM.
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