Ballast resistor

[TulsaFred]

I've rebuilt a 1275 and have it on an engine test stand (this one: https://www.northerntool.com/shop/tools/product_200334501_200334501).

I'll be wiring the ignition soon and am wondering about a ballast resistor. I know many cars have these to reduce the voltage going to the points so as to improve their longevity.

I don't know of one on my bugeye though.

Should there be a ballast resistor in the circuit, or should I send direct battery voltage to the coil?

thanks
Fred

 

[jlaird]

No ballast resistor on a BE. Yep. direct to dizzy from battery.

 

[aeronca65t]

Assuming you are using a stock, Lucas coil for that car (which is internally ballasted), you do not need an external ballast resistor.

If you are using an aftermarket coil that is not internally ballasted (and most aren't), then a ballast resistor should be used. You can get away without one for a while....the engine will run, but it'll just increase points wear if you don't use one with a non-ballasted coil.

 

[dklawson]

As stated above, the BE would not have had a ballast ignition system from the factory.

If you are in doubt about the coil, mark and remove its low tension wires. Use a multimeter to check the resistance across the low tension terminals. If you measure close to 3 Ohms you have a standard coil. If you measure between 1 Ohm and 2 Ohms you have a ballast coil. If you have a ballast coil you need to use the ballast resistor.

On the assumption that you still have a standard (non-ballast) coil I would be more concerned with the car's (engine's) polarity relative to the coil connections. If you are leaving the BE as positive ground and have the original Lucas coil with low tension terminals marked SW and CB, the power in from the SWitch goes to the SW terminal and the dizzy points connect to the CB terminal. IF you have switched the car's polarity to negative earth, the power from the ignition switch goes to the CB terminal and SW is connected to the dizzy.

 

[regularman]

Take an ohm meter to the coil. If its 3 ohms you are good. If its 1.5 then you need a resistor to keep from burning it up.

 

[MichiganTed]

Great tip guys! I was wondering the same thing last week.

 

[TulsaFred]

I looked on the SummitRacing website and searched for ballast resistors.
There are 19 different ones listed with a range of resistances, eg. 0.25 Ohm to 1.6 Ohm or so.
Can anyone put this into perspective?

thanks
Fred

 

[JPSmit]

I looked on the SummitRacing website and searched for ballast resistors.
There are 19 different ones listed with a range of resistances, eg. 0.25 Ohm to 1.6 Ohm or so.
Can anyone put this into perspective?

thanks
Fred

Yes, IIRC, a Bosch Blue coil doesn't require a resistor and is what Jeff Schlemmer recommends.

 

[regularman]

There are basically 2 types of coil. A 12 volt coil and a 6 volt coil. The 6 volt coil gets 12v to it to give a hotter spark when starting and then goes back to 6 volts when its running. They use a resistor to knock it down from 12v to 6v. The resistor should be about the same ohms as your coil that way it drops have the voltage and you get the 6volts at the coil. Thats why its called ballast, it matches the coil reistance and cuts the voltage to it in half under normal running conditions.

 

[TulsaFred]

Not to get too far into the weeds, but Ohm's Law is operative here,
V=IR
where V=voltage, I= current (amps), and R = resistance (Ohms).

The ballast resistor increases the resistance thus altering either volts, amps or both.
I believe the potential difference (volts) is fixed at 12 volts and the ballast resistor therefore decreases the current. This would then reduce points wear.

However, that is my amateur attempt at understanding. Any engineers out there to comment or correct me?

The various Ohms ratings of the available ballast resistors would result in a range of currents. I suppose the difference has to do with theresistance of the coil used, which I assume varies. Both Jegs and Summit Racing sell a "universal" ballast resistor with 1.6 Ohms resistance.

Is there a target current or current range across the points that is generally aimed for?

If so then total resistance (coil+ballast resistor) would be a constant. The correct ballast resistor would then be a easy to determine by measuring your coil resistance.

Thanks
Fred

 

[regularman]

The condenser is what keeps the spark from happening at the coil. The current through the either type of coil would be about the same. The 3 ohm coil at 12v= 4 amps. The 1.5 ohm coil at 6v=4amps. Its all about doing the temporary bypass of the ballast resistor and hitting that 6v coil with 12v when you first crank it. 12v at 1.5 ohms=8amps for the time you are cranking the engine over. This gives it one heck of a hot spark but the coil cannot take that for long. The other 12v 3ohm coil just gets 12v either starting or running.

 

[rkep01]

The difference in potential (voltage) between the input terminal of the coil and ground is 12V. If you place a resistor the same value as the resistance of the coil in series with it, the potential between the input lead of the resistor and ground will still be 12V. The potential between the input terminal of the coil and ground would then be 6V, because the resistor is dropping 6V, and the coil is also dropping 6V. I hope this helps in your understanding.

 

[TulsaFred]

OK, it's kind of silly, but being still confused I did a google search and came up with this interesting thread over at the Corvette Forum:
https://forums.corvetteforum.com/c1-and-c...d-readings.html
Lots of disagreement whether the Ballast Resistor alters voltage, current or both. An ignition design engineer with 24 years in Detroit at an OEM is pretty firm in his opinion that it limits current, not voltage. When a voltmeter shows a drop to 6 V, the engineer says you must use an oscilloscope to measure and that the coil always sees 12V.
Here is one of his comments:
<<You are under the significant misconception that an ignition system is a resistive circuit. It is not. It is primarily an inductive circuit, with a small amount of resistance in there (via the ballast resistance and the coil windings). Borrow an oscilloscope sometime and attach it to the C+ lead. You will get an amazingly different reading than you will get from a voltmeter. You can't get an accurate reading of an inductive PWM switching circuit using a voltmeter! It's not the voltmeter's fault, it wasn't designed to do that task. Get an oscilloscope, and see for yourself what the ignition primary circuit (and its components) are actually doing. Please!
In my travels in the car hobby world I've run into three key wive's tales. They are:
1) Road hugging weight. (Mostly dead, but some still revert back to it on occasion.)
2) Exhaust backpressure helps bottom end torque. (This still pops up more often than it should.)
3) A ballast resistor limits/regulates/sets the voltage that the coil sees. (The coil is driven by 12 volts, and sees 12 volts at the start of every dwell period. Every time!)
Get that oscilloscope, and come back and tell me what the voltage is that is pushing the current through the coil. You will get a different answer than what your simple voltmeter is telling you. If you're an honest sort, I'm betting you will agree with me.>>

At least I'm not the only one confused

Fred

 

[tomshobby]

There are basically 2 types of coil. A 12 volt coil and a 6 volt coil. The 6 volt coil gets 12v to it to give a hotter spark when starting and then goes back to 6 volts when its running. They use a resistor to knock it down from 12v to 6v. The resistor should be about the same ohms as your coil that way it drops have the voltage and you get the 6volts at the coil. Thats why its called ballast, it matches the coil reistance and cuts the voltage to it in half under normal running conditions.

At one time cars used a 6V system. Starters turned slower and the spark was weaker in cold weather. Then 12V systems came about. Starters turned faster and the spark was stronger especially in cold weather.

Soon after the switch to 12V systems it was realized the hotter spark also burned points much faster so the ballast was introduced to allow the full voltage from the battery while the starter was engaged and the reduced voltage after the resister while running.

 

[Sarastro]

Your engineer is correct but unfortunately his comments don't help you figure out what to do.

The simplest approach is to recognize that you need 3-4ohms total resistance to prevent frying the coil or points. If your coil is around 1.5-2 ohms, you'll need that 1.6 ohm ballast. The thing that saves your tush is that none of this is critical--after all, you use coils and other parts from allmanner of different manufacturers, and they're all at least a little different, and they all work.

Actually, a better solution is just to get the right coil to begin with.

Or, even better, get an electronic ignition.

 

[TulsaFred]

Thanks Steve
That is succinct.
Fred

 

[Hap Waldrop]

Assuming you are using a stock, Lucas coil for that car (which is internally ballasted), you do not need an external ballast resistor.

If you are using an aftermarket coil that is not internally ballasted (and most aren't), then a ballast resistor should be used. You can get away without one for a while....the engine will run, but it'll just increase points wear if you don't use one with a non-ballasted coil.

Yep Nial nailed it, it toally depends on the coil you are suing, on the MGB race car I use the Lucas sport coil (it packs a powerful punch) with my Mallory Unilite dizzy, no ballst resistor, because the Lucas sport coil is internally ballasted.

 

[BlueMax]

Lucas Sport coils produce 40,000 volts, if you grab hold of a bad wire you will most assured have a bad day, no pace makers please!

 

[bug_sixty]

This is basic but.. on pos ground cars does the the neg or pos coil terminal wire go to the distributor?

 

[Billm]

Positive- consider the distributor to be the ground
BillM