Coil Confusion.

I have a MSD 8202 blaster coil. It says it's for electronic ignition.
I am running a Pertronix so will this coil work and do I need to run a ballast resistor?

 

Don't think so. That coil is listed with resistance.

• Primary resistance is .7 OHMs and the secondary is 4.5K OHMs

 

I'll give it a try. The resistance part is what confuses me.

Thanks...

 

A coil either has an internal resistor or needs an external resistor, your coil has internal resistance so no need for a separate resistor.

 

Well, I thought total resistance needs to be more than 0.7 ohms for points ignition. If you add a ballast resistor, the coil will still work and it won't cause any harm. For a 0.7 ohm primary, I would use one.

Looking at ballast resistors, they have a typical resistance of 1-1.5 ohms. This would be added to the resistance of the coil primary. With your coil resistance less than the resistance you would add with a ballast resistor, that suggests you need to be somewhere in the 2-3 ohm range. https://www.summitracing.com/search/part-type/ballast-resistors?SortBy=Default&SortOrder=Ascending&keyword=ballast resistor&kr=ballast resistor

You should be able to buy a ballast resistor at your FLAPS. When someone asked for a ballast resistor, we would sell them one for a '70s Dodge. 1970 DODGE MONACO 5.2L 318cid V8 Ballast Resistor | RockAuto One of many that will work.

What does Pertronix say? It's the big transistor in the Pertronix module that is at risk.

 

The Pertronix Ignitor modules, when used with an internally resisted coil, do not require an external ballast resistor. The ignitor module will operate just fine with a full 12VDC applied. The Ballast Resistor is for the coil, not the ignitor module, it will operate just fine without one, and is, in fact, designed to work that way.

 

It's not an internally resisted coil. 0.7 ohms is the winding resistance. An internally resisted coil will have ca 2-3 ohms total resistance. No added internal resistor.

 

Then the coil might require a ballast resistor. The Pertronix module doesn't have anything to do with it. The resistor is only for the coil. You can hook up the coil to a resistor, and the Ignitor module to a full 12VDC, and everything will work just fine, the coil voltage and module voltage don't need to match for the system to work. That being said, the ignitor module will operate just fine at the lower voltage that the ballast resistor provides, if you want to wire it up that way. The module literally doesn't care, and is happy to operate just fine at either voltage.

 

Reconsidered ... I think looking at voltage is a red herring here.

The added resistor will act as a voltage divider between the winding resistance and the ballast, but the purpose of the added resistance is to limit current. When grounded, the switch side of the coil will drop to the divided voltage, but the distributor side will fall to zero. When the points are open, both terminals of the coil rise to full operating voltage, ie 12.6 from the battery or 13.4 from the alternator. This does not drop with added resistance because it's floating.

Also realize that this is an RLC circuit, with complicated exponentially-decaying behavior. Voltage over time depends on the load (secondary), the switching rate and the values of R (total resistance), L (coil inductance) and C ("condenser" capacitance). The resistance acts like a damper on the waveform, but the voltage still switches from grounded (0) to full operating voltage (13.4), regardless of the value of R.

Adding a ballast resistor to R will limit the peak current, protecting the transistor in the module from excess load, overheating and failure. The coil will still spark.

 

This is true. But I can say, I've got Ignitor modules wired up 12VDC direct in a couple vehicles, one that's been there for close to 20 years, the other close to a decade, and I've not had any failures, or even need for adjustment on either of them. Is it *possible* to cook an ignitor module? Probably, but in my experience, it's such an unlikely possibility that it's not something I even worry about.

 

The PO hooked the Pertronix directly to the coil after the resistor. I wasn't sure about that so this is why I questioned it.

Thanks.

 

This is usual with a ballast resistor. Nothing remarkable here.

Ignition switch -> ballast resistor -> coil (+) primary
coil (-) primary -> distributor with Pertronix module.

 

Not to hijack this thread but I'm now wondering about the coil I have waiting to install and figured this was the place to ask.

I purchased a Holley Sniper Coil from Summit (https://www.summitracing.com/parts/sne-556-153) which lists primary resistance as 0.700 ohms, secondary as 4.70K ohms.

For a distributor I have this unit from Top Street Performance (Buick Odd Fire V6 HEI Distributor). I've asked them several times for specs on it and they say they don't have any....

I'm planning to replace the carb with a 2BBL Holley Sniper + master kit. I shouldn't need an external ballast resistor, right? I also did get a Classic tach filter (https://www.summitracing.com/parts/cin-sn20) for that line from the HEI to the Sniper.

 

You understand that the HEI distributor has its own coil under the cap? You have to modify the distributor to run with an external coil.

 

Obviously I do not....so I guess that means I can return the coil?

 

No external ballast needed.

 

Up to you. Seems like the right move at this point.

 

I guess I picked the right thread to post in since the title includes the word "confusion."