Ballast resistor question

[Bruce B.]

I'm in the process of removing a HEI distributor from my 62 (a modified car) and need to install a ballast resistor to use with a standard coil and points distributor.
I have 2 ballast resistors and they test as anticipated at approximately 1.8 and 0.4 ohms. My problem is that when I put power to either one, I read approximately 12 volts on both sides of the resistor.
I feel like I'm missing something .
Any advice will be appreciated.
Bruce B.

[Ted S.]

Re: Ballast resistor question

Bruce, with the points open the resistor would measure 12 volts on both sides because there is no path to ground (aka current flowing). In other words the circuit is open. Once the circuit closes then you have two "resistors" in the circuit, the ballast resistor and the resistance from the windings in the coil. The two resistors will "consume" the full 12 volts. That is if you measured across each of them with the circuit closed the two voltages would sum to equal 12 volts. Hope this helps, Ted

[Duke W.]

Re: Ballast resistor question

I believe the 0.4 ohm (GM specs it at 0.3) was OE on all '62s, but they also had "special" coils. If you use a generic 12V coil use the higher resistance ballast. For a given coil resistance, the lower the ballast resistance the higher the primary current, and the "low ohm" ballasts were known to burn points.

If you only have one resistance load in a circuit, you will always read full circuit voltage drop across that one load regardless of its value. What changes with reisistance is current. I=V/R, which is Ohm's Law.

Wire each of the ballasts in series with a 12V coil and now measure the voltage drop across both the ballast and coil. Such a two load circuit is called a voltage divider.

Duke

[Dennis A.]

Re: Ballast resistor question

Bruce...
Don"t forget to rewire the coil. Check the green to the starter, the brown from the ign. switch and reinstall the black to the Dist. A new engine harness would have all this...

[Bruce B.]

Re: Ballast resistor question

Thanks for all the help.
Everything went together nicely and the car runs great.
Bruce B.