Testing a 12V resistor coil.
 

How do you you test the voltage the points see in operation from a 12V internal resistor coil ? Thank you.

Re: Testing a 12V resistor coil.
 

Volts at the points is not important. Current (amps) is.

If you measured voltage at the points, I think the voltmeter would just see 12 volts while points are open, 0 volts when closed connected directly to ground. You could measure this at the distributor terminal of the coil. But not sure what that would tell you.

Re: Testing a 12V resistor coil.
 

Skip suggests 3.8 to 4.2 volts from the coil to achieve good point life. Does that convert to amps which would affect the life of the points. Thank you.

Re: Testing a 12V resistor coil.
 

The basic battery ignition system needs a breaker point set to provide a pulsating direct current so the inductor coil will function. Induction can only happen in a coil if there is either a rapid change of polarity such as AC current provides or a rapid on off half wave pulsating direct current to simulate AC. It's the build up of a magnetic field in the core followed by the collapse of that field when the points open that provides induction of a low tension voltage in the primary to a high tension voltage in the secondary. The condenser has to be there in the circuit to catch the back flow of voltage during the induction process or it will jump the point gap and burn the points up. The back flow voltage can reach as high as 200-volts in some cases but it's the condenser's job to insure that doesn't affect the breaker points.

The ignition system is a complete circuit on its own. The primary will draw current from the battery when the points are closed in order to build the magnetic field in the coil core. It would be this current that a person should measure with an amp meter in the circuit. Once the points open, the magnetic field collapses and there is no further current draw till the points close again.

Depending on the design of a coil, it will draw whatever current it is designed to depending on system voltage. Coils with a 1.5-Ohm primary were designed to be able to take 6-volts with no adverse effects. The use of 12-volts on this type of coil will allow a current draw that will overheat the primary so a ballast resistor is required to control the current in flow to this type of coil. A modern type coil with a 3-Ohm primary will function on 12-volt with no need for a ballast.

When external ballasts are used with any coil induction ignition system design, they are there to protect the coil from excessive current draw and make the system more reliable. Some designs can take more heat than others. Modern materials have made coils designs more reliable without the need for any form of current control

Ohms law shows the relationship between the carrier voltage and the current as well as wattage and resistance. A change in any one will affect the other three.

Re: Testing a 12V resistor coil.
 

Excellent post "rotorwrench". It makes it easy to understand. Using Ohm's Law (I=V/R), you can see that 12/3=4 amps and that 6/1.5= 4 amps as well. Points like to operate carrying current in the 4 amp vicinity, no matter what the voltage.

Re: Testing a 12V resistor coil.
 

Thank you rotorwrench & tubman. That was a great explanation of how, why and what affects the system.