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B_man_Al · 08-09-2013, 07:22 AM

Gasoline … hope you dont glaze over here, it’s a long one!

I think the digital meter is giving you false readings when the car is running. This is because the meter is set to DC (I presume) , but the voltages are not a steady DC as the current is being interrupted by the points. Also you can get some pretty high inductive kicks back from the coil which really confuses a digital meter. That's why I keep telling people to measure with the engine off, and the points closed. Only then will you get accurate DC readings.

I asked about the ballast and coil resistance readings because I'm trying to figure out if your numbers are MEASURED or are they the mfg SPECS.

NO BALLAST RESISTOR CASE, ENGINE OFF:

Let’s assume the coil is 1.4 ohms, and the ballast is 0 ohms (not there) as stated.

From your measurements the coil voltage is 5.7v and the resistance is 1.4 ohms. This makes the steady state DC coil current 4.07 A. (I =5.7/1.4)

If there is no ballast resistor, then the loom resistance must be dropping the rest of the voltage = 12V-5.7V = 6.3V. This makes the “loom” resistance = 6.3V/4.07A= 1.5 Ohms

.7 OHM BALLAST RESISTOR CASE, ENGINE OFF:

Let’s assume the coil is 1.4 ohms, and the ballast is .7 ohms as stated.

From your measurements the coil voltage is now 4.7V and the resistance is 1.4 ohms. This makes the steady state DC coil current 3.36 A. (I =4.7/1.4)

The voltage across the ballast resistor becomes .7ohms x 3.36A = 2.35V

Now the loom resistance must be dropping the rest of the voltage = 12V-4.7V-2.35V = 4.95V . This makes the “loom” resistance = 4.95V /3.36A = 1.5 ohms

In both cases, the “loom” resistance comes out to 1.5 ohms, so that must be the correct value, and all the numbers make sense.

I would suggest that you make an actual measurement of the current with the multimeter to confirm, (with the engine off and the points closed).

If you confirm these numbers, then it is easy to calculate the peak coil current when running as follows:

I coil running = I coil stopped x 14/12 (assuming the generator/alternator is putting out 14v)

Then :

I pk with Ballast resistor =3.36A x 14/12 = 3.92 A
I pk without Ballast resistor =4.07A x 14/12 = 4.75 A

Finally to get the average running current (which determines the heat of the coil), take the peak currents above X dwell angle/45 (assuming a V8 engine)

Assuming the dwell is around 30 degrees, the numbers become:

I avg with Ballast resistor = 3.92 A X 30/45 = 2.61A
I avg without Ballast resistor = 4.75 A X 30/45 =3.16A

These current numbers seem quite reasonable to me, and its not clear if the ballast resistor is required. From the above calculations, the majority of the current limiting is in the “loom resistance” of 1.5 ohms, and not the actual ballast resistance of .7 ohms

Hope this isn’t too complicated, but that’s life!

08-09-2013, 07:22 AM	#72
B_man_Al Senior Member Join Date: Feb 2011 Location: Ottawa ON Canada Posts: 301	Re: Mallory tech answer to coil/ballast ? Gasoline … hope you dont glaze over here, it’s a long one! I think the digital meter is giving you false readings when the car is running. This is because the meter is set to DC (I presume) , but the voltages are not a steady DC as the current is being interrupted by the points. Also you can get some pretty high inductive kicks back from the coil which really confuses a digital meter. That's why I keep telling people to measure with the engine off, and the points closed. Only then will you get accurate DC readings. I asked about the ballast and coil resistance readings because I'm trying to figure out if your numbers are MEASURED or are they the mfg SPECS. NO BALLAST RESISTOR CASE, ENGINE OFF: Let’s assume the coil is 1.4 ohms, and the ballast is 0 ohms (not there) as stated. From your measurements the coil voltage is 5.7v and the resistance is 1.4 ohms. This makes the steady state DC coil current 4.07 A. (I =5.7/1.4) If there is no ballast resistor, then the loom resistance must be dropping the rest of the voltage = 12V-5.7V = 6.3V. This makes the “loom” resistance = 6.3V/4.07A= 1.5 Ohms .7 OHM BALLAST RESISTOR CASE, ENGINE OFF: Let’s assume the coil is 1.4 ohms, and the ballast is .7 ohms as stated. From your measurements the coil voltage is now 4.7V and the resistance is 1.4 ohms. This makes the steady state DC coil current 3.36 A. (I =4.7/1.4) The voltage across the ballast resistor becomes .7ohms x 3.36A = 2.35V Now the loom resistance must be dropping the rest of the voltage = 12V-4.7V-2.35V = 4.95V . This makes the “loom” resistance = 4.95V /3.36A = 1.5 ohms In both cases, the “loom” resistance comes out to 1.5 ohms, so that must be the correct value, and all the numbers make sense. I would suggest that you make an actual measurement of the current with the multimeter to confirm, (with the engine off and the points closed). If you confirm these numbers, then it is easy to calculate the peak coil current when running as follows: I coil running = I coil stopped x 14/12 (assuming the generator/alternator is putting out 14v) Then : I pk with Ballast resistor =3.36A x 14/12 = 3.92 A I pk without Ballast resistor =4.07A x 14/12 = 4.75 A Finally to get the average running current (which determines the heat of the coil), take the peak currents above X dwell angle/45 (assuming a V8 engine) Assuming the dwell is around 30 degrees, the numbers become: I avg with Ballast resistor = 3.92 A X 30/45 = 2.61A I avg without Ballast resistor = 4.75 A X 30/45 =3.16A These current numbers seem quite reasonable to me, and its not clear if the ballast resistor is required. From the above calculations, the majority of the current limiting is in the “loom resistance” of 1.5 ohms, and not the actual ballast resistance of .7 ohms Hope this isn’t too complicated, but that’s life!