Posted by Texasmark1 on May 22, 2012 at 07:19:22 from (67.142.175.22):
In Reply to: 12 Volt coil posted by Bud Higginbotham on May 21, 2012 at 16:03:03:
Points are designed to operate with 4-5 amperes of current (at least that's what the manuals say that I read; I don't design conventional ignitions).
For a 12v battery, 5 amperes is 12.75 or lets say 13.5volts if your gen is working, divided by 5 amperes = 2.7 ohms.
When the points are closed, with the proper dc resistance (the thing you read with a multimeter on ohms scale and the engine not running), normal current flows through the primary of the ignition coil for the duration of the charging cycle (dwell). The points open and the energy now stored in the coil is released into the spark plug....bang!
Soo, depending on the dwell time, number of cylinders, rpm and all, Ohms law says that the coil resistance x the current flow (squared) flowing through it will generate heat in the coil; the normal process with a little additional heat added from the fact that they are usually conveniently attached to the side of the engine block.
Soooo if you have no/too small a ballast resistor to limit the current, you can over "wattage" your coil and it can be destroyed from heat, along with your points burning up prematurely from the over current/over sparking that your "condenser" may not catch due to the excessive energy stored in the coil [w (in watt-seconds) = 1/2 L (inductance of the primary winding) x I (dc current flowing through it when the points open) squared].
I know you didn't ask for that but if I don't use it from time to time I will forget it.
Sooooooooo you might get out your ohmmeter on the OHMS, 20 is a popular number with DVM's for the scale, and measure from the cold side of your ignition switch (switch off) to the black wire that goes into the side of your distributor. If you measure too few ohms, you have found your smoking gun.
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