Does anyone know why the ballast resistor with a dual point distributer gets hot and if this is normal or not?
1961 Corvette - 283 cui / 270 Hp
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Re: 1961 Corvette - 283 cui / 270 Hp
Power dissipated in an electrical circuit is I**2*R... that's current squared times resistance.
All C1s use the 0.3 ohm ballast, and primary coil resistance is about 1.0 ohm, so total circuit resistance is about 1.3 ohm.
I = V/R, so at 14 volts I = 10.7 amps, but the circuit is intermittent and the average is about 5 amps, so total power dissipated by the ballast is about 5**2 (0.3) = 7.5 Watts and about 25 Watts in the coil. With appropriate unit conversions, Watts (power, which is energy per unit time) can also be expressed as BTU per unit time, which is the rate of heat generation.
7.5 Watts is a lot of power dissipation for the small mass of the wire, so it gets hot.
You can run the calculation for the 1.8 ohm ballast.
The ballast wire is a special alloy that has increasing resistance with increasing temperature, so it acts as a current limiter to prevent excess current that can burn the points and excess coil temperature that can damage the coil.
Look up posts by the late Dale Pearman from the early days of the TDB, and you will find that he refers to the ballast as an "analog computer", which is a correct representation albeit, it's about as crude an analog computer as can be imagined.
DukeLast edited by Duke W.; July 29, 2016, 10:30 AM.- Top
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