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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / December 2004internal resistor in coil question
In the old, conventional distributor/breaker point systems, how does the internal coil resistor let full voltage go through for starting, and then drop it down for running? Is it a temperature driven thing (i.e., as the resistor warms up, the resistance drops)? And how does it work with the external resistor? Is there a separate leg on one position of the starter switch that flows direct to the coil for starting, then switches to the resistor for running? Just curious. > In the old, conventional distributor/breaker point systems, how does the > internal coil resistor let full voltage go through for starting, and then > drop it down for running? Is it a temperature driven thing (i.e., as the > resistor warms up, the resistance drops)? what internal coil resistor? > And how does it work with the external resistor? Is there a separate leg on > one position of the starter switch that flows direct to the coil for > starting, then switches to the resistor for running? That is how all the older resistance wire circuits I've seen work. Ted I think the "internal resistor" coils have an extra reactance coil in them that offers little resistance when cranking, but after start the pulsating field creates inductive resistance that limits the current flow. All coils will have some reactance, of course, but the internally-ballasted type will have much more. Dan > In the old, conventional distributor/breaker point systems, how does the > internal coil resistor let full voltage go through for starting, and then [quoted text clipped - 7 lines] > > Just curious. The ignition switch applies power to the resistor in the "ON" position. Further rotation applies power to the "START" contact, which applies power directly to the coil and the starter relay. This effective bridges the resistor during the few seconds of START mode, while the starter motor is drawing such a heavy current that system voltage drops considerably. This is all done to help give you a good, hot spark even during cranking. I have never seen an "internal" resistor. Since the resistor usually has to drop about 6 volts while supplying about 2 amps of average current, the resistor will dissipate about 12 watts. Run resistors often look like a little ceramic brick, and are often mounted to the firewall to help dissipate the excess heat. The resistance element is often something like nichrome wire, like in your toaster. It actually has a small positive temperature coefficient; that is, resistance goes up with temperature. Ed To answer your original question, there obviously is no provision for 12volt bypass for starting in an internally resisted coil. And for you other guys up there, I don't know how common they are or for how many years they were produced or by whom but, yes, there is such a thing as an internally resisted coil in which no external resister is required. Good luck. > To answer your original question, there obviously is no provision for > 12volt bypass for starting in an internally resisted coil. And for you > other guys up there, I don't know how common they are or for how many > years they were produced or by whom but, yes, there is such a thing as > an internally resisted coil in which no external resister is required. > Good luck. These coil were (are) used on a lot of industrial applications, such as tractors or forklifts. A full 12 volts is on the + side of the coil at all times, whether it is starting or running. I'm trying to recall a part number on the top of my head. I think it is Stardard ignition products #12 or #15. Something like that anyway. > In the old, conventional distributor/breaker point systems, how does the > internal coil resistor let full voltage go through for starting, and > then drop it down for running? It doesn't, per se. > as the resistor warms up, the resistance drops)? Other way round. As the resistor heats up, its resistance *increases*, which reduces voltage to the coil, which causes the coil to run cooler, which reduces the resistor temperature, which increases voltage to the coil, which causes the coil to run hotter...and thus the coil temperature is controlled to prevent coil damage while providing the hottest spark possible. These internal-resistor setups give shorter breaker point life than systems with external coil resistors. > And how does it work with the external resistor? Is there a separate > leg on one position of the starter switch that flows direct to the coil > for starting, then switches to the resistor for running? Yes. The resistor bypass terminal may be on the starter switch, on the starter relay, or on the starter solenoid. DS }}} Other way round. As the resistor heats up, its resistance *increases*, which reduces voltage to the coil, which causes the coil to run cooler, which reduces the resistor temperature, which increases voltage to the coil, which causes the coil to run hotter... {{{ Of course! My mistake. Should have caught that. Thanks. the reason for the resistor is to shorten the time for a full buildup of the magnetic field (current ) in the inductor. (coil). the resistor limits the current in the coil to a liveable value while increasing the rate of buildup of the current in the coil. this gives the capability for a full output at higher engine speeds. the coils are still about the same as a six volt coil but the resistor prevents overheating and failure. sammmm > }}} Other way round. As the resistor heats up, its resistance *increases*, > which reduces voltage to the coil, which causes the coil to run cooler, > which reduces the resistor temperature, which increases voltage to the > coil, which causes the coil to run hotter... {{{ > > Of course! My mistake. Should have caught that. Thanks. |
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