Ignition coils and ballast resistors???????
Discussion
I'm looking to replace the ignition coil on my tasmin race car as I suspect its a bit iffy. Why do some coils required the installation of a ballast resistor?
My car runs on an old MBE electronic ignition box (blue aluminium box) and a breakerless dizzy. Can anybody suggest the best coil to fit to it?
My car runs on an old MBE electronic ignition box (blue aluminium box) and a breakerless dizzy. Can anybody suggest the best coil to fit to it?
I can answer your first question, its to reduce the voltage to the coil once you release the ignition switch from the start position. When the switch is in the start position the coil sees full battery voltage, to provide a bigger spark for starting, once the switch is released (presumably because the engine has started) the coil sees a reduced voltage because the supply then goes through the ballast resistor, this is meant to reduce wear on the points.
Ballast resistors came about when ignition systems were a bit simpler (in fact everything was a bit simpler). The were introduced back in the good old days when it was simply a set of points in the distributor switching a 12v supply to the coil which produced a high voltage pulse to fire the plug. The problem came when cranking the engine, the battery voltage would fall which reduced the spark right when you were trying to get a rather suspect mixture to ignite. The simple solution was to use a coil with a lower impedance primary so it needed less voltage to produce a decent spark, this got the engine going but when you stopped cranking and the supply came back to 12v it was too much for the coil, particularly if the engine was stopped with the ignition switched on. So as to use the lower voltage coil and not have it burn out during normal running it was supplied via a ballast resistor which was bypassed when the starter motor was engaged.
There are two main uses for ballast resistors.
One is a cold start aid - the coil runs on say 9 volts and the ballast resistor drops the battery voltage to this. When cranking the full battery voltage is applied to the coil - it may go down quite a bit on a cold start due to the current taken by the starter.
Second use - with some electronic systems - is to limit the maximum current to avoid destroying power transistors, etc.
One is a cold start aid - the coil runs on say 9 volts and the ballast resistor drops the battery voltage to this. When cranking the full battery voltage is applied to the coil - it may go down quite a bit on a cold start due to the current taken by the starter.
Second use - with some electronic systems - is to limit the maximum current to avoid destroying power transistors, etc.
trakkies said:
One is a cold start aid - the coil runs on say 9 volts and the ballast resistor drops the battery voltage to this. When cranking the full battery voltage is applied to the coil - it may go down quite a bit on a cold start due to the current taken by the starter.
To put some numbers to it:A non-ballasted coil would normally run on a nominal 12V supply. A ballasted coil would run on a nominal 6V. During cranking the battery would typically drop to around 9V so the ballasted coil sees a higher voltage than normal and produces a much stronger spark.
This mucking around is really only necessary on simple mechanical ignition systems. As soon as you go to an electronic ignition system you can control the dwell electronically and produce a stronger spark without burning the coil out. Hence you won't normally see ballasted coils with a modern ignition system.
bertelli_1 said:
I ran without a capacitor on my megasquirt setup for a while but after reading the manual decided it prudent to fit one. Now I can't remember why it was needed or where that particular bit of info was. Not much help really was I?
There can be an issue with spikes on the 12v side resetting the ECU if the coils are driven directly - is that what you're thinking of?Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff