Ballasted and Non Ballasted Ignition Coils
This is probably the most misunderstood part of our ignition circuits! What does it mean and what are the differences?
Well to understand both these systems we firstly need to understand a little
about what a coil does in the ignition circuit. The coil is a device that
produces the spark at our spark plugs. It does this by amplifying the voltage
applied to it from our battery, Thus it multiplies the battery voltage to
about 18,000 volts that then sparks across the spark plug.
Ok so in the early system a non ballasted coil is fed with about 12 volts from our battery when the ignition is turned on. Then each time the points open and close a spark is produced across the spark plug. The intensity of the spark is directly proportional to the battery voltage applied to the coil. So if we get 12 volts from the battery then multiply that up to 18,000 under normal conditions, what would happen if our battery voltage reduced? Well the battery voltage does reduce when we turn the engine over with the starter motor. During the starting routine using the heavy starter motor the battery voltage can drop to about 6 to 8 volts especially when it is cold in the winter. So when the battery voltage reduces to 6/8 volts then the spark voltage will drop to below 10,000 volts and will substantially reduce the power of the generated spark.
This therefore is the problem with older ignition systems where when the starter motor is engaged during start up the spark capacity reduces because of this voltage drop. However engineers a good few years ago decided to sort out this problem and came up with a splendidly simple solution The Ballasted Coil.
In the Ballasted coil system the normal mode of operation is that a resistor reduces the battery voltage to the coil and this reduced voltage is multiplied up by a stronger coil to our desired 18,000 volts. The resistor actually reduces the battery voltage to the coil to about 6 volts under normal conditions so works well. Now what happens when we operate the starter motor on a cold morning? Well the resistor voltage would possibly reduce to less than 6 volts and we would have subsequent reduction in generated spark voltage. However the tricky bit now happens, because in this system when we engage the starter motor on start up the system “bypasses” the resistor circuit and applies the full battery voltage to the low voltage coil. So even if the battery voltage falls to say 7 volts it is still high enough to make the correct spark voltage at the coil and helps the car start by ensuring full spark is available. Then when the engine is running and the starter switch is released the supply to the coil is back through the resistor and is again operating in normal mode.
From a Spridget point of view the change from non Ballasted to Ballasted
happened about 1978 thus only the later 1500cc MK1V Midgets were fitted
with the better starting system of Ballasted coils. If you are not sure
of which system is fitted to your car then check how many wires are fitted
to the (SW) (+) contact on the coil. If there are 2 wires coloured white
with light green tracer connected then it can be assumed you have the ballasted
system. A further check is to simply switch on the ignition and with your
multi meter measure the voltage between this connection and earth (engine).
You should measure about 6/8 volts confirming a ballasted system, if you
measure 12 volts (full battery voltage then the system may have been modified
by a previous owner and if you are not sure about how to proceed then seek
competent advice.