tech53

I think the ignition coil is one of the least understood things on Brit bikes following electrics in general. But what is the coil and how does it work in an ignition system? Which coils for a Boyer on twins? I’ll try to keep the technical part of it all down to layman terms.

COILS AND BOYERS:

The coil is simply an Alternating Current (AC) transformer. It has N1 number of turns in the primary winding and N2 number of turns in the secondary winding. If an AC voltage is put into the primary it comes out the secondary multiplied by the turns ratio of N2/N1. In our case the voltage comes in as a switched 12 volts (switched by the points or by the Boyers output transistor; switched DC acts like AC under some circumstances, like in an ignition system) and goes out at thousands of volts to the spark plug. There is more to it than a simple transformer, but more on that later.

The primary winding wire has resistance and that resistance limits the current flow through the primary winding. Typical Brit 12 volt coils end up with about 4 ohms. So instantaneously, when the switched 12 volts is applied, about 3 amps flows through the primary windings. Typical Brit 6 volt coils end up at about 2 ohms, hence the same 3 amps flows through their primary.

The Boyer Electronic ignition was designed to operate one 12 volt Brit coil of about 4 ohms. That limits the current out of the Boyer to about 3 amps. In a points system the primary resistance is not all that important as the points can handle many more times the current of 3 amps. But a Boyer uses a transistor to switch the voltage onto the coil and transistors have voltage and more importantly current limits. Too little current (too high of a resistance) and the coil may not generate enough energy to reliably spark the plug. Too much current (too low of a resistance) and the transistor melts from the heat and the Boyer is toast.

Many combinations of coils have been tried for twins but it has come down to only a couple of choices: two 6 volt, 2 ohm coils or one dual spark plug output 12 volt, 4 ohm coil. Whatever the combination, the resistance the Boyer sees can not fall out of the 3 to 4.5 ohm range. Ballast resistors have been suggested but the energy the Boyer
instantaneously delivers is fixed at about 36 watts (3 amps times 12 volts). If you turn part of that energy into heat in the ballast resistor, then the coil(s) get robbed of energy they need to jump a spark gap in the secondary circuit.

How about two 12 volt coils connected in series? The resistance in series circuits is additive and the resistance the Boyer sees would be 8 ohms. That limits the current flow through the coils to 1.5 amps and the voltage across each coil to 6 volts. They were designed for 3 amps at 12 volts or 36 watts and they get 1.5 amps at 6 volts or 9 watts, 1/4 the energy for which they were designed. Power in is power out for the coil so the spark plugs get starved for energy. The bike may run fine at low RPMS but may miss in high RPM running or under heavy loads.

How about two 12 volt coils connected in parallel? The resistance in parallel circuits is not additive and the resistance the Boyer sees would be 2 ohms. That results in the Boyer having to supply 6 amps amps which exceeds the current specification of the Boyer and the Boyer is burned up toast. The bike may run fine, for a while, but eventually the output transistor of the Boyer will overheat and melt, resulting in a potentially long walk home.

So for a Boyer installation choose some combination of coils connected in series or parallel that limit the Boyers output current to 3 amps (the resistance the Boyer sees must be in the 3 to 4.5 ohms range) and puts the rated voltage to each coil (that is, 12 volts to 12 volt coils and 6 volts to 6 volt coils). It matters not how you get to that situation but the above two combinations of two series connected 6 volt, 2 ohm coils or one 12 volt, 4 ohm dual output coil flat work.

HOW COILS WORK IN AN IGNITION CIRCUIT:

Ignition circuits include a condenser (capacitor in modern terms), even inside a Boyer. It is connected so that it is placed across the coil(s). This makes the coil circuit into an oscillator. That is, when the circuit is instantaneously supplied with a voltage, over time it oscillates from positive to negative building with each transitions through zero volts until it reaches a peak voltage dependent on the values the circuit components. In the case of the ignition circuit, that primary coil terminal to terminal voltage reaches about 300 volts peak to peak. Given the turns ratio, the output of the coil has the potential of 30, 000 volts. But long before that the spark gap is jumped (dependent on cylinder conditions, compression ratio, spark plug gap), at about 15,000 volts. So we end up with about a two to one safety factor to accommodate worn plugs with wide gaps, excessive engine loads and compression ratios enhanced by carbon buildup in the cylinder head. Bad condenser? Then maybe you’ll get no spark or at least a very weak spark. The Boyer’s condenser is internal but modern condensers (now called capacitors) by nature are very reliable and don’t wear out. Old automotive condensers and Lucas condensers are not modern capacitors. Hence they probably should at least be tested if not regularly replaced.