DIY Transistorized Ignition

My 73 Honda CL175 has a very simple points ignition system. There is one set of points which fires a single coil connected to both cylinders. (wasted spark) This setup renders an exceedingly small spark. While it is true only a little spark is needed to ignite the hot air and gas having a larger spark is always fun and if I can increase the reliability, even better.

The easiest and cheapest upgrade to a points setup is to transistorize them. A transistorized ignition still uses the points but places a transistor in-between the point and the coil. This reduces the damaging current flow across the points and puts it on the transistor which (if properly sized) easily dissipates it as heat. By reducing the current across the points the wear becomes almost entirely mechanical, reducing the maintenance period and extending their useful life. If reduced maintenance is not incentive enough a heavy duty transistor can reliably switch a coil with a lower impedance. Generally speaking lower impedance coils produce a larger more aggressive spark. A larger spark can make for easier starting, improved idling and a greater chance of proper combustion in any condition. A final, albeit small, benefit is a transistor switches faster than points. A faster on off transition results in a sudden discharge of the coil and consequently an improved spark.

So, what did it take? I pulled the heat sink off an old intel PII I found in the garbage. If I am going to run a serious coil I want to make sure the transistor says nice and cool.

The circuit uses a high power PNP transistor. Any will do, the transistor needs to handle at least 10 amps and the higher the voltage level the better. Positive side of the coil goes to the transistors collector. 12V switched to the emitter and a the base connects to a ~40 ohm 10W resistor that continues to the points. My final setup was pretty small, it tucks nicely under the gas tank so the stock look is maintained.

In the future I would like to use two COP (coil on plug) units... just have to wait till an equipped car shows up in the local junk yard!








Research!
http://www.modelflight.com/ignition.html
http://www.5bears.com/tim4.htm
http://en.wikipedia.org/wiki/Bipolar_junction_transistor