Warning! Pointy headed speak below
I've been making some calculations based on the manufacturers datasheet.
http://www.datasheetcatalog.org/datashe ... Xyywus.pdf
The maximum temperature that the transistor as photographed above is capable of operating at is 175 degrees. From this data sheet, the component manufacturer claims that the junction-case temperature that is generated is 2.5 degrees/Watt. This is Using a hypothetical coil of 3.0 ohms, this coil will consume 4 amperes at 100 percent duty cycle. So 4 amperes x 12 Volts is 48 watts, and 48 Watts x 2.7C/W is 129.6 degrees Celsius (lets call it 130 for fun).
Add this temperature to the ambient temperature in the distributor (approx 80 degrees, ie. block temperature) and you have an transistor operating temperature of a fraction under 200 degrees. This makes for a fairly stressed transistor and will explain the occasional failure. I can see why it is not a good idea to leave the ignition on with this going on. When the engine is running the coil is only "on" for about 50 percent of the time, so this reduces the current by the same amount thus reducing the running temperature to 145 degrees including the ambient temperature.
Not to bore anyone, but with these figures, don't leave this sucker on without the engine running, and despite my thoughts in the post above, don't use an electronic (or resistor rated) coil using this module.It is running above it's rated temperature. These figures won't destroy the transistor outright. But it will reduce its lifespan dramatically. My thoughts would be to purchase (from Jaycar) or create a heatsink and bind this onto the outward surface of the transistor using heatsink compound. This will reduce the temperature by 1.7 degrees/watt, thus the unit will run at a much reduced temperature of around 100 to 110 degrees running and 120 to 130 degrees with the ignition on but not running.... A much better option.
Not bad....
