I was going to fit a Velleman K2543 electronic ignition system to my motorcycle (12V) but I thought I’d do a bit of analysis first. Looking at the circuit, R5, R6, and R7 (all 150 ohm) are either connected from the +ve supply to the -ve supply via T1 (so will see V+ - 0.2V) or via D3 and the base of T2 (so will see V+ - 1.4V)
So with the engine running V+ is about 14.4V, so the resistors see 14.2V or 13V so dissipation IN EACH ONE is 1.34W or 1.12W. They are only 1W resistors. This is a potential reliability issue, either from resistors burning out or heat melting the solder holding them in.
I’m not sure why they need to be 150 ohms. They will supply almost 0.25Amps into the base of T2.
Only 0.02A is required. I am thinking of replacing them with 680 ohm items.
Hi,
You would be right in the case of a DC voltage
In this case the transistor is switched according to the engine RPM and number of cylinders
So the average power dissipation is less then 1W
PS we sell this kit for over 30 years, no problem.
Hi
Perhaps my post was unclear…
When the points are closed, T1 is switched off and current flows through R5/6/7 into D3 then into the base of T2. The base emitter voltage of T2 will be around 1.4 (sorry I forgot it was a darlington in my last post) and the voltage drop across D3 will be about 0.7V. So the total voltage drop across R5/6/7 is 14.4-2.1V and the dissipation is just over 1 watt in each resistor.
When the points are open T1 is switched on and current flows therough R5/6/7 into the collector of T1. The stauration voltage will be about 0.2 V so the total voltage drop across R5/6/7 is 14.4-0.2V and the dissipation is 1.3W.
So when the engine is running the dissipation in each resistor is somehwere beween 1W and 1.3W.
I think the reliability of the kit is a testimony to the safety margins used by resistor manufacturers.