1/3 Scale Ford 289 Hi-Po

[ddmckee54]

Perhaps a better way to think of it as something like "voltage times RPM sort of equals heat", as heat is probably the killer here. Higher RPM means more pulses/second so you have a higher duty cycle for the switching transistor(%ON time vs. %OFF time). All solid state electronic devices have an internal resistance that will generate waste heat when power is applied. During the ON time of that duty cycle, the device is going to be generating heat. The device can only bleed that heat off at a certain rate, which will happen mostly during the OFF part of the duty cycle. That's why you'll find heat sinks on computers and most electronics, it helps bleed the heat off faster. But there are still limits to the device's internal temperature, and how fast you can get rid of that heat. The weak link is usually the connection from the device to the outside world.

Hitting a capacitor with a higher voltage allows it to charge faster, which is great for a CDI system. But that higher voltage also means more waste heat that needs to be bled off, volts x amps = watts. Roy said "I would not try 7.4 volts because the switching transistor in the DC-DC circuit would not handle the current long term." It's just a guess, but eventually you probably get enough heat built up in that transistor that it cooks itself.

Don

 

[rsholl]

Terry and Don, you are both correct. Heat is the enemy in the DC-DC converter but not a problem in the HV section. As you increase the input voltage you also increase the high voltage to the Capacitor, SCR, and Coil. The most vulnerable component here is the SCR. I have toasted many of them in my early design work. We use a 500 volt SCR in the system but even 1 volt over that limit and its history. The main reason I went to the 500 Volt SCR was per chance someone did inadvertently apply 7.4 volts.

At a fixed RPM if you increase the input voltage, say from 4.5 to 5 the current draw will go up. At a fixed input voltage if you increase the engine speed (RPM's) you also increase the current draw. Increasing both of those at the same time will increase the current draw even more and as Terry mentioned, it is not linear in nature. If memory serves me correctly the CDI with 6 volts input and Maximum RPM or Sparks per Second the current draw is approaching 500ma or 1/2 amp.

Roy

 

[ddmckee54]

The voltage limit for the SCR also makes sense. Solid state devices are grown/built in layers, the layers are normally separated by a non-conductive layer. As it's the easiest to grow, this is usually silicon dioxide for silicon devices. The thickness of the layers is precisely controlled, and the layer thickness of the insulating determines the maximum voltage rating. Exceeding the maximum voltage rating will allow current to arc through the insulating layer, and things will go boom. (Mostly on a really small scale, but bigger devices=bigger boom.)