TheMonkey wrote:this is great. then, with injectors that measure 2.4 ohms each or
higher.... just treat them the same as high impedence? no difference in wiring or in config?
i just read that each circuit can handle 5 amps. if running voltage is 13.5v rather than 12v, then wouldn't that be 2.7 ohms or higher? reason i ask, is that the injectors i'm referring to measure exactly 2.4 ohm, and runs at 13.5v, which is 5.6 amps.
OK, the driver in all of this is the driver
The VND5N07 starts current limiting at 5 amps. What this means is that when the pass current exceeds 5 amps (or thereabouts, its not exact) it will limit the current at 5 amps. How does it do this? The driver starts to operate in a linear mode where it will drop voltage such that it maintains the 5 amps.
You are correct on the injector at 12 volts, the current = voltage / resistance = 12V/2.4 ohms = 5 amps on the nose. In this situation the current is right at the threshold and the driver stays saturated (i.e. not in linear mode).
Now lets say the battery voltage is 14 volts. If we run ohms law, this is 14 Volts/2.4 ohms = 5.8333 amps. Since the current has exceeded the driver threshold of 5 amps, it will go into a linear mode. In linear mode, the driver basically "throws away" voltage to maintain the 5 amp current. We know from the first example that 12 volts is all the injector needs to flow 5 amps. The supply is 14 volts - so this means the driver has to "throw away" or drop 2 volts. So you have the 14 volts potential of the battery, 12 volts are dropped by the injector and two are dropped by the driver itself. Another way to look at it is the driver becomes a resistor, and to flow 5 amps at 2 volts is a 0.4 ohm resistor equivalent.
Of course when there is a voltage drop there is heat. The power dissipation in watts is current times voltage. In this case we have two volts dropped multiplied by 5 amps, this is 10 watts. It is alot of heat, but the D-Pak package is pretty good in this regards, having a junction-to-case value of 3.75 deg C/watt. So there is heat increase which derates the max temperature you can run this.
What happens if the die becomes too hot? The driver simply shuts down, no damage done. In fact, there was a setup using microsquirt where they were running 1.0 ohm injectors per bank. The problem was that they could not get enough fuel in the engine no matter how high they jacked up req_fuel. Turned out that the drivers were overheating and shutting off after about 3 ms. More interesting is that they cooled down fast enough to re-enable for the next pulse!
Its interesting to do the same exercise with a peak-and-hold circuit. Use the same 2.4 injector, but this case assume the hold current is 1 amp. Assume a battery of 14 volts. It takes 2.4 volts of potential to make a current of 1 amp flow thru a load of 2.4 ohms (2.4V = 1a * 2.4ohms). The rest of the voltage the driver has to throw away. In this case it is 14 volts - 2.4 volts = 11.6 volts. Power dissipation is also 11.6 watts, very similar to the above situation.
Note that this same driver is used in another aftermarket controller that run low-Z injectors, apparently with success. Of course if in doubt add the external resistors.
- Bruce