Low Z Injectors on Sequencer

[TheMonkey]

if using low impedance injectors with sequencer, can PWM be used or do resistors need to be installed?

i'm not sure if the flyback board can be used for 6 or 8 injectors in sequential, or do you need separate circuits for each injector?

[Bruce Bowling]

The drivers are the same as microsquirt - each channel can handle a single 2.4 ohm injector. Anything lower will require series resistance.

There is no PWM on the sequencer for injector current control.
- Bruce

[TheMonkey]

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.

[racingmini_mtl]

Another question about this is the current drawn from the charging system. If you have 50% duty cycle and 8 injectors that's 20A and at 80% it's 32A of continuous current. Also this has to go through the ground plane, the connector and the wiring. Is it safe?

Jean

[TheMonkey]

ah yes... duty cycle...

it didn't occur to me that the particulars of running low-z vs high-z comes down to current flow.... and as far as grounds and current, i'll have to re-read the discussion we had previously on this, but low-z injectors certainly up the ante.

[Bruce Bowling]

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

[Bruce Bowling]

Another question about this is the current drawn from the charging system. If you have 50% duty cycle and 8 injectors that's 20A and at 80% it's 32A of continuous current. Also this has to go through the ground plane, the connector and the wiring. Is it safe?

Jean

A lot of this has to do with the voltage drop in the ground plane, connector, and wiring. Run 32 amps thru 0.01 ohms and this is 0.32 volt drop. Its easy enough to measure with a scope under load.

As a comparison, look at a regular megasquirt. Each channel people can (and often do) hang four 1.2 ohm injectors in parallel. Thats 0.3 ohms equivalent. And often people forget to turn on PWM current limit or set it incorrectly. If there is no PWM, then 12 volts thru 0.3 ohms is 40 amps!... per channel! I have personally witnessed this setup more than once. And this is thru the (perceived) crummy DB connector shared with two pins. And thru a groundplane distance of 4 inches because of the drivers on the top of the case having to route to the side connector position.....

Peek at the layout of the sequencer. The drivers are at the far right, necked up right against the ampseal connector (which has monstrous pins). The groundplane distance is less than an inch worst-case and there are multiple ground points.

This is why, way in the beginning, I indicated to use lots of parallel ground connections, and to only connect up the right-hand connector grounds.

- Bruce