Calling all EE's - Check my logic

[3D914]

OK before I possibly fry my injectors, I wanted to have someone check my thinking here. I've wired up my Megasquirt injection system and ready to do the pre-start testing.

I have a 10-watt/6-ohm resistors inline with each of the injectors power connection. This is done since we have low-voltage injectors (3.5 v), and Megasquirt is a 12v system.
As a preliminary check, I was expecting to see less than 12 volts after the resistor. But it was 12v. Am I missing something here?

If I do a current test, I do need to load the line I'm testing - correct?

I also need to double-check the resistence values to see if they're within tolerance.
Checked: All resistors are within tol - reading 6-ohms.

Sorry I don't speak fluent electronicease - I'm "eletronically challenged!"

[mike95125]

To measure current through the system you do need to have everything hooked up othrewise you will just be measuring the current throught the resistor. Thus, you would need to break the circuit and insert the meter in series (in line).
As for total resistance, the word is impedance (Z). It is a lot like regular resistance, but it varies with the applied current/voltage.
Easy stuff: You have 12V with (X)amps coming to the injector. You want 3.5V @ (Y)amps max. You may need to find out what the maximum impedance is on the injector.
- You want to drop 8.5V before the injector
- You need to know how much current (amps) are flowing through the system.
- Current is constant in a circuit so here comes the math;

The value of the resistor you need to use=8.5V/current (amps) flowing through circuit.

If I recall correctly: Injector impedance is 3ohms, and you have a 6ohm resistor, so 9 ohms total( I am guestimating here).
Current=12V/9 ohms, so with the resistance that you are using you would expect a current of 1.33amps (quite a lot for most resistors! Any bad smells?)
Power (watts) = current * Volts, in your case if these values were correct you would need a 20watt resistor because you would be using almost 16W.
Just a couple of things to consider.
HTH
(hope that I was clear and correct)
-Mike

[Dave at Pelican Parts]

It was confusing as heck, that's what it was!!

I think the first reply addressed the question more directly. You need the circuit hooked up to measure the voltage drop across the various bits.

--DD

[mike95125]

Yay, Dave....
You have now figured out what the individual drops are in the circuit (I hope that they are right!). But, what about the required power ratings, and values of resistance required to drop the prescribed voltage. If the drops are what you want then you still have the problem of over loading the resistor with too much power. Then your measurement will be 12v across the resistor, and 0 across the injector as you have effectivly burnt out your resistor.
3D914- if you need clarification feel free to PM me, I would be glad to help explain.

Dave, your evaluation and your great contribution has been noted....

[JohnB]

How many EE's does it take to....

Yes, I am an EE also. Nice work, Mike.

[seventythree]

OK guys, I am afraid it is not that easy. You may want to double check with Brad Anders, but last time I checked, the stock ECU uses an over-drive scheme to speed up the actuation of the injector solenoid. In other words, 12V is applied initially to snap the solenoid open. Then, the voltage is taken to hold level of ~3.5V for the remainder of the duty cycle. For more detail, either dust off you oscilloscope, or consult someone with fresher or better memory.

[mike95125]

914GT, really? I am not infallible. I figured that the pulsating DC would generate the necessary contracting and expanding flux to generate the impedance(?). Perhaps I was over thinking, but there is an internal resistance of the coil used in the injector that needs to be considered (unless it is negligible). As well, I also figured that the tempo of the duty cycle for the injector would warrant the use of a higher wattage rating. But, I am definetly not trying to pontificate here, and know enough to know that I am just educatedly guessing (Read, I have measured nothing).
For the capacitor idea, that sounds like it would work provided they did the scoping. The good thing about the MS system is they would be able to add this cap in the box and may only need two since there are only two freq. generators. = much more calc.

JohnB - thanks for the Props.

Im not an EE, just a AVT with some years of experience
Hope that any/all of this is helping (not just tying up band width)
-M2