Measuring low frequency average voltage

I have a question about measuring the average voltage being delivered to a circuit. Basically what I want to do is tap into the line coming from the ECU to the fuel injectors in my car. The ECU sends a pulse signal to the injector to signal the injector to open and start spraying fuel into the intake. I figure that if i can measure the average voltage on this line then i can know what percent of the time the injector is open. then knowing the fuel delievery rate of my injector and my speed i should be able to calculate an instantaneous gas milage value. Eventually i'm going to do this digitally with A/D's and a LCD output so that i can monitor a number of variables. But right now i want to do it all analog. That it's I'd just like to have a (analog) voltmeter that reads a voltage that is proportional to the average voltage being sent to the injector. I figure that for high frequencies that any typical analog voltmeter is going to read the average voltage, but what happens at idle when the frequency is going to be something like 4-8Hz? does a typical analog voltmeter average over that range too or does it start to look DC at 8Hz?

(for the car experts out there: i can't use OBD-II and get this info, as my car is an '89 and predates OBD-II by many years).

if anyone has any ideas that would make my life easier, i'd appreciate it!

mike

Reply to
Mike Deskevich
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A/D's

What you possibly want to do is to integrate the pwm signal going to the injectors into a dc voltage. You can do this with a resistor in series with a capacitor to ground. The other end of the resistor connects to your injector signal. Start with about a 10k resistor and a .47uF to

1uF cap. At the junction of the resistor and cap you will have a fluctuating DC voltage. By playing with the cap values you will affect the response time of the derived DC voltage, a scope will let you see the ripple. If you need to draw any current from this junction, then use an op-amp with the gain set to 1 to boost the available current.

There are two ways to calculate the optimal values for your cap:

1) Use a bunch of math and factor in a ton of variables you probably don't know yet 2) Tinker with it like a "real" engineer and watch it on a o-scope or DMM till you like what you're seeing

Guess which way I would do it. ;-)

Depending upon the accuracy required, the "more correct" way to do this may be to measure the number and width of pulses over a known time interval; PIC chips come to mind. You should also factor in the instantaneous voltage applied to the injectors since the ECU uses that when calculating the pulse width to use. The injectors will open faster at higher voltages letting in slightly more fuel for a given pulse width and fuel pressure. IOW, just knowing the average voltage may not be accurate enough for you. OTOH, it may be more than accurate enough.

Good luck, sounds like lots of fun. YMMV

Reply to
Anthony Fremont

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