DIY_EFI Digest V5 #566

[Bruce Bowling]

> ----------------------------------------------------------------------
>
> Date: Thu, 05 Apr 2001 14:04:52 -0600
> From: Ken Bauman <303print at northrim.net>
> Subject: Interesting P&H Driver Link and a MAP Question
>
> A link I came across for making your own Peak and Hold drivers.
>
> http://www.ednmag.com/ednmag/reg/1995/090195/graph/18di5fg1.htm
>

This circuit controls average current using a PWM signal, with the duty cycle
yielding an average current, as explained in the text. But, it does not measure the
actual current in the power transistor circuit - it has a fixed

You can do this function directly from a microprocessor port which has PWM capability
- make the initial PWM duty cycle 100%, and after so many milliseconds (like 2 - 3),
switch the PWM duty to something like 25% or so to invoke the current limit. The
actual number for the duty cycle can be computed from the injector impedance, PWM
frequency, and transistor saturation voltage drop (or Rds for a MOSFET), or better
yet measure the current during current-limit mode and adjust the duty cycle to
achieve the desired holding current - if the micro has a spare ADC channel then you
are golden. Simply hook up a sense resistor in series (like 0.1 ohms)  with the drive
portion Emitter circuit), measure the current across the resistor, and set up a
feedback loop. Done.


--
- Bruce

---------------------------------------------
             Bruce A. Bowling
         bbowling at earthlink.net
   http://home.earthlink.net/~bbowling
---------------------------------------------


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