[Diy_efi] Re: P/H hammer driver circuits

[Marcell Gal]

Hi, 

Btw I uploaded a revised circuit schematic to
http://x-dsl.hu/genboard/injector/injdrive.pdf
for your pleasure.
See the injdrive.txt at the end of this mail. 

Greg Hermann writes:
> on it a little bit. Maybe hold voltage choices of .75, 1.0, 1.5, 2, 3, and
> 8 volts ??

It seems possible, but why would you do that?
Can you not maintain a reasonably steady current with
pulsing the injector voltage and using a flyback schottky?
Remember that nobody cares injector voltage. Only current.
If you build a pwm supply to maintain the voltages above
you waste components (therefore decrease reliability).
If you go analog, you waste power (and decrease reliability),
grow cooling requirements. The only advantage I can think of
is less electromagnetic contamination. (but you can put your
driver FET close to the injector as well) 

> Don't forget that a circuit to dissipate the fly-back voltage--energy as
> QUICKLY as possible (and also safely) when the driver is turned off is
> another critical design element in maximizing injector response and dynamic
> range.

why?
I thought that requirements are
 - monotonously growing injected quantity with growing pulsewidth
 - function predictibility (pulsewidth, FuelPressure, Ubattery, Temp ...) 

If the current decay is slow (because of using a low voltage
schottky diode), you have to switch off the master pulse earlier.
Remember that linear (pulsewidth -> injected quantity)
is out of question at small injection rates anyway, because at
injector opening it seems to have a 2nd order flow-buildup. Look at
http://www.diy-efi.org/efi332/equations/algorith.htm
I think a few lines of extra calculation does not hurt. 

I have a related question:
does it significantly help us if we first use a bigger injector
current for opening the injector, than fall back
to a lower hold current? This can result in faster opening
OR lower dissipation, but if we use efficient pwm limiting anyway,
this does not seem that big of a deal.
However, to make everyone happy, I added the peak and
hold function. Those that don't need it will leave off the
4 simple components. 

 Marcell 

 -------------- injdrive.txt ---------- 

This is an efficient current limiting
peak and hold injector driver
from simple components. You do not have to chase fancy
components that are hard to buy in small quantities.
Works for low or high impedance injectors. 

Having 4 or 8 pieces of this on a PCB is preferred. 

http://x-dsl.hu/genboard/injector/injdrive.pdf
(the geda .sch and .ps is also there, this
helps if you're designing a PCB. Please
drop me a mail if so.) 

====== Circuit operation
The circuit works by masking the master injection signal
(look at Qfeedback and U1)
when injector current exceeds a threshold
(measured on Rcurrent). It has a histeresis
comparator. The histeresis can be set with Rhisteresis.
(200k is the minimum, 470k seems better)
The frequency will not be very stable, but it's not a
problem. We want specified current, not specified frequency
after all. 

The limiting current is basically wired in by Rcurrent (so
that voltage drop is ~0.6V), you can change
that to match your desired injector current. 

Peak current is determined by R11, R12, C13 and Z14.
It works by charging C13 to 5V (or 7V depending on
S15 which is not a real switch but wired according to user taste)
before master pulse becomes active. Therefore a higher
injector current will be maintained for about a
millisecond, before voltage of C13 decays (exponentially).
The Z14 zener makes a voltage offset, so that given
voltage drop on R11's 'input' pin (appr. 5V) is relatively more significant.
Note that dropping injector current to about 70% in hold mode will
result in 49% power dissipation. 

Q2, Q3 and Q4 is a simple 2 stage bipolar driving for the
power FET. Nothing fancy. C3 was put in after Alexei Pavlov
pointed to a motorola appnote. It helps when Q3 and Q4 bases
go down, but worsens when they go up. (so don't make its capacitance much 
higher) 

Rlimitn and Rlimitp be small, like 47..100 Ohm. 

That's all, simple, isn't it?
====== End of Circuit operation 

Some concerns:
This circuit relieves the ECU from high freq switching
of injector signals (eg. from software) for current-limiting
which is nice if you drive many sequential
injectors and your particular uC does not have enough hardware PWM outputs. 

The ECU can have diagnosis data about proper (or electrically failed)
injector operation by looking at one of the digital signals,
eg. the gate of Q1 or the masking input of U1 which is the output
of Qfeedback. If it's not flipping back and forth within a freq limits
after master injector pulse is applied (and longer than a minimum value)
than something is wrong (too high freq if injector is short-circuited.
No masking if injector is not connected at all. Other problem
if mask freq is way too low). No need to monitor it continuously,
just once in a while.. 

This driver relies on a Dschottky diode to dissipate the energy of
the injector at switchoff, so switchoff isn't particularly fast.
However if you want less flow, use a shorter master pulse. Simple.
The turnoff is deterministic. 

Do you have comments on the circuit? 

thanx:
Marcell
cell at x-dsl.hu 

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