ecu voltage problem

[Webb]

S. Lastuka wrote:
> 
> We are using a 7805 1 amp voltage regulator.  We also tried a 1000 uF cap
> between 12 volts and ground to no avail.  I am going to run over to the
> ECU and analzye the ground situation since we had the computer
> running the engine on a stand but as soon as it was in the car it
> crapped out.  Other information: the regulator seems to put out 5 Volts
> down to a 7-8 volt input.  The cmos in the 68hc11 will work down to 4 1/2
> volts with little problem and can probably drop a bit below that
> sometimes.
> Sean
> On Wed, 30 Apr 1997, Tom Cloud wrote:
> 
> > >I am an FSAE student and we built an EFI computer for the competition.
> > >There seems to be a problem running it off the car's power supply, ie the
> > >battery.  When it is hooked up to the battery the computer holds pulse
> > >widths at around 1 ms per revoltution regardless of throttle position.
> > >When we hook it up to the standard voltage regulator used for testing in
> > >the lab the car works perfectly with full rpm and throttle range, but
> > >can't move because it is attached to an extension cord.  We're using a
> > >68hc11 processor.  If anyone has had any weird voltage problems like this
> > >any help would be appreciated.  Thank you,
> >
> > What kind of regulation circuitry are you using to drop
> > the voltage from the automotive 12 volts to your circuit.
> > Keep in mind that your regulator needs to be able to deal
> > with voltages as low as 8 or 9 volts (cranking) to over
> > 15 volts, with voltage spikes maybe as high as several
> > hundred.  A resistor/zener or varistor input could squelch
> > the spikes (lo-R so you don't lose voltage from current
> > drain of circuit).  Put this before your regulator.
> > In-line inductors (chokes) work also, but they can
> > get big.  Also consider you may have some sort of
> > ground loop going on.
> >
> > Tom Cloud <cloud at peaches.ph.utexas.edu>
> >
The 7805 wants a small (~.1uF) cap on the 5V output side as well. This
is to get rid of high frequency transients, so a big electrolytic won't
work on the output. 

The big cap on the input will not do much. The electrical model for the
auto battery contains the worlds largest cap (farads). Another 10,000 uF
is insignificant.

If all the power you are using is 5V from the 7805, you might consider a
power resistor on the input side of the 7805. The idea is to resistively
isolate the circuit. Size the resistor to drop 4 volts or so, at the max
current the ecu will draw (this also keeps the 7805 cooler).

I'd suggest the following circuit. 

                     ______
+12V>------R1----|---|     |---|--->5V out
                 |   |7805 |   |
                 C1  |_____|   C2
                 |             |
                 Grnd          Grnd
         
    
R1=(4V desired drop)/(100mA - max current draw)
C1=10,000 Electrolytic, in parallel with a .1uf ceramic
C2=.1uf ceramic

I used to install radios in road graders. Sometimes nothing works, but
this approach has worked for me before. Be sure the metal box the ECU is
in is RF tight, and all lines going into the box use the little studs
that have caps to ground built in.

Good luck.
RTW