[Diy_efi] LM1815 problem

[Garfield Willis]

On Mon, 8 Jul 2002 14:35:34 +0200, Arnoud van der Wel
<A.P.vanderWel at utwente.nl> wrote:

>For a 1987 Moto Guzzi V75/4

Great classic, I envy you.

>In particular, the LM 1815 is not really doing what it should be. I use =
it
>in the adaptive mode (pin 5 open), and it is (mostly) working fine. =
However,
>with input noise, I get short pulses at the output.

Question, is this mostly at higher rpm where the vrs amplitude gets
fairly high? Tell ya why below.

>I can understand that if I produce a trigger condition at the input, =
that
>the one-shot triggers and produces an output pulse (this would be my own
>fault), but this is input noise that apparently is *not* sufficient to
>trigger the one-shot but *still* generates some junk (pulses far shorter
>than the programmed one-shot duration) on the output. I don't like this:=
 as
>long as I don't trigger the one-shot, I feel there should be *no* pulses=
 at
>the output.=20
>...
>Does anyone have similar experience using the LM1815? solutions? better
>workarounds? Do they all do this?=20

These are yeoman parts. We use them to good success in a couple of our
products. Unless you have a bad lot with horrid leakage onchip, I
wouldn't suspect the part.

Suggestion: put a pullup of 10k or so on pin 12 and attach scope. Look
for runt pulses there. That will tell you if they're coming from the OS.
Make absolutely sure you have a good ground on pin 11. I would hardwire
a jumper directly from pin 11 to pin 2 right smack on the package just
to eliminate any chance you have noise on pin 11.

Lastly, this is infact a fairly old design; the output structures are a
bit odd. Mind the 1k load R to ground on pin 10 in Fig. 1. It's there
for a reason. Pin 10 is NOT a totem-pole output. Also, you say you have
10k inline to pin 3 (input); is that because you have weaker than
expected sensor output levels at low rpm? Otherwise, their recommended
typical value is 18k. Doubt if that is your problem, but it's wise to
realize that at higher rpms, VRSs put out hundred + volt spikes. The
resistor there is to whack those down to safe size. You should select
the input series R to be as high a value as will give you reliable
output from the 1815 at cranking rpm.

Do try swapping out the part with one from another lot, if all else
fails.

HTH,
Gar Willis
Principal Engineer
EGOR Techno
3491 Edison Way
Menlo Park, CA 94025
650-216-9874
garwillis at msn.com (e-mail & PayPal transfers)
www.egortech.com (best viewed with IE)


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