In a message dated 6/15/2006 10:49:30 AM Central Standard Time,
tr3driver@comcast.net writes:
> "Most people" not including Dan Masters, I guess. Compared to the constant
> under-engineering our cars had originally, it's kind of refreshing to work
> with
> an over-engineered kit !
Dan is a retired controls engineer who worked for TVA in their nuclear power
plants. It's natural for him to over-engineer things.
>
> > But if you are using a
> >relay then
> >indeed a diode will solve that sneak circuit.
>
> A diode will also work for the pump, just takes a bigger diode. Still
> cheap.
Depends. If the pump has an onboard power supply bypass capacitor then no
problem. But they may cut a few pennies counting on a more intimate connection
to the battery and isolating it with a diode may cause additional stress on
the electronics leading to premature failure. Not having cut one open and
expecting different pumps from different manufacturers to vary I'd err on the
side
of caution and hang a capacitor across the power leads if I isolated it with a
diode.
>
> > But then you should put in an
> >MOV across the relay coil (or the diode) to prevent the inductive kick back
> >from over stressing the diode.
>
> A MOV (metal oxide varistor) seems both unreliable and overkill, IMO.
> Easier,
> cheaper, better, IMO, to just use an oversize diode that can take the
> stress.
> For the relay, a 1 amp, 50 volt diode should be plenty.
What's unreliable about an MOV? No less reliable than the diode. Much more
so than the diode if you don't have some sort of spike protection. Here's
what happens: when the oil pressure switch opens up the energy stored in the
magnetic field in the coil of the Anti-Run-On valve will cause the voltage at
the
switch to drive negative in an effort to maintain the current level. Normally
the voltage will rise high enough to jump the OP switch contact gap and the
energy will be dissipated in the arc. But add in a diode and an alternate
current path the voltage will rise until the reverse withstand voltage of the
diode is exceeded and it avalanches. The problem with this is with very fast
rise
time situations, which this will be, the current doesn't spread across the
whole cross section of the diode and will concentrate about the point where the
bond wire is attached and the silicone at that point will become over stressed
and convert to carbon (not really but it will fail and usually as a short).
I've been burned and burned bad (metaphorically speaking) for not using
MOV's. If you don't like MOV's you should at least tie a .1 ceramic capacitor
across the diode. This will slow down the rise time and let it avalanche at a
slower rate.
>
> For the pump, you could go really overboard with 25 amps, 200 volts, and
> still pay only a dollar or two
> for the diode. If you really want to be cheap, rob a diode from an old
> alternator <G>
For some reason this brings to mind the picture I saw of Bob Pease using a
Volkswagen cylinder head as a transistor heat sink. I don't know why.
Dave
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