> Depends. If the pump has an onboard power supply bypass
> capacitor then no
> problem.
Or is designed to withstand whatever transients it generates.
> 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.
Could happen, I suppose. But given that the pump is apt to be on the end of
some rather thin wires run the length of the car, tied in with who knows
what other junk and powered through an overloaded switch, it seems unlikely
to me that they are relying on the input to be low impedance. Batteries
have pretty lousy high frequency impedance anyway.
> I'd err on the side
> of caution and hang a capacitor across the power leads if I
> isolated it with a diode.
The "belt and suspenders" approach. Certainly won't hurt.
> What's unreliable about an MOV?
Pick up any "MOV protected" power strip. Odds are very good that at least
one of the MOVs are non-functional. I've seen estimates as high as 90% "in
the real world". They aren't entirely "one-shot" devices, but they come
pretty close. If you dig through the spec sheet, you'll find that the
stress that they will take more than 1000 times is many orders of magnitude
less than the transients they are supposed to protect against. Even the
"1000 lifetime" stress is many orders of magnitude lower than the advertised
protection.
> No less reliable than the diode.
Even so, having them both in the circuit doubles the chance of failure. And
the diode is required.
> 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.
Only if the diode reverse voltage is lower than the switch contact gap at
that instant in time. This is why you need a diode with a voltage higher
than the nominal supply voltage, so it doesn't break down when the switch
opens. 50v might not be enough, but I think it is. Don't forget that the
switch contacts move very slowly, meaning they are just barely cracked at
this point ... doesn't take much voltage to jump them.
If not, $.70 at Radio Shack will buy you a pair of 200 piv diodes rated for
30 amps surge. Still not enough ? How about $.60 each (pack of 3) for 1000
piv ? And you still haven't spent the price of a MOV (assuming you can even
find one, Radio Shack doesn't carry low-voltage MOVs).
> 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.
I suppose it's a mindset thing ... some people see junk, others see
possibilities. Big chunk of aluminum with fins ... sounds like a great heat
sink to me! And I've been using a battery charger for many years now that
was given to me with a blown diode pack ... works just fine with alternator
diodes.
I suspect we lost all the non-EE types on the list long ago, so I'll shut up
now. Time for lunch anyway.
Randall
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