Re: Use low ESR caps in G08 ?

Christopher X. Candreva wrote:

>On Sun, 8 Feb 2004, Rodger Boots wrote:
>
>
>
>>The only reason to NOT use low ESR capacitors is cost. The only reason
>>NOT to use 105 degree capacitors is cost.
>>
>>
>
>OK. I know I've asked the low ESR question before -- found my own question
>in the archive, but I also keep getting bit by something that is supposedly
>'better' not being correct for a particular application.
>
>So, there isn't a chance of a circuit depending on the resistance of the
>cap, and not working correctly with an ESR one ?
>
>

If an engineer needs series resistance he/she will use a resistor. You
can't rely on the internal resistance of a capacitor for several reasons:

    1) It isn't well defined.

    2) If you design for a certain manufacturer the parts buyer will
manage to find another cheaper part and switch it on you without
consulting you.

    3) Changes with temperature.

    4) Changes over the lifetime of the part.

In them old days we didn't have low ESR capacitors (not advertised as
such) so much until the advent of switching regulator power supplies.
Then low ESR became important to allow for low output ripple voltage
without using extra capacitance.

And over the years the size of the capacitors kept shrinking which
reduced the ability to radiate heat. So the parts ran hotter, life went
down. The ability to not handle ripple current is why Wells-Gardner
vector HV supplies keep blowing those capacitors on the incoming power.

Funny story (in a real sad way---demonstrates parts buyers ignorance of
electronics), I once needed a replacement for Big Blue in an Atari power
supply. Wico had them listed, good price, too. So I ordered one and
they sent me a tiny PC board style capacitor. Right capacitance, right
voltage, not even close to handling ripple current. But nobody thinks
of handling ripple current (but they should), never mind there was no
way to mount something like that.

I still say use the best available part. For one thing it will probably
be rated for a higher ripple current which means longer life. For
another thing you probably won't have to replace it again, which is a
good thing because game PC boards don't hold up well to changing parts
often. AND DON'T USE TANTALUM CAPACITORS UNLESS SPECIFICALLY CALLED
FOR, they have a tendency to catch fire and burn up boards unless used
properly. They are great for timing circuits but just aren't the best
for high power use. And before someone starts arguing that just keep in
mind I can produce the scorched board test fixtures from my Real Job to
prove how nasty they can get.

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Received on Mon Feb 9 01:15:18 2004