"Phil Allison" wrote in
:



Our buddy Phill composed, in an unusually non-berating way today
thusly :

"Large electros kill spikes just like sledgehammers kill flies. "

And they got better aim. As long as you have full wave rectification,
and the rectifiers can handle alot of peak current, they are quite
effective. However they are not effective so much against having
simple overvoltage applied.

Thanks for the correction. I mistyped, omitting "and surges" as you may
have guessed by my reference to surges later on. However I do wonder
whether the self-inductance of the typical wound foil construction high
voltage electrolytic is enough to cause problems with very short
impulses if one omits the varistors.

Littelfuse's Varistor page (which contains more than yoever wanted to
know about them):
http://www.littelfuse.com/products/varistors.aspx
states:


*** Speed of Response and Rate Effects ***

The varistor action depends on a conduction mechanism similar to that
of other semiconductor devices. For this reason, conduction occurs
very rapidly, with no apparent time lag – even into the nanosecond
(ns) range. Figure 18, shows a composite photograph of two voltage
traces with and without a varistor inserted in a very low inductance
impulse generator. The second trace (which is not synchronized with
the first, but merely superimposed on the oscilloscope screen) shows
that the voltage clamping effect of the varistor occurs in less than
1.0 ns.

It does qualify that by stating this speed is not achivable in a leaded
package, but I'd bet its still far far better than the electrolytics
capability to deal with fast risetime transients.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL