"Rick Frazier" bravely wrote to "All" (14 Oct 05 07:08:21)
--- on the heady topic of "High Voltage Caps for Plate Supply ??"

RF From: Rick Frazier
RF Xref: core-easynews rec.radio.amateur.homebrew:88241

RF I have an Ameritron AL-1200 amplifier, and it came (used) with a
RF capacitor problem.
[,,,]
RF I was in high tech engineering (computers and peripherals) for 20
RF years, and we never ran electrolytics at anywhere near their rated
RF voltage. Permissible margins for low voltage DC circuits were 50% or
RF more margin. Typical rule of thumb was 100% margin (a 10 volt cap
RF running on nominal 5 volt line, etc.). Even in an industry where every
RF penny of component cost was significant, submitting a design with
RF electrolytics running with as little 25% margin typically got a less
RF than stellar response during design reviews, to say the least.
[,,,]
RF Thanks
RF --Rick AH7H


Rick, it is okay to run electro's at about 80% of their voltage spec.
Electro's are formed at up to 140% of their final spec and can thus
tolerate turn on surges. It is the turn-on surge spec that one should
worry about because that is when the electro is vulnerable. At turn on
an electro re-forms a little bit which draws a larger current than
usual. If that current doesn't fall off rapidly enough the electro can
become damaged and leak permanently. In the old days electros used to
be stamped with both surge and working voltage ratings.

Isn't using an electro at 50% of spec rather conserative? I might
understand being this prudent with semiconductors. An electro also has
a leakage current spec at the rated working voltage. There isn't a
direct linear relation between voltage and leakage. The relation is
more like a reverse exponential and the leakage current drops off a
lot more at a lower percent of rated voltage.

A*s*i*m*o*v

.... Paul's Law: You can't fall off the floor.