15 May
2007
15 May
'07
4:53 a.m.
From: "Chuck Guzis" <cclist at
sydex.com>
Would it perhaps be better to insert a series resistance with the
mains supply rather than a variac? This would allow full line
voltage to be present at the input of the PSU, yet provide a bit of
protection should something start to draw a too-large amount of
current.
I'll usually insert an appropriately-sized incandescent lamp in
series with the line when I'm powering up an older piece of equipment
in unknown condition.
Cheers,
Chuck
Hi
It is almost impossible to reform properly with a variac alone.
To do it correctly one needs to isolate the capacitor and put
a serious current limiting resistor in series. The trick is to
limit the current to a value that the forming will not outgas
faster than the normal diffusion that the seal can handle. Using
something like a 70W lamp would be way to fast. Most times,
a large capacitor will need to be formed with a current limit
on the order of 5 to 15 ma.
Even then, I recommend limiting the voltage with a variac
and watching the current by measuring the voltage across
the resistor. Went the current stabilizes at a particular voltage,
I turn the voltage up.
As for burning up a variac, they can handle a high current
load long enough to blow a fuse, even a dead short.
For 25 to 50 volt caps, I recommend starting with the voltage
limited to about 5 volts, for the first part of the cycle.
If it forms too fast, the seal will burst and the cap will
soon dry out.
All that being said, I've just turned on powersupplies that
have been off for upto 5 years without worry. I've had a
few tantalums pop but these go with or without forming.
Dwight
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