[Heathkit] SB-601 capacitor voltage ratings

[Mike McCarty]

Dave Cook wrote:
> Hi guys, I'm recapping an SB-610 and am a little confused about the voltage
> rating of C600, C601 - the 40 uf'ers in the 600V supply. There's no voltage
> rating in the parts list, and the caps themselves show 350V, but looking at

That looks fine to me. As already commented by others, that's a
(full wave) voltage doubler. I'd add a 680K ohm 1/2 W resistor in
parallel with each of them. Otherwise, you are relying on
"matching" the leakage current through them to keep them under voltage.

I'd use 1/2 W not because of dissipation, but because of voltage
stand of issues.

Here's how I came up with the 680K value.

First, I calculated a "reference" current as

Iref = sqrt(C*V)

with

Iref = reference current in uA

C = rated capacitance in uF

V = WVDC of the cap

So, I used sqrt(30*350) = 102 uA

Now, for brand new caps, we can expect that

Iref/5 <= Ilkg <= Iref/2

I used the upper limit, and get the expected leakage
to be less than approximately 51uA. I multiplied this
by 10 to allow for "swamping", even as the caps age,
and get 512uA.

Now, 350V/512uA = 684K ohms. I used the next lower
standard value.

The power dissipation must be considered, and in this
case is

	300*300/680K = 0.132 W

Doubling, and preferably tripling, this, gives us the necessary
rating for power. That would be a 1/4 W resistor acceptable.
However, a 1/4 W resistor shouldn't have 300V on it. Hence, the
1/2 W resistor.

The power ratings on resistors are based on no nearby heat
radiating components, in calm room temperature air, with
free convection. For inside a cabinet, one needs to derate
by a factor of 2, and preferably a factor of 3.

HTH

Mike
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