When the cap gets too big...

[Harry Bissell]

Sure. The other problem is when the rectifier diodes get fried by too much
inrush current. There are three good things to do...
1) Put a small value resistor in series with the diodes... even an ohm or two
will limit currents from near infinity to practical values...
2) Consider the Choke input filter... the series inductance limits the rate of
rise of current to a step function...
3) Add an active "soft-start" circuit, which is easily made from a Power MOS
transistor, a resistor and a small cap. The transistor is initially "off" and
slowly turns on as the voltage at the gate (limited by R-C constant of your
choice) rises...  Other soft start circuits might consist of series resistors
paralleled with relay contacts... put some time delay before the relay closes
and you have it...

IMHO I'd use series resistance. This will also make the RC time constant of the
filter caps a little bit longer so they may actually get more effective. Hell
we're talking linear regulators so who cares about just a little more heat ???

:^) Harry

Bill Layer wrote:

> Hi,
>
> Saw the thread about cap size and needed to throw in my two cents. Big
> isn't always more when dealing with cap size. As a cap passes it's maximum
> effective size, another issue comes up, that of xfmr saturation on the
> power cycle.
>
> As I know it, there comes a point where the cap is so large (and so low in
> resistance compared to the Zsec of the xfmr) that the turn-on point for the
> rectifier causes such a heavy current inrush that the transformer core
> saturates trying to push against the brick wall (low as it may be :) that
> the too-large cap presents. I don't need to expound on the non-plus nature
> of core saturation, or the noise and other issues it introduces.
>
> Anyone have experience with this?
>
> +----------------------------------------------------------+
> |      "The" Bill Layer - Frogtown, Minnesota. U.S.A.      |
> | Vacuum tubes, Analog, Motorcycles and Other Alternatives |
> +----------------------------------------------------------+
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> | <blayer at uswest.net> |  | <b.layer at vikingelectronics.com> |
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