When the cap gets too big...

[Magnus Danielson]

From: Bill Layer <blayer at uswest.net>
Subject: When the cap gets too big...
Date: Wed, 08 Dec 1999 07:33:00 -0600

> 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?

If I recall things correctly did some larger (classic type of powersupply
based) power amplifiers in the 80thies have a resistance and delayed relay
in order to bootstrap the caps with a series resistor and after some time
the resistor was shortened by the relay and you would see the full line
current (and thus voltage) available and the caps would load up from the
intermediate level. This method reduces the current-spike as you switches
the amp on.

Actually, basically the same type of mechanism exists in electromechanical
motors which you also need to bootstrap in a similar fashion before the
rotation energy (analog to the capacitors stored charge) high enougth.

Cheers,
Magnus (In the land of the Julbord (Christmas time version of smörgåsbord))