[Milsurplus] AN/GRR-5 Power Supply Questions

[WF2U]

ABSOLUTELY not true! The voltage rating of electrolytic capacitors, just
like that of any other capacitor indicates the insulation level between the
two electrodes which form the capacitance. The capacitance has nothing to do
with the voltage rating. "Forming" an electrolytic capacitor also has
nothing to do with "attaining their full capacitance". It is intended to
prevent the breakdown of the /dielectric insulation (the composition of
which in an electrolytic capacitor is an electrolyte)
when an old capacitor is connected to a voltage approaching the original
rating, due to chemical changes in the electrolyte if the capacitor is not
exposed to a voltage for a long time.
In other words, capacitor forming simply is a way to prevent an old, unused
for a long time electrolytic capacitor from blowing up when voltage is
applied for the first time.
BTW even in a formed or used and aging capacitors the electrolyte may break
down and fail.
And lastly, capacitance in ANY capacitor is a function of the area of the
electrodes and the distance between them - it's constant!


73, Meir WF2U
Landrum, SC

> -----Original Message-----
> From: milsurplus-bounces at mailman.qth.net
> [mailto:milsurplus-bounces at mailman.qth.net]On Behalf Of Neil Barnett
> Sent: Monday, August 21, 2006 7:43 AM
> To: milsurplus
> Subject: Re: [Milsurplus] AN/GRR-5 Power Supply Questions
>
>
> >    4. C101, C110 and C111 are 1,200 mfd/15 WVDC caps used as filters.
> > Can I use 2,200 mfd/50 WVDC? How about 1,000 mfd/50 WVDC?
>
> I presume these are electrolytics, in a 12-13.8 VDC system.
> Unlike ceramic and polyester capacitors, where it is permissible
> (and even desirable) to use units of high voltage rating so as to
> avoid breakdown of the dielectric, electrolytic capacitors must
> not be run at less than about two-thirds of their rated working
> voltage.
>
> If you attempt to use 50 WVDC electrolytics in a 12-13.8 VDC
> system, they will fail to attain their full rated capacitance during
> operation, thus reducing their effectiveness as filter elements.
> Additionally, I'm sure research has shown that such operation
> leads in the long term to the total failure to form any capacitance,
> i.e. a cumulative deterioration in the ability to "form" the correct
> dielectric.
>
> My advice is to forget the 50 WVDC units.  Try to find units of
> the correct capacitance in the now-standard 16V rating (should
> not be too difficult!).  Use more capacitance, rather than less, if
> you can't get an exact match, for example 1,500 mfd/16 WVDC.
>
> 73 de Neil ZL1ANM
>