Ned Seith wrote:
>=20
> Roman,
>=20
> I had the good fortune, in the early 1990's, of consulting with Ralph
> Kerrigan, the Chief Capacitor Design Engineer with Sprague Electric due=
 to
> the quantity of capacitors that I was ordering. I will share a few thin=
gs
> that I learned from Mr. Kerrigan. At that time, the high voltage capaci=
tors
> manufactured by Sprague were available with either a "foil" conductive
> material or with a "metalized film" conductive material. Mr. Kerrigan
> indicated that the "foil" conductive material had a superior reliabilit=
y
> for handling "higher currents" while the "metalized film" conductive
> material had a superior reliability for "voltage transients". The
> "metalized film" also had a "self healing" characteristic allowing for
> recovery from minor transient voltage damage. At that time, the Sprague
> high voltage capacitors were also available with a "polyester" film
> dielectric material,
> a "polypropylene" film dielectric material or a "polycarbonate" film
> dielectric material. The "polypropylene" film was superior to the
> "polyester" film and cost 150% more than the "polyester" film.. The
> "polycarbonate" film was superior to the "polypropylene" film and cost =
650%
> more than the "polyester" film. After weighing all the pros and cons, I
> decided to go with the "metalized film" conductive material with the
> "polypropylene" film dielectric material. This combination was availabl=
e
> off the shelf in voltage ratings from 400V to 2000V in standard capacit=
ance
> values. However, I had to special order because my specification of 1.6=
8uF
> was not a standard value. I could have used an off the shelf  1uF and a
> 0.68uF combination, however, that would have doubled the cost and the
> physical space required. The body of these capacitors was approximately=
 1"
> in diameter and 3" in length. These capacitors had the typical oval
> geometry, typical white body and typical yellow outer wrap. In quantiti=
es
> of 10,000 the capacitors were $3.90. These capacitors were used to repl=
ace
> malfunctioning solid state controllers that provided 120 Vac to "Exit
> Lamps" from a 277 Vac main in all the international airports throughout=
 the
> free world! Prior to this project, I didn't know that the typical
> international airport has 2,000 "Exit Lamps".
> The capacitors that you have are probably "foil" and "polyester" film w=
hich
> are fine for non critical applications. You should be able to purchase
> small quantities of "metalized" "polypropylene" film capacitors from a
> capacitor manufacturer's distributor and might even acquire "free"
> engineering samples!
> It should be noted that a 1,000 V "foil" and "polyester" film capacitor
> will provide a similar level of reliability as a 630 V "metalized" film=
 and
> "polypropylene" film capacitor. This is why I emphasized raising the
> voltage ratings of the capacitors. The most important thing is that the
> voltage rating is more than adequate and that the capacitor is not
> generating heat. The capacitors surface temperature should never be all=
owed
> to reach 75=BAC. The "Exit Lamp" capacitors while enclosed in the lamp
> fixtures experienced a maximum  increase in surface temperature of 3=BA=
C
> after 72 hours of continuous operation.
> I eventually upgraded the "Exit Lamp" design with SIDACs (SYDACs) that =
were
> manufactured by RCA and Motorola at that time. The SIDAC is a very
> interesting thyristor that has many useful applications. A simple but
> inaccurate description of the SIDAC would be that it is a two terminal
> TRIAC (DIAC) that must be activated by the application of a voltage tha=
t is
> greater than the typical 130 Vac main. Placing a SIDAC in series with a
> 120V lamp on a 120 Vac main will accomplish nothing. However, placing a
> 120V lamp in series with a SIDAC on a 277 Vac main will illuminate the =
lamp!
>=20
> One last bit of advice and hocus pocus . . .  When ever dealing with hi=
gh
> voltages, I sincerely think that it is best to be in a Nikola Tesla sta=
te
> of mind!
> Good luck with the killer volts!


Hi Ned, wow thanks for the info. I have saved your
post for prosperity! :o)

I'm an old hardware guy too, and have played with lots
of caps, but generally i'm using the milspec polyester
ones for size vs capacity.

Let me say I really appreciate you sharing your experience
here, like others I was a bit sceptical about your first
post re not using the series resistor, as this is required
by some certification standards, and series resistors
always appeal to me as a TV repairer, i've seen what mains
spike do to hardware! :o)

I have a lot of polyprop caps, but generally don't use them
as the 630v polyester are smaller and cheaper, but like
I said before I don't sell HV cap products. I have seen
some polyprops used in personal defence (electric discharge)
products. And i've never seen one fail.

Please keep sharing any wisdom you have on this matter,
the whole Xc deal interests me a lot, I don't see why
Xc should be any less respected than Xl but that always
seems to be the case. In the future I see big improvements
in caps but not much changing in inductors, and after being
abused by my lecturer many years ago about my 45mA Xc supply,
then seeing them become the modern standard some 15 years
later, i'm now keeping an open mind as to where the future
is headed. Capacitive transformers anyone??
-Roman

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