Hay all,
I have been experimenting with series electrolytic capacitors, and when
the
capacitors are high grade, and of equal capacitance, they seem to drop the
DC voltage pretty evenly when used without equalizing resistors.
I have two Central electronic 20a's, and the HV 40-40 Uf @ 450 wvdc can
electrolytics needed to be replaced. This brings up another issue where with
today's line voltages, these filter capacitors often see higher than rated
voltage during warm-up when the 5U4G heats up faster than the indirectly
heated tubes in the radio. We often need 500+ volt electrolytics for bullet
proof reliability. Digging through my junk box I found eight NOS Sprague 100
uf 250V radial leaded capacitors date code 82 something (figure 20+ years
old). These were unused.
I took these capacitors and with a series resistor of about 10k (5 watt
WW)
I charged them to 250 volts. leakage currents were nil, and later (after
discharging!) I measured the capacitance. All 8 were the same on my 'C'
meter at 109 Uf. I found it strange that they were so close. I kept
remembering tolerances like +80%, -20%, but not in my case.
I then fiddled around with some clip leads, and series two of the caps
up,
and placed them across 450 vdc with no resistors. With my 10 meg input DVM,
I measured 200 volts across one of the caps, and then 200 volts across the
other. Wait a minute! 200 + 200 does not equal 450. Apparently the 10 meg
DVM impedance was influencing my reading by increasing the leakage current
across the capacitor being measured. Since I did not have two identical
DVM's I conclude that the capacitors were sharing the 450 volts (225 + 225)
pretty evenly. So it looks like I made two 50 Uf 500 Vdc capacitors out of 4
pieces rated 100 uf, 250 vdc.
These capacitors are now in both my 20a's with no problems. One change
was
that even though one of the can electrolytics was still working, after
replacing it with my 4 caps ( pi filter ) the HV B+ rose some, and the 120
hz ripple voltage diminished considerably. I guess the can capacitor,
although still working had higher ESR, and maybe diminished capacitance. My
'C' meter showed over 40 Uf capacitance, so I guess it was the higer
effective serial resistance (ESR) that degraded over the past 50 years.
I realize that this debate over equalizing resistors with series
capacitors
is controversial, and to some extent is similar to equalizing resistors
across series up HV diodes. In the later case, it is now becoming accepted
that with modern high quality diodes, that these resistors don't help
protect the diodes. I am beginning to think that the same is true with
series up modern high quality capacitors. I am talking about modern computer
grade capacitors that are small, cheap, and reliable up to voltages of 250
vdc. I am not referring to no-name 450 volt units from various suppliers.
Mouser & Digikey have a wide selection of good parts that I am referring to.
It would be interesting to see how the voltages would imbalance if the
capacitors were poorly matched in terms of capacitance (assume equal leakage
currents).... Any thoughts?
Regards,
Jim Candela
WD5JKO
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED]
> Sent: Saturday, February 22, 2003 5:52 AM
> To: [email protected]
> Subject: Re: [AMRadio] DRIVER XFMR UPDATE
>
>
> In a message dated 2/21/03 9:23:48 PM Eastern Standard Time,
> [EMAIL PROTECTED]
> writes:
>
>
> > If it's impossible to find 125 mfd's (which I doubt...they can
> > be found), then you can put two, say 65's or 75 mfd, in series
> to replace
> > each
> > 125mfd's.
> >
>
> Nope... capacitors are not like resistors. putting them in
> series will halve
> the total value, not double them. To get the equivalent value of two in
> series, you have to use caps twice the value. In this case,
> you'd put (2)
> 250 ufd caps in series to get the equivalent of 125 ufd. It's
> advisable to
> also put equalizing resistors across each cap to make sure that
> the voltage
> division is equal to prevent one cap from having a
> higher-than-rated voltage
> across it.
>
> Eric, W2CAU
>
