> > I realize that this debate over equalizing resistors with series > >capacitors is controversial
Jim, I am at a loss. What controversy? It has always been engineering practice to include equalizing resistors in series cpacitor filters. Even with the new capacitors initial low leakage these caps will eventually start to leak. Yes, the new caps are much better than the old ones but its not the same as semiconductor technology. Can anyone point out ANY commercial gear that uses series filter capacitors without equalizing resistors? Well, without the use of equalizing resistors in your series capacitor filter bank you'll be the one cleaning up all the goop that blew out of the electrolytics when one of them starts to leak and the divider goes out of balance. Won't happen today. Won't happen tomorrow.... But it will happen. Maybe with luck the worst you'll see is a blown fuse. Or perhaps these are'nt necessary either? HI! 73 de KA4JVY Mark --- Jim Candela <[EMAIL PROTECTED]> wrote: > > > 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 > > > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/
