On Monday, October 14, 2019 at 2:13:58 PM UTC+11, m...@uga.edu wrote:> The rest of the story... > > For those who tuned in late, I'm replacing the power supply filters in a Heathkit IT-11 capacitor checker. In the original circuit they are two 40-uF 350-V in series (to handle 600 V). I ordered a 20-uF 1-kV film capacitor to replace them. > > Well, there just isn't room for that film capacitor under this chassis! So I had to resort to two 47-uF 450-V in series, like Heathkit's original. > > There was debate in this thread about whether to add balancing resistors, and I didn't. John Larkin said "they take care of themselves." Indeed they do, sharing the voltage to within a few percent of equal. > > I wonder if Mr. Larkin could explain further. Is there some effect that makes them equalize if they have different leakages to start with? I would expect the opposite, that the leakier one would carry a lower voltage and would form less, causing the difference in leakage to increase -- but I might be wrong.Leakage currents have a tendency to increase exponentially with applied voltage, so you often don't need much voltage difference to compensate for significant differences in initial leakage current. Some manufacturer's application note that I read back in 1970 was a full bottle on the subject. <snip> -- Bill Sloman, Sydney
Film capacitor as power-supply filter
Started by ●October 7, 2019
Reply by ●October 14, 20192019-10-14
Reply by ●October 14, 20192019-10-14
m...@uga.edu wrote: ---------------------------> > > There was debate in this thread about whether to add balancing resistors, > ?and I didn't. John Larkin said "they take care of themselves." > Indeed they do, sharing the voltage to within a few percent of equal. > > I wonder if Mr. Larkin could explain further. Is there some effect that makes them equalize if they have different leakages to start with?** The mid point voltage will be offset such that the same leakage current flows in both caps. The total voltage of the caps should exceed the supply by 25% or more to allow this. Adding bleeders increases the leakage through the leakier example. Both caps will get more leaky when hot, IME more so for the leakier one. I've been doing is for decades with no issues at all. Usually with 2 x 350V or 2 x 400V caps in series on a 500V supply with up to 600V surges at switch on. ..... Phil
Reply by ●October 14, 20192019-10-14
On 2019-10-14, mc@uga.edu <mc@uga.edu> wrote:> The rest of the story... > > For those who tuned in late, I'm replacing the power supply filters in a Heathkit IT-11 capacitor checker. In the original circuit they are two 40-uF 350-V in series (to handle 600 V). I ordered a 20-uF 1-kV film capacitor to replace them. > > Well, there just isn't room for that film capacitor under this chassis! So I had to resort to two 47-uF 450-V in series, like Heathkit's original. > > There was debate in this thread about whether to add balancing resistors, and I didn't. John Larkin said "they take care of themselves." Indeed they do, sharing the voltage to within a few percent of equal. > > I wonder if Mr. Larkin could explain further. Is there some effect that makes them equalize if they have different leakages to start with? I would expect the opposite, that the leakier one would carry a lower voltage and would form less, causing the difference in leakage to increase -- but I might be wrong. > > I am going to look back at the bleeder network and see if there's a good way to split it in half.because they are in series the capacitors share the voltage in such a way that the leakage current is the same in each. thus the leakier (more prone to leaking) has a lower voltage, typically leakage increases with voltage, so the capacitors self-balance. -- When I tried casting out nines I made a hash of it.
Reply by ●October 14, 20192019-10-14
On Monday, October 14, 2019 at 12:17:39 AM UTC-4, Bill Sloman wrote:> > Leakage currents have a tendency to increase exponentially with applied voltage, so you often don't need much voltage difference to compensate for significant differences in initial leakage current. > > Some manufacturer's application note that I read back in 1970 was a full bottle on the subject. >Thanks. If the increase is exponential (more than linear), that explains why they self-regulate.
Reply by ●October 14, 20192019-10-14
On Sun, 13 Oct 2019 21:31:09 -0700 (PDT), Phil Allison <pallison49@gmail.com> wrote:>m...@uga.edu wrote: > >--------------------------- >> > >> There was debate in this thread about whether to add balancing resistors, >> ?and I didn't. John Larkin said "they take care of themselves." >> Indeed they do, sharing the voltage to within a few percent of equal. >> >> I wonder if Mr. Larkin could explain further. Is there some effect that makes them equalize if they have different leakages to start with? > >** The mid point voltage will be offset such that the same leakage current flows in both caps. The total voltage of the caps should exceed the supply by 25% or more to allow this.The leakage current must be the same in series caps, no matter what the voltage.> >Adding bleeders increases the leakage through the leakier example. > >Both caps will get more leaky when hot, IME more so for the leakier one.If they are in series, the leakier one has less voltage drop so dissipates less power. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 14, 20192019-10-14
On Sun, 13 Oct 2019 20:13:53 -0700 (PDT), mc@uga.edu wrote:>The rest of the story... > >For those who tuned in late, I'm replacing the power supply filters in a Heathkit IT-11 capacitor checker. In the original circuit they are two 40-uF 350-V in series (to handle 600 V). I ordered a 20-uF 1-kV film capacitor to replace them. > >Well, there just isn't room for that film capacitor under this chassis! So I had to resort to two 47-uF 450-V in series, like Heathkit's original. > >There was debate in this thread about whether to add balancing resistors, and I didn't. John Larkin said "they take care of themselves." Indeed they do, sharing the voltage to within a few percent of equal. > >I wonder if Mr. Larkin could explain further. Is there some effect that makes them equalize if they have different leakages to start with?The one with the highest I:V leakage curve gets a smaller share of the total voltage, and the other gets more voltage, automatic equilibrium. They don't self-equalize on voltage; that's the point. They must have the same leakage *current* If the caps are similar, and leakage in each one increases radically with voltage, they will settle down with nearly equal voltage drops. If they don't, that's OK too. As Phil points out, forcing the series cap voltages to be equal breaks the equilibrium and increases cap leakage.>I would expect the opposite, that the leakier one would carry a lower voltage and would form less, causing the difference in leakage to increase -- but I might be wrong.Forming is an electrochemical reaction that is driven by current, not voltage. And the current is necessarily equal in all caps of a series string.> >I am going to look back at the bleeder network and see if there's a good way to split it in half.-- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 14, 20192019-10-14
jla...@highlandsniptechnology.com wrote: -----------------------------------------> > >** The mid point voltage will be offset such that the same leakage > > current flows in both caps. The total voltage of the caps should > > exceed the supply by 25% or more to allow this. > > The leakage current must be the same in series caps, no matter what > the voltage.** Really ?> > > >Adding bleeders increases the leakage through the leakier example. > > > >Both caps will get more leaky when hot, IME more so for the leakier one. > > If they are in series, the leakier one has less voltage drop so > dissipates less power. > >** The actual leakage current with HV electros is normally in the micro-amp range so dissipation causes no discernible temp rise. Ambient temp and rises in same entirely swamp it. Where series bleeder resistors are used ( eg 100,000 Fender tube amplifiers ) they pass about 1mA which is enough to reduce and nearly eliminate mid point voltage offset. The main benefit however is in bleeding off charge so the caps do not present a hazard if the AC supply is removed while there is no load on the PSU. There are a couple of ways this can happen. See typical schem: https://schematicheaven.net/fenderamps/fender_bassman50.pdf ..... Phil
Reply by ●October 14, 20192019-10-14
On Mon, 14 Oct 2019 14:24:33 -0700 (PDT), Phil Allison <pallison49@gmail.com> wrote:>jla...@highlandsniptechnology.com wrote: > >----------------------------------------- > >> >> >** The mid point voltage will be offset such that the same leakage >> > current flows in both caps. The total voltage of the caps should >> > exceed the supply by 25% or more to allow this. >> >> The leakage current must be the same in series caps, no matter what >> the voltage. > >** Really ? > > >> > >> >Adding bleeders increases the leakage through the leakier example. >> > >> >Both caps will get more leaky when hot, IME more so for the leakier one. >> >> If they are in series, the leakier one has less voltage drop so >> dissipates less power. >> >> > >** The actual leakage current with HV electros is normally in the micro-amp range so dissipation causes no discernible temp rise. Ambient temp and rises in same entirely swamp it. > >Where series bleeder resistors are used ( eg 100,000 Fender tube amplifiers ) they pass about 1mA which is enough to reduce and nearly eliminate mid point voltage offset. > >The main benefit however is in bleeding off charge so the caps do not present a hazard if the AC supply is removed while there is no load on the PSU. > >There are a couple of ways this can happen. > >See typical schem: > >https://schematicheaven.net/fenderamps/fender_bassman50.pdf > > >..... Phil > > >Ole tube gear seldom had fans. It just cooked itself. BLDC fans are great, small and quiet and cheap. The fans back when were big shaded-pole motor things.
Reply by ●October 14, 20192019-10-14
John Larkin wrote: -------------------> > >> >** The mid point voltage will be offset such that the same leakage > >> > current flows in both caps. The total voltage of the caps should > >> > exceed the supply by 25% or more to allow this. > >> > >> The leakage current must be the same in series caps, no matter what > >> the voltage. > > > >** Really ? > > > > > >> > > >> >Adding bleeders increases the leakage through the leakier example. > >> > > >> >Both caps will get more leaky when hot, IME more so for the leakier one. > >> > >> If they are in series, the leakier one has less voltage drop so > >> dissipates less power. > >> > >> > > > >** The actual leakage current with HV electros is normally in the micro-amp range so dissipation causes no discernible temp rise. Ambient temp and rises in same entirely swamp it. > > > >Where series bleeder resistors are used ( eg 100,000 Fender tube amplifiers ) they pass about 1mA which is enough to reduce and nearly eliminate mid point voltage offset. > > > >The main benefit however is in bleeding off charge so the caps do not present a hazard if the AC supply is removed while there is no load on the PSU. > > > >There are a couple of ways this can happen. > > > >See typical schem: > > > >https://schematicheaven.net/fenderamps/fender_bassman50.pdf > >** For an unexplained reason JL introduces a new topic here. Keeping the wacky guy on-topic is a battle.> Ole tube gear seldom had fans. It just cooked itself. >** Not one bit true - the cooking itself bit I mean.> BLDC fans are great, small and quiet and cheap. The fans back when > were big shaded-pole motor things.** But could be very quiet. Many tube scopes had fans fitted to the back panel and with a series dropping resistor could be made very quiet indeed. Some famous tube instrument amps had AC fans as standard, but there was a big nasty waiting to catch any designer who was stupid enough. Ampeg released a new version of their famous SVT, a 300W tube bass amp in the late 1990s. It had an AC fan that ran continuously. The designer made two bad errors: 1. He made the air flow go under the chassis, over the main PCB with all the output tube sockets and up though holes to cool each 6550 tube's glass envelope. A similar method is used in high power RF transmitters. 2. He also fitted octal sockets that had 2 inch long pins - so the PCB could be remain deep under the chassis with the socket's flange secured to the metal deck as usual. The result was that dust and fluff collected around all the long pins until a massive 700VDC arc-over occurred between an adjacent pair ( plate and heater ) causing extensive damage to the PCB, tubes and many other parts. Took hours to clean up and fix. Later production examples had plastic sleeving fitted to the long pins. The PCB still became so badly contaminated it was no cure. .... Phil
Reply by ●October 14, 20192019-10-14
On Mon, 14 Oct 2019 15:21:17 -0700 (PDT), Phil Allison <pallison49@gmail.com> wrote:>John Larkin wrote: > >------------------- >> >> >> >** The mid point voltage will be offset such that the same leakage >> >> > current flows in both caps. The total voltage of the caps should >> >> > exceed the supply by 25% or more to allow this. >> >> >> >> The leakage current must be the same in series caps, no matter what >> >> the voltage. >> > >> >** Really ? >> > >> > >> >> > >> >> >Adding bleeders increases the leakage through the leakier example. >> >> > >> >> >Both caps will get more leaky when hot, IME more so for the leakier one. >> >> >> >> If they are in series, the leakier one has less voltage drop so >> >> dissipates less power. >> >> >> >> >> > >> >** The actual leakage current with HV electros is normally in the micro-amp range so dissipation causes no discernible temp rise. Ambient temp and rises in same entirely swamp it. >> > >> >Where series bleeder resistors are used ( eg 100,000 Fender tube amplifiers ) they pass about 1mA which is enough to reduce and nearly eliminate mid point voltage offset. >> > >> >The main benefit however is in bleeding off charge so the caps do not present a hazard if the AC supply is removed while there is no load on the PSU. >> > >> >There are a couple of ways this can happen. >> > >> >See typical schem: >> > >> >https://schematicheaven.net/fenderamps/fender_bassman50.pdf >> > > > >** For an unexplained reason JL introduces a new topic here. > > Keeping the wacky guy on-topic is a battle.This is called a "discussion group." And isn't heat related to electrolytic cap life?> > > >> Ole tube gear seldom had fans. It just cooked itself. >> > > >** Not one bit true - the cooking itself bit I mean. > > >> BLDC fans are great, small and quiet and cheap. The fans back when >> were big shaded-pole motor things. > > >** But could be very quiet.The ones in the Tek 500-series scopes were noisy. I wonder why Tek used fans.> >Many tube scopes had fans fitted to the back panel and with a series dropping resistor could be made very quiet indeed. > >Some famous tube instrument amps had AC fans as standard, but there was a big nasty waiting to catch any designer who was stupid enough. > >Ampeg released a new version of their famous SVT, a 300W tube bass amp in the late 1990s. It had an AC fan that ran continuously. > >The designer made two bad errors: > >1. He made the air flow go under the chassis, over the main PCB with all the output tube sockets and up though holes to cool each 6550 tube's glass envelope. A similar method is used in high power RF transmitters. > >2. He also fitted octal sockets that had 2 inch long pins - so the PCB could be remain deep under the chassis with the socket's flange secured to the metal deck as usual. > >The result was that dust and fluff collected around all the long pins until a massive 700VDC arc-over occurred between an adjacent pair ( plate and heater ) causing extensive damage to the PCB, tubes and many other parts. > >Took hours to clean up and fix. > >Later production examples had plastic sleeving fitted to the long pins. > >The PCB still became so badly contaminated it was no cure. > > > >.... Phil >It didn't help thermally that some audio gear was literally upholstered.
