Re: [Drakelist] Testing Caps for Leakage
Richard, That is very true. Paper caps can leak yet look OK for capacitance and not too bad for ESR. I think 5uA is too high for a paper cap. If that cap is being used for plate to grid coupling and the grid circuit is 1 Mohm, there will obviously be a problem. I have a common Heathkit IT-11 cap tester and I've calibrated the eye closures for 1uA/10uA/1mA respectively on the leakage switch. It tests caps just fine and will form electrolytics. Dennis AE6C On Sat, Apr 2, 2011 at 10:07 PM, Richard Knoppow 1oldle...@ix.netcom.comwrote: For years I thought it was sufficient to test caps for dissipation factor, another name for ESR, I discovered the hard way its not. Many will tell you that electrolytic caps must be tested with polarizing voltage and that may make a difference in the measured capacitance of some but that is not the problem I am discussing. I discovered that many caps develop low _parallel_ resistance AKA leakage. Not just electrolytic but also paper and probably other types of caps. I found this when I was getting an old General Radio signal generator going and discovered that the modulation monitor didn't work right. The coupling cap was a 0.05 uf high quality molded paper cap. It measured fine on a GR impedance bridge but a new cap fixed the problem. The new cap measured about the same as the old one on the bridge. So, I decided to measure the leakage current. I did this with a small GR regulated and adjustable power supply and a DMM capable of measuring micro-amps. Most DMM's and VOM's are sufficiently sensitive to work. I found the bad capacitor to have a lot of leakage current (can't remember how much) where a new one had none detectable. I used about 300V on that one because it was rated that high. I then checked a bunch of discarded caps of all sorts plus some new ones. Bad electrolytics of course have very high leakage (right up to being sort circuits). I think the limits are around 5 ua for paper or other non-electrolytic types and perhaps 15 ua or a bit more for electrolytics. The current for an electrolytic will drop as the virtual electrode forms, if it increases the thing is no good. I later obtained a General Radio megohmeter,which will make a similar measurement but it has a fixed bias of 500V which is too much for many caps. It cqan be rigged to work with a lower voltage external supply, I have not tried that. The only advantage it has over the supply and meter method is that it indicates resistance directly. Anyway, the point is (and you know what's coming) that many bad caps will test good on a bridge or capacitance meter that is not set up to measure _parallel_ resistance with sufficient voltage. Note that _leakage_ or parallel resistance is also why bridging a bad cap with a good one will often NOT make a difference, its _not_ loss of capacitance but increase in leakage that's the problem and putting two caps in parallel won't change that. To investigate a cap it really is necessary to lift one end and substitute another. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Testing Caps for Leakage
-Original Message- From: Dennis Monticelli <dennis.montice...@gmail.com> Sent: Apr 3, 2011 1:22 AM To: Richard Knoppow 1oldle...@ix.netcom.com Cc: Drake List <drakelist@zerobeat.net> Subject: Re: [Drakelist] Testing Caps for Leakage Richard, That is very true. Paper caps can leak yet look OK for capacitance and not too bad for ESR. I think 5uA is too high for a paper cap. If that cap is being used for plate to grid coupling and the grid circuit is 1 Mohm, there will obviously be a problem. I have a common Heathkit IT-11 cap tester and I've calibrated the eye closures for 1uA/10uA/1mA respectively on the leakage switch. It tests caps just fine and will form electrolytics. Dennis AE6C I don't remember where I got the 5 uA value, you could well be right. It doesn't take much leakage to cause problems with coupling caps. Modern film caps have leakage so low its difficult to detect. ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Testing Caps for Leakage
Richard, Any readily detectable leakage is too much for a grid coupling cap IMO. A cap that even partially closes the eye on the 1uA setting of my Heathkit is grounds for replacement. Modern caps are cheap and easy to obtain. If you don't like the bright colors of new axial caps, just slip a little heat shrink over them and viola you have a mini black beauty :-) Dennis AE6C On Sun, Apr 3, 2011 at 5:43 AM, Richard Knoppow 1oldle...@ix.netcom.comwrote: -Original Message- From: Dennis Monticelli Sent: Apr 3, 2011 1:22 AM To: Richard Knoppow 1oldle...@ix.netcom.com Cc: Drake List Subject: Re: [Drakelist] Testing Caps for Leakage Richard, That is very true. Paper caps can leak yet look OK for capacitance and not too bad for ESR. I think 5uA is too high for a paper cap. If that cap is being used for plate to grid coupling and the grid circuit is 1 Mohm, there will obviously be a problem. I have a common Heathkit IT-11 cap tester and I've calibrated the eye closures for 1uA/10uA/1mA respectively on the leakage switch. It tests caps just fine and will form electrolytics. Dennis AE6C I don't remember where I got the 5 uA value, you could well be right. It doesn't take much leakage to cause problems with coupling caps. Modern film caps have leakage so low its difficult to detect. ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Testing Caps for Leakage
Dennis Thanks for your posting on the cap testing. I have no idea what my older C-3 does for leakage tests but I found this interesting paper on it... http://www.kb2ljj.com/Manuals/37-Heath%20Kit/Heathkit%20C-3%20%5Bnotes%5D%20WW.pdf It sounds like the C3 is supposedly set for 4.5 ma I may try the restoration that is suggested in the paper... Several more new caps on order for the R4B. I replaced the bias filter cap for the AVC/Mute circuit that had excessive leakage and it fixed it fine. The C-3 did show leakage but I am still learning how to use it and a more sensitive leakage test would be useful for audio coupling caps. It sounds like the sensitivity of the C-3 meter could be increased with use of the notes by w7EKB Paul K3PG http://pgerhardt.blogspot.com Message: 4 Date: Sun, 3 Apr 2011 11:31:24 -0700 From: Dennis Monticelli dennis.montice...@gmail.com To: Richard Knoppow 1oldle...@ix.netcom.com Cc: drakelist@zerobeat.net Subject: Re: [Drakelist] Testing Caps for Leakage Message-ID: BANLkTi=34a_6copu05uz7r4m4vrzr8x...@mail.gmail.com Content-Type: text/plain; charset=iso-8859-1 Richard, Any readily detectable leakage is too much for a grid coupling cap IMO. A cap that even partially closes the eye on the 1uA setting of my Heathkit is grounds for replacement. Modern caps are cheap and easy to obtain. If you don't like the bright colors of new axial caps, just slip a little heat shrink over them and viola you have a mini black beauty :-) Dennis AE6C ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] Testing Caps for Leakage
For years I thought it was sufficient to test caps for dissipation factor, another name for ESR, I discovered the hard way its not. Many will tell you that electrolytic caps must be tested with polarizing voltage and that may make a difference in the measured capacitance of some but that is not the problem I am discussing. I discovered that many caps develop low _parallel_ resistance AKA leakage. Not just electrolytic but also paper and probably other types of caps. I found this when I was getting an old General Radio signal generator going and discovered that the modulation monitor didn't work right. The coupling cap was a 0.05 uf high quality molded paper cap. It measured fine on a GR impedance bridge but a new cap fixed the problem. The new cap measured about the same as the old one on the bridge. So, I decided to measure the leakage current. I did this with a small GR regulated and adjustable power supply and a DMM capable of measuring micro-amps. Most DMM's and VOM's are sufficiently sensitive to work. I found the bad capacitor to have a lot of leakage current (can't remember how much) where a new one had none detectable. I used about 300V on that one because it was rated that high. I then checked a bunch of discarded caps of all sorts plus some new ones. Bad electrolytics of course have very high leakage (right up to being sort circuits). I think the limits are around 5 ua for paper or other non-electrolytic types and perhaps 15 ua or a bit more for electrolytics. The current for an electrolytic will drop as the virtual electrode forms, if it increases the thing is no good. I later obtained a General Radio megohmeter,which will make a similar measurement but it has a fixed bias of 500V which is too much for many caps. It cqan be rigged to work with a lower voltage external supply, I have not tried that. The only advantage it has over the supply and meter method is that it indicates resistance directly. Anyway, the point is (and you know what's coming) that many bad caps will test good on a bridge or capacitance meter that is not set up to measure _parallel_ resistance with sufficient voltage. Note that _leakage_ or parallel resistance is also why bridging a bad cap with a good one will often NOT make a difference, its _not_ loss of capacitance but increase in leakage that's the problem and putting two caps in parallel won't change that. To investigate a cap it really is necessary to lift one end and substitute another. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
