XPost: sci.electronics.design   
   From: cd@notformail.com   
      
   On Mon, 4 Mar 2024 15:29:02 +1100, Bill Sloman    
   wrote:   
      
   >On 4/03/2024 5:03 am, Cursitor Doom wrote:   
   >> Hi all,   
   >>   
   >> Using my Peak ESR/Capacitance meter, I was carrying out in-circuit   
   >> checks on large electros in the linear PSU I've previously mentioned   
   >> here. I was getting a lot of "in-circuit/leaky" warnings for two of   
   >> them, so I pulled them out completely and am getting the same warning   
   >> when they're checked out of circuit, which surprised me as it's   
   >> unusual IME. Anyway, the leak would have to be very bad to result in   
   >> ripple, would it not? AFAIK, the leading culprit for ripple is caps   
   >> which have lost a significant amount of capacitance or else developed   
   >> a very large ESR.  Do I have that right?   
   >   
   >What you have wrong is the imagined association between leakage and ripple.   
      
   Eh? I did say leakage was probably about the least likely cause of   
   ripple!   
      
   >   
   >The "ripple" on the voltage across a capacitor reflects the charge that   
   >is being taken out of and fed into the capacitor over the mains cycle.   
   >More capacitance means smaller ripple.   
   >   
   >Leakage is just the current flowing through the oxide layer on top of   
   >metal conductor surfaces inside the electrolytic capacitor. If the   
   >capacitor has been un-used for a long time, some of the oxide layer may   
   >have diffused away, making the capacitance and the leakage current   
   >higher. Applying the working voltage will re-form the oxide layer by   
   >making it a bit thicker.   
   >   
   >> I'd like a "second opinion" as it were on the leakiness of these caps.   
   >> What's the best old-school method for testing for this? I just want to   
   >> ensure the ESR meter isn't faulty (highly unlikely but the possibility   
   >> must be eliminated to be sure).   
   >   
   >If an electrolytic capacitor hasn't been used for some time, it's going   
   >to be  leaky. If you want to measure how leaky, measure the direct   
   >current flowing through the capacitor as you increase the bias voltage   
   >across it. It should drop as the oxide layer re-forms, perhaps over hours.   
      
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