Capacitor shelf life?

Discussion in 'General Electronics Chat' started by Nicholas, Sep 19, 2015.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Hi all!

    I used to fix CRT video boards, but recently I got so sick and tired of it that i quit that hobby :) However, I have managed
    to hoard a lot of caps, everything from noname to Rubycon and Panasonic etc.

    Now I am thinking; do they have a shelf life? Do they get old by just...being?

    Thanks,

    Nicholas
     
  2. recklessrog

    Member

    May 23, 2013
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    The electrolyte in electrolytic capacitors can dry out, best thing to do is to try and re-form them, something we used to do a lot years ago when salvaging parts from old equipment. It's done like this:-
    If say the capacitor were 16mfd 250volts, first check its not short circuit. Then connect it to a variable power supply via a w/w 5-10k (not critical, just to limit the current in case it goes short circuit)and a 500ma fuse. Connect your volt meter directly to the capacitor and apply about 30 volts. After a moment (time constant of C/R) the meter should show the same voltage as the power supply. Leave it for half an hour and if all is ok slowly increase the voltage to around 100v while keeping an eye on the meter, if at any point it starts to drop, it indicates a short, if this happens, lower the voltage until you have a steady reading again and leave it for another half hour to "heal".
    When all appears ok, repeat the process in 50volt steps and increase the time to about an hour until you reach 2/3rd the working voltage of the cap, then slowly turn up to full rated voltage and leave for 2-3 hours,
    If all is good, turn off the power supply and discharge the capacitor with a 10k 5w resistor, do not just short it with a screwdriver!
    As I mentioned in another post, I have 3x2.5k resistors in series in an insulated and ventilated case connected to a pair of multimeter probes that I made 45 years ago for discharging capacitors. keep the probes connected for 20-30 seconds to fully discharge high value caps.

    DO REMEMBER THAT HIGH VOLTAGE AND CHARGED CAPACITORS STORE ENOUGH ENERGY TO KILL YOU, WORK SAFELY AND USE ONLY ONE HAND AND RUBBER SOLED SHOES!!!

    (I spent many years working both professionally and as an amateur radio ham on circuits that were at many thousands of volts and drawing lots of current! imagine the power available from a 60 mfd capacitor charged to 15Kv!!!!)

    P.S someone once said, "the reliability of any electronic equipment is dependant on the quantity and quality of the electrolytic capacitors it uses"!
     
    Last edited: Sep 19, 2015
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    They do dry out but usually because of heat and/or some reverse bias.

    I would not do anything to them until you want to use them. Then, apply a bias voltage to them to "heal" the dielectrics. These capacitors are an electrochemical device and can fail but keeping them cool and away from circuits is the best way to store them.

    Note that manufacturers do not put expiration dates on them, they just expect them to last a significant time once installed in a device (10-2pm years). Note that I have a power supply from HP built in 1974 that still works perfectly. Never changed a cap.
     
    Last edited: Sep 19, 2015
  4. recklessrog

    Member

    May 23, 2013
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    Stir? Ive got a big wooden spoon you can have ha ha. Think he meant store!
     
  5. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    121
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    Thanks guys, and thanks for the 'healing'-recipe! I think for now that I will just store them
    in a dark and cold place :)

    This: "DO REMEMBER THAT HIGH VOLTAGE AND CHARGED CAPACITORS STORE ENOUGH ENERGY TO KILL YOU, WORK SAFELY AND USE ONLY ONE HAND AND RUBBER SOLED SHOES!!!"

    Was one(among others) reason that I got out of the CRT-repair hobby.

    Thanks guys!
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    This is from a Cornell Dubilier application guide for electrolytic caps:
    capShelfLife.jpg capStorageTemp.jpg

    Failure mode is usually open circuit for electrolytic caps and short for tantalum.
    capFailure.jpg
     
    Last edited: Sep 19, 2015
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    In aluminium electrolytic capacitors; the actual dielectric is an oxide film on the anode foil and the electrolyte is a caustic solution.

    In normal use, the applied voltage will always push a small leakage current - that is what maintains the oxide film.

    In neglected capacitors, the caustic electrolyte gradually etches away the oxide film. As the dielectric gets thinner; the capacitance increases - but the capacitor won't stand its rated voltage without breaking down and/or overheating.

    You need to re-form capacitors that have stood idle, with a current limited supply that can ease them up to their rated voltage over a 24h period. Probably a few mA for smaller caps and a few tens of mA for bigger ones.
     
  8. recklessrog

    Member

    May 23, 2013
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    I can't completely agree with the last paragraph, as in over 55 years of being involved in electronics in one form or another including servicing, design development, commissioning etc etc, I personally have replaced many hundreds of short circuit electrolytic capacitors, as well as thousands that have raised ESR due to drying out. I think that the Taiwanese or some similar country of manufacture, stole the design of some capacitors but omitted to use an important chemical inhibitor resulting in a rash of equipment failure.
     
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  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Not a "hard and fast" rule, but I've encountered very many tantalum capacitors that are shorted or at least very leaky, on the other hand; aluminium electrolytics do tend to fail open circuit fairly often, I have found shorted aluminium caps - but not all that often.

    Stored aluminium electrolytics often increase in capacitance, but if you apply the full rated voltage all at once - they can go bang!
     
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  10. recklessrog

    Member

    May 23, 2013
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    Just for the hell of it, today I dug out from my shed an old mono record player amplifier from about 1963. the power supply uses an EZ80 rectifier and a dubillier 8+ 8 mfd 250volt smoothing capacitor. As my forum name suggests, I recklessly just plugged it into the mains and switched on half expecting fireworks or instant confetti from the cap exploding, but guess what? it works perfectly and is driving a nice 1kHz sinewave into a large 10 ohm resistor. I did connect a speaker and there is very little hum and noise!! its now been running 4 hours. So there you have it, some survive!!

    P.S Ive had it at least 30 years and never plugged it in before!
     
    Last edited: Sep 19, 2015
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    That quote came from a Cornell Dubilier application guide; they've been making capacitors for over 100 years. If you think you know more than them, more power to you.

    Personally, I've never replaced an electrolytic that failed short (though I agree that it can happen) but I've replaced many shorted tantalums. When I was a technician and had to work on power supplies, I replaced a number of electrolytics that had exploded or leaked; usually from voltage overstress, including many that were installed backwards and failed on first power up (which we always did with a variac).
     
  12. recklessrog

    Member

    May 23, 2013
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    Maybe Dubillier were referring to their own capacitors and had not included poorly made capacitors from dubious sources that found their way into a many cheap cassette, video recorder and t.v's etc. that were around in abundance during the 80's and 90's. No i would never dare to assume i know more than such a well respected company, I can only relate my own personal experience.

    Just for info, I was a partner in a service company that had contracts with major consumer electronics sales outlets for 19 years, our average weekly repair inventory was around 300-350 faulty items a week and we also were a major manufacturers U.K based service center so we saw a very wide spectrum of equipment pass though, some excellent and tons of "junk"
     
    Last edited: Sep 19, 2015
  13. recklessrog

    Member

    May 23, 2013
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    You got me thinking about why my experience is at odds with others, so a quick phone call just now to my ex senior engineer has thrown some light on it.
    There were two major companies that produced the cheapest of cheap items and used the lowest cost components they could get away with. Both had circuit boards drilled to accept either axial or radial capacitors, except that due to the height of radials, they were laid flat and failed in the same way. They were not really like modern caps that bulge and split open along scored lines at the top, instead they "relied" on the rubber bung at the bottom blowing out.
    Now it seems that due to the strain put on the bung by bending the leads so close to the body, when they pressurised due to their very poor quality and not really being fit for the job, (and high temperature environment) the bung would come half out ( the side nearest the pcb staying in place) and the innards spew over the exposed internal ends of the leads causing a short. Hence our disproportionate number of s/c capacitors.
    Whilst the above does explain things, I can recall other instances of shorted electrolytics (or so leaky as to cause the accompanying circuit to behave as if they were short)
     
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  14. RichardO

    Well-Known Member

    May 4, 2013
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    Thanks for the clarification! It is always nice to learn from others experience.
     
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  15. #12

    Expert

    Nov 30, 2010
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    I see you remember the stolen formula fiasco. That's almost street cred around here. :D

    Just a passing thought...some percentage of shorted electrolytics go on to spew or explode, thus converting a short to an open and concealing the original nature of their death. If you don't think about it, your superficial impression is that they all failed open.;)
     
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