Voltage ratings on electrolytic capacitors?

Discussion in 'General Electronics Chat' started by spinnaker, Jan 3, 2011.

  1. spinnaker

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    I hate to question the opinion of another forum member but I guess it is all about sharing and knowledge so here goes.

    In another thread a forum member said that in order for electrolytic capacitors to maintain their properties, their voltage ratings should not exceed more than twice the voltage expected across them, also allowing for any spikes.

    I deal a lot in the 5V range and 3.3V range. with 78XX regulators. I usually use a walwart. the 78XX regulators usually call for a tant cap on the input or output. But the smallest voltage ratings on most of these are 50V. Mouser has lower ratings but they are something like $2+ a piece.

    So what is the deal? Should I still use the 50V tants or go into bankruptcy buy the more expensive caps?
     
  2. #12

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    Tiny point here...tantalum capacitors are not electrolytic capacitors.

    I've been corrected.
     
    Last edited: Jan 3, 2011
  3. thatoneguy

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    Lower voltage rated caps generally have a lot lower ESR and overall better response. Putting a 50V in where a 10V would do will result in higher ripple.

    This has to do with the "Forming" of the electrolyte, google "Capacitor Forming" for more than you'd ever want to know.

    P.S. Tantalum are a type of electrolytic cap, they are just called Tantalum and the other type is "Aluminum Electrolytic". Tantalum are a lot more fussy about polarity and voltage, but have great specs (and a pricetag to match).
     
  4. spinnaker

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    Sorry newb here. I thought they would be in that family. The previous post mentioned only tants.

    Are tants have similar problems to electrolytics in that they will degrade over time?
     
  5. thatoneguy

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    They don't degrade much, they simply explode instead. :eek:
     
  6. spinnaker

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    I'm such a newb I should have looked at the datasheet before posting. They don't require tants at al. I think I used them once in another PS project.

    OK what about electrolytics then? Last couple of 5V supplies I built, I but a 10uf 50v electrolytic on the output. Is this too low of a rating? Or can I use it?


    And what about the tants anyway? Can I use 50v tants on 5V? Will I have problems?
     
  7. marshallf3

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    You need 0.1 uF caps right on the input and output pins, as far as electrolytics it depends on what's going in and out as far as supply current, frequency and the amount of ripple you can put up with.

    See the sticky post at the top that talks about bypass caps.

    A 50V cap is way too high of a voltage rating to be using in a 5V circuit, 10 or 16V is more the norm even if you need to go up in value which will never interfere with a power supply.

    I'm hoping someone will get this into the e-book some day if it isn't already there, electrolytics need around 50% or a bit more of their rated voltage to hold their polarization and life.
     
  8. SgtWookie

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    I used to say to go double the expected highest voltage for electrolytic capacitors.

    What happens is that as you get near the capacitors' rated voltage, the leakage current increases significantly. If you actually get to the caps' rated voltage, it acts as somewhat like a voltage clamp ala Zener diode. This can cause the electrolyte to boil due to high power dissipation (along with high current usage), and can result in the capacitor exploding, raining alkaline conductive confetti over your circuit board - generally something to be avoided.

    You'd be OK to select a cap with a voltage rating ~20% higher than expected.

    Tantalums are somewhat similar in that they are generally polarized (the + terminal marked instead of the - terminal), and that they lose their dielectric over time - albeit more rapidly than aluminum electrolytic caps. The attraction is low ESR and small physical size. The negative component is loud explosions if they lose their dielectric.
     
  9. marshallf3

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    Nice to see that we agree on this matter but I'm sure you've had to reform dielectrics on old caps just as often as I have over the years resurrecting old equipment. Common electrolytics, NP electrolytics, tants, micas, films, ceramics etc all have their own properties and peculiarities but common aluminum electrolytic and tant caps are probably the least understood.

    One thing I'll add - compute the peak voltage of the input to your supply, not the RMS you expect from it under load.
     
  10. spinnaker

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    So lets say I want to use an electrolytic for say a 9V walwart that requires some filtering. With no load the particular one I have puts out around 15V.

    Would I select the cap for a no load condition? If so that would be 15+ 3 (20%) correct? A total of 18V but my choices are 16V and 25V? Which would be best?

    marshallf3 is right. A sticky thread should be created in this very subject.

    Perhaps additional into regarding the size of the filter as it has to do with current. I know Sarge posted it once before and I can look it up but it would be nice if it was in an ebook or sticky.


    I would have saved a small fortune in caps. Anyone need some 50v electrolytics? :)
     
  11. SgtWookie

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    Yes, I have.
    People don't realize that one of the worst things you can do to old equipment is to simply power it up. The odds of survival for old electrolytic caps under such treatment are not good at all. They need to be charged slowly via a high-impedance source to their rated voltage until the leakage current is acceptable, or until one's patience wears out and they are rejected as unserviceable.

    The forming of the dielectric is not well understood by most, nor the degradation of same. The very chemicals that aided the formation of the dielectric in the first place tend to eat away at it over time if left discharged for long periods. Simply charging a cap to it's rated voltage using a 10k Ohm resistor while monitoring the leakage current could save a lot of them.

    That's why I recommended doubling the voltage rating. Easy to remember, and not likely to cause problems. If you go too low in voltage specifications you are much more likely to experience problems.
     
  12. spinnaker

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    After thinking about what Sarge mentioned about the voltage approaching the rated voltage and what marshallf3 had to say about an the voltage rating on an electrolytic being too high, I guess it is kind of a balancing act.

    But I assume for the newb hobbyist, it is probably best to error on the high side to avoid that shower of metal? :)
     
  13. SgtWookie

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    Depends on your expected load, and the impedance of the source. Many commercial manufacturers for cheap consumer goods would use 16v caps in that application. Mil-spec applications might require 100v caps.

    It would likely help to avoid lots of confusion. However, there is a great deal already written, and even more to be written about capacitors. It's like - the more you know about them, the more you want to know about them.

    Do me a favor - call me Wook instead of "Sarge", as "Sarge" is an Army thing.

    With 50Hz input to a full-wave bridge, a 1A load with an ideal 10,000uF capacitor will have 1v of ripple. That ignores lots of things like transformer secondary impedance, cap ESR, etc - but is still a useful starting place.

    No thanks, I have lots of 'em. ;)

    They are still very useful - just physically larger than you might like.
     
    Last edited: Jan 3, 2011
  14. thatoneguy

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    For most stuff you have been working with, the 50V will be fine, but if the circuit acts odd in the future (years, not days), check the caps first.

    For rectified 12VAC, the peak voltage is just short of 17V, so 25V caps would be good, while 16V wouldn't be rated for the peaks, and would eventually fail. If running on an already regulated 5V line, then 6V rated caps are nice due to their small size for capacity, as well as low ESR.

    It comes down to the load and definition of "acceptable performance". Some circuits can stand a bit of ripple in the power supply, especially if decoupling caps are all around on the circuit board. Some switching power supplies will fail when the caps get barely past the high end of their expected ESR rating.

    For most hobby circuits, using an over-rated cap will be fine to assemble the circuit for a prototype, and used in something else later. It would get too expensive to stock 10 different voltage ratings of the same value cap.
    The biggest part to pay attention to: Values around 1uF or smaller, I always use film caps instead of aluminum or tant. They may cost a bit more, but cause a lot less headaches in the long run.
     
  15. spinnaker

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    Sorry wook did not know that. I never served in any branch of the service. Especially considering most join at a very young age. I really admire those that do. Had I known then what I know now I would have but in several years myself.

    I have got to write this down. :)



    Sorry for being so dense. I am confused by what marshallf3 said about the voltage ratings on caps being too high and using too high of a rating will cause the cap to degrade.


    Can I still use my 50V caps in my lower voltage applications (5-25V)?

    I did understand your mention of the procedure for reforming caps. Do the two have something to do with one another?
     
  16. spinnaker

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    Ha! TOG just answered most of my questions! :)

    You don't care if I call you TOG? :)


    Years don't matter. Most of my stuff won't be around then. I may not be around then. :)


    You can call me Spin, you can call me Spinnaker but ya doesn't have ta call me Johnson. :)
     
  17. thatoneguy

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    TOG is fine, it is a bit of a pain to login... All the easy names were taken.
     
  18. marshallf3

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    raining alkaline conductive confetti over your circuit board - generally something to be avoided.

    I'm sorry, just can't stop laughing from seeing this in mis-designed circuitry from some of the best companies out there. It usually wasn't due to under specifying the voltage but more due to not taking the ESR into consideration given the frequency.

    Worst experience I ever had was in an antique scope, the huge metal can vented but it took a day to get the smoke and smell out of the apartment I lived in at the time. It was so bad that it started at the ceiling and went down to within about 2' of the floor before I could kill the power to it, had to crawl to the door to open it as the smoke as so acrid you couldn't breathe it. Had that thing not vented it probably would have blown the windows out.
     
  19. marshallf3

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    Had no idea we'd create such a stir here but there's been a ton of valuable info posted.

    With 50Hz input to a full-wave bridge, a 1A load with an ideal 10,000uF capacitor will have 1v of ripple. That ignores lots of things like transformer secondary impedance, cap ESR, etc - but is still a useful starting place.

    The problem I have with huge initial filter caps is that they draw such a large inrush current that often a small resistor is necessary between the bridge and the actual filter cap so you don't blow your main fuse, transformer or rectification method. I'm sure there's a rule for this somewhere but anything over 4,700 uF I'll tend to put a very small resistance before it. Considering it's going to be regulated later on it doesn't give you any penalty unless it's a high current circuit that already puts a decent load on things.
     
  20. marshallf3

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    Some circuits can stand a bit of ripple in the power supply, especially if decoupling caps are all around on the circuit board.

    Another bingo and I'm a stickler for this. It's too bad the tants didn't turn out as promising as they first appeared to be but when you look at http://www.allaboutcircuits.com/vol_1/chpt_13/2.html some light will be shed on the subject.

    If only making hobby circuits you can get by with a lot of things, if designing something you may eventually want to share it never hurts to include a few extra bypass caps in addition to those that are absolutely required. I tend to put 0.1 or 0.01 caps all over the place with an occasional electrolytic if a component is a bit away from the main power supply trace wise.

    RF, which I occasionally deal with, is a totally different subject.

    Anyone care to look over this? It's in somewhat raw form as I never finalized the documentation on it. I'm sure a few final details missing but you'll get the idea. Modified a design, chose a far newer FET and I can get 22 dB of gain with less than 1 dB of noise. I challenge anyone to better that:

    http://www.innoengr.com/images/FM_preamp/
     
    Last edited: Jan 4, 2011
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