Tantalum Capacitor Question

Discussion in 'General Electronics Chat' started by SteveM99, Oct 19, 2011.

  1. SteveM99

    Thread Starter New Member

    Feb 10, 2011
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    I have a board that uses a 100uf/20v Tantalum capacitor being used on the +15v power supply line.

    Occasionally the cap blows when power is turned on. The +15 volt is probably spiking causing it to blow (switching power supply) or it could be the Chinese cap with mis-marked voltage rating.

    I can not find a 100uf Tantalum cap with a voltage rating higher than 20 volts (neither Digi-Key or Mouser).

    Can I substitute an Aluminum Electrolytic with a higher voltage rating (35v) to replace the Tantalum?

    Appreciate the help, thanks.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The answer is maybe. Tantalum caps have superior specs overall, but they also have the unfortunate tendency to go up like a match head if provoked. They also get very noisy before failing. When they work (which they usually do) they are superior in every way.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Aluminum electrolytics generally have a higher ESR than tantalums, thus the concern would be if it is filtering a significant amount of ripple current from the switching supply which could cause it to get hot. You can try the aluminum but check to see if it gets warm after several minutes of operation.

    Here is a 100μF, 30V tantalum.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Either the voltage is spiking, or there is a large turn on current spike as the cap tries to charge from zero to 15 with little source resistance to limit the current.

    For a power supply filter an electrolytic is probably OK. In the olden days when tantalum caps were pricy that's what was used there.

    Your test results will prove if it is acceptable.
     
  5. SteveM99

    Thread Starter New Member

    Feb 10, 2011
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    Thanks guys.... forgot to mention that its not Surface Mount.

    I'm still trying to find who the Vendor is (no markings other than 100L20).

    In Tantalum datasheets (Kemet), I see charts which list the ESR based on Capacitance vs. Voltage Rating (100khz@+25C) but I don't see the same breakdown with the Electrolytic Capacitors ?

    Do you have to calculate the ESR with the Electrolytics?

    Thanks.
     
  6. colinb

    Active Member

    Jun 15, 2011
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    What is the source of your +15 V supply?

    Maybe you can just try an aluminum electrolytic cap instead and see how it works. Or you could spend $65 on that 100 µF, 30 V tant cap... although it if is massive inrush current surge destroying your current cap then you will likely blow up this $65 one too.

    Maybe it's tantalum for a good reason, then again maybe not. Is it your circuit design or is it designed by someone else? Can you share the schematic? Maybe someone here can give better advice after looking at your design.
     
  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    20V is too low for a tantalum on a 15V line, it should be at least a 25V part and preferably 35V. However, a 20V or 25V electrolytic should be fine.

    Or find two tantalum 220uF @ 16V, and place them in series.
     
  8. colinb

    Active Member

    Jun 15, 2011
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    Why is a tantalum rated at 20 V too low for 15 V use when a 20 V aluminum electrolytic is ok?

    Are you suggesting that the voltage actually has exceeded 20 V and the electrolytic caps would be more forgiving when their voltage is actually exceeded?

    Or, are you saying the tantalum caps are failing even though the voltage has not exceeded their 20 V rating?
     
  9. Lundwall_Paul

    Member

    Oct 18, 2011
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    I would consider installing a I would install a TSV Transient Voltage Suppressor. Such as a Microsemi 1N5637 or 1N5636.
     
  10. Evil Lurker

    Member

    Aug 25, 2011
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    Tantalums are great for applications where high frequency and ultra-reliability are needed, but the downside is they are very sensitive to overvoltage.

    What is the ESR on that cap that blew? You might could get away with a low ESR/high ripple electrolytic or even go to an organic polymer cap.

    I would probably recommend going with a Panasonic FM series cap @25 volts or if you need more ripple bump it up to 35v.
     
  11. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Tantalum caps are notorious for failing due to insufficient voltage derating and in-rush current (especially as ESR in newer ones become lower and lower). To be careful you should double the working voltage, or worst-case not go over ~ 70%. For instance 12V / 16V = 75%, so on a 12V line a 20V tant cap would be better than a 16V. Although it is quite possible you can put a 16V tant cap on a 12V line and never see a problem, it just isn't a good design practice.

    On the other hand, electrolytic caps like to be run at about 80% of their rated voltage, but don't go below ~60% or over ~90% if practical.
     
    Last edited: Nov 18, 2011
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  12. SgtWookie

    Expert

    Jul 17, 2007
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  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That's what blows tantalums. We were strictly forbidden to use solid tantalum in military supplies unless some form of turn on surge current limiting was built in. Their ESR is so low they can blow up when powered up from a low impedance voltage source.
     
  14. colinb

    Active Member

    Jun 15, 2011
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    Thanks for the insight, that does seem to explain things.

    However, are you suggesting that it is detrimental to use an electrolytic capacitor with too high a voltage rating? Sure, a very over-rated capacitor will be larger and waste some space, cost a little more money, but is it actually harmful or otherwise a true problem to use, e.g., a 100 V capacitor for a 12 V application??
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Aluminum electrolytic capacitors have increased allowance for leakage current as their voltage rating increases. This logically follows because since the distance between the plates needs to be increased for the higher voltage, the plate area needs to increase, so the leakage rate also increases.

    This can cause problems in some circuits; for example if you are trying to create a 555 timer circuit with a long duration, the cap leakage current may be greater than the R1/R2 network can supply for the calculated time duration - or it can cause the timing to be much longer than calculated.
     
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  16. colinb

    Active Member

    Jun 15, 2011
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    Thanks, SgtWookie. I understand now.
     
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