Tantalum capacitor problem

Discussion in 'General Electronics Chat' started by gotumal, Jun 30, 2008.

  1. gotumal

    Thread Starter Active Member

    Mar 24, 2008
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    0
    Hi reader,

    In my board I have used a 22uF/35V tantalum capacitor (125E ESR). It is across the filtered output of bridge wave rectifier. So it is rippled DC. The peak voltage across it is 25V under worst condition.

    When I turned on the power supply, the capacitor got burnt immidiately (shorted). Now I am analysing what went wrong? I have checked, max volateg across it, temperature etc. I am told to check the ripple voltage to ripple current. How to measure it with CRO?

    Thanks in advance.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That sounds like a classic reversed polarity problem. Tantalums die instantly when you put them in circuit with the leads reversed. Check the polarity of the voltage before trying another capacitor.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Adding to Beenthere's suggestion, you will find that while most aluminum electrolytic capacitors have their - side marked, tantalum capacitors have their + side marked. This can easily lead to confusion (and loud BANGs) if you're simply looking for a mark.

    It is best to check the manufacturer's datasheet to verify what their particular marks mean.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    Another factor to consider is whether the voltage specification of the tantalum was appropriately derated for the intended application.

    See:

    http://www.nec-tokin.com/english/guide/cap/pdf/notes.pdf
    http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/KO-CAPUpd/$file/KO-CAPUpd.pdf

    John
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Is this tantalum capacitor being used to pre-filter the voltage at the input to a switching mode power supply?

    hgmjr
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    I don't use tantalum capacitors anymore because they all blew up and disappeared.
    I have never had the same problem with electrolytic and ceramic capacitors.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    You obviously didn't have the same batch of ceramics I had - they blew up and disappeared, too.
     
  8. rherber1

    Member

    Jan 6, 2008
    15
    0
    Firstly, I will qualify my remarks by saying I don't like dipped tantalum caps at all and won't use them at all if I can help it. The tantalums I have experience with are hermetically sealed (wet) types and I haven't used the polymer types yet.

    Tantalum capacitors (wet) are characterised by extremely low ESR, high temperature operation (>125C), with long and reliable life (are used in medical and space apps). I do use them in DC filtering, but only with certain provisos;

    They don't like high ripple or where the likelihood of high surge currents are present and I never use them on their own as a filter directly after an ac rectifier arrangement. I use them only on a regulated DC supply as an adjunct to a good quality aluminium electrolytic bulk storage capacitor and I haven't ever had one fail in this application mode.

    This Kemet paper gives some pertinent information as to the failure of tantalum caps in power on surge situations and the use of Surge Step Stress Testing.
    http://www.kemet.com/kemet/web/homepage/kfbk3.nsf/vaFeedbackFAQ/B015828EE4AFF1808525722500694AD4/$file/2001%20CARTS%20SSST.pdf
     
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