Reverse polarity Electrolytic

Discussion in 'General Electronics Chat' started by Art, Dec 9, 2015.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi Guys,
    I am wondering if a polarised electrolytic capacitor was connected across a power supply for a decade and did not blow up,
    could it ever become a working electrolytic capacitor of polarity reversed from the marking on the package?

    If not, assuming a polarised electrolytic capacitor is totally discharged by being shorted,
    why does it have a positive and negative terminal?
    Thanks :)
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
  3. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    Electrolytic capacitors will tolerate small reverse voltages, on the order of 1.5V.

    Reverse biasing them can cause dielectric breakdown, any that were abused should not be relied upon for normal usage.
     
  4. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    I know they should be replaced,
    But have real examples that have not shorted or blown up (but don't expect they are nessecarily being capacitors) after more than a decade.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    The terminal polarity tells you where you should apply the positive and negative voltage when you charge it.
    If you apply a reverse polarity voltage greater than a volt or so it will start conducting current (somewhat like a diode) and can be damaged.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    Yes, it's possible for reverse biased electrolytic caps to not blow up. If the reverse voltage is small, it won't even harm them.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    it depends on the type, a cap that has set around discharged for a long time can be re"formed" with reverse voltage sometimes. when caps are made, they arent polarised, they are formed by putting a lower voltage across them and ramping the voltage up tol the rated voltage over time. note,---- tantalum and other high tech caps arent done this way, they can and will explode.
    the plates of the caps are identical aluminum untill they have been formed.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    Theoretically: you can re-polarise a capacitor. In aluminium electrolytics, the anode foil is "formed" during manufacture to grow an oxide layer (the actual dielectric) - but they usually do all that before rolling the 2 foils and separator and sealing them in the can.

    In most cases; reverse voltage will break down the oxide layer and the fault current boils the electrolyte - with predictable results!

    AFAIK: Solid tantalum capacitors are the least tolerant of reverse voltage - at the very least, they go leaky - I've seen bead tantalums go off like a match head.
     
  9. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    If you apply more than a few volts they will explode.
    When I was learning electronics the class all built up a pcb each.
    One student put in his electrolytic in the wrong way around.
    When he applied power it exploded and hit the ceiling.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    If you limit the current low enough, it wont boil the electrolyte and build up a head of steam.

    Electrolytics stored for a long time eventually lose the oxide layer on the anode foil - as it gets thinner; the capacitance increases, but the breakdown voltage becomes less. Its the leakage current from the application of within normal voltage that maintains the oxide layer.

    Reverse voltage will speed up the dissolving of that oxide layer, and cause a new one to form on the other foil - but the current has to be limited to avoid destruction of the capacitor.
     
  11. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    That’s where I was going, but I didn’t want to lead the thread there myself, and influence the answer.
    I have 2 retro computer Commodore Amiga CD32, manufactured 1993-94.
    They have an external linear PSU, and for at least one of two board revisions,
    two SMD decoupling electrolytics are installed reversed.
    The silkscreen is correct, but they are mounted opposite.

    I have two units and there are two incorrect caps in each, so that’s four caps, no leakage,
    and both units still chug along nicely. I used them a good six months before ever finding out about the error.
    Something I can test with the old ones when the new caps arrive.

    Ian, thery wouldn’t have been current limited in any way,
    they all see the power supply and ground plane of the main PCB :D
    Cheers, Art.
     
  12. SLK001

    Well-Known Member

    Nov 29, 2011
    811
    224
    Are you sure that it isn't the SILKSCREEN that is in error? If the error was discovered after the PCBs were fabricated, the manufacturer might have just placed the caps properly and made the correction down the line.

    It is unlikely that a electrolytic capacitor was mounted backwards without catastrophic results.

    Also, what do you mean by "decoupling"? Electrolytic capacitors are generally used as filtering capacitors (mounted VCC to gnd) as opposed to signal decoupling.
     
  13. Techno Tronix

    Member

    Jan 10, 2015
    140
    10
    Hello,

    I don't think it is silk screen error. There is something else which i can not find out. Keep me update.
     
  14. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    It's very well known in the community that collects these computers about the error.
    I have also checked, and the silkscreen is correct, but components wrong.
    I'll follow up here when the new replacement caps arrive.
    There are plenty of other caps on the board mounted correctly,
    But the two in question have positive terminal connected to ground plane.

    By decoupling I mean the PSU is a brick seperated from the unit with 3 meter cable. The caps are local on the computer PCB across the power supply input.
     
  15. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    This would be why.. optional SMD or thru mount caps confusing for the assembler.
    [​IMG]
     
Loading...