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Electrolytic caps in series

Discussion in 'Hobby Electronics' started by Rheilly Phoull, Jun 22, 2005.

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  1. Terry Given

    Terry Given Guest

    mentioned to answer the OPs original question/
    therein lies the rub. Sometimes thats not so easy to do. And a reduced
    margin may mean cheaper caps.


    You've probably already noticed that good cap manufacturers also have a
    surge voltage rating. very handy, especially with regenerative loads.
    bloody good.

    as the caps get bigger, leakage also increases - ISTR proportional to C.

    it also doesnt help that the data sheet values can be wildly
    pessimistic. I know with Hitachi that the quoted lifetime figure is
    actually the process -3 sigma point, and is about half the average lifetime.
    Cheers
    Terry
     
  2. Guest

    Oh dear! Here comes the language again And up until now he was making some
    very intelligent contributions to an interesting thread - which proves he can do
    it.

    This masterpiece of understatement said twice for some reason (timeless literary
    quality?). Again one should lok past it to see real ability, but it's not
    shining through - it is well hidden and must be dug out
     
  3. One day Phil Allison got dressed and committed to text
    Yup, thats the one. The coupling caps were all grey jobs (this is an asian
    'knockoff') but as I recall the fenders were like that.
    Funny how it kept going since all the caps were leaky and you could hear the
    DC on the pots.
     
  4. GM

    GM Guest

    What ? I think Phil hit the nail on the head with his first post in the
    thread - Why bother with resistors as they are absolutely not required.
    He then explained his reasoning for this in later posts, after most had
    ignored the first one. Give us a break nospam.
     
  5. Folks,

    The generally accepted practise is to always use the same capacitance
    and voltage ratings of HV elec caps. They should even be the same
    manufacturer and type.

    If there are no voltage sharing resistors and one of the capacitors has
    a DC leakage current which is much greater than the other caps, it will
    put a high voltage stress on the other caps, and they may go BANG.

    For HV caps, which are usually rated in the range 350 - 550 volts, the
    voltage sharing resistors whould be at least 1 Watt types, to have
    sufficent voltage rating. 2W, 5W and 10W are also fine (carbon or wire
    wound). The resistance value should be such that at least a few mA of
    current is flowing and about 50% to 75% of the wattage rating is used
    (for long term reliability). Higher power resistors improve the voltage
    regulation a bit, but they produce more heat and waste some of the
    power transformer capability (could be an issue if its running close to
    its full rating).

    eg. Design for 450 V across each cap, using 2W resistors.

    R = E^2 / P = 450 x 450 / 1.5 = 135000. Use 150K resistors,
    (I = E / R = 3mA). 120K would also be OK (1.7W approx dissipation).

    HTH,
    Glenn.


     
  6. Phil Allison

    Phil Allison Guest

    "Glenn Baddeley = top posting bullshit artist

    ** Correct - then you can expect to get close matching.


    ** For a cap to do as you suggest, it must be a faulty cap - ie one that
    is not able to meet maker's specs for max voltage or leakage at rated
    voltage.

    Ballast resistors are no help if a cap is faulty.


    ** Wrong - WW resistors are only readily available in values up to about
    10 kohms - far too low a value for use in parallel with high voltage
    electros.

    Sustained high DC voltages make carbon or metal film resistors go high in
    value or open circuit - even if used well below their power rating .
    Composition resistors also go high value with time, heat and the application
    of high DC voltages.

    The point is simple - you are better off without ballast resistors across
    electros unless they are 100 % reliable.





    ............ Phil
     
  7. Terry Given

    Terry Given Guest

    and the more caps in parallel, the better the matching (variations tend
    to cancel out)
    LHS of bell curve. not terribly likely, but if you use 1,000,000 caps
    per year, you *will* see some.
    bingo. lots of people seem to like throwing money at equipment so that
    "in the event of a failure, such-and-such happens." In the event of a
    failure, the costs associated with returning the equipment for repair
    are often far in excess of the actual replacement parts cost, so why bother.
    depends on your definition of "readily available". From a hobbyist
    perspective, yes. For a real company, no.
    whats the mechanism?
     
  8. Phil Allison

    Phil Allison Guest

    "Terry Given" = kiwi bullshit artist

    ** What an asinine and pointless comment - faulty caps get removed from
    gear at the factory.

    ** What an asinine and pointless comment - faulty caps get removed from
    gear at the factory.


    ** What an asinine and pointless comment - faulty caps get removed from
    gear at the factory.


    ** Same one everyone else uses - fuckhead.


    From a hobbyist

    ** What an asinine and pointless comment..


    ** Just trolling - or you need to know ??


    ** What a pointless comment.



    ............. Phil
     
  9. Hi Phil,
    Maybe, but there is a low % of failure rate that will be encountered
    eventually, especially after 20+ years. I would rather "insure" for that
    possibility that risk a catastrophic failure and expensive repair.

    If one cap has 1mA leakage and another cap has 2mA leakage,
    the additional voltage across one of the caps may lead to its
    premature failure.
    It depends on the type of fault. If it is an excessive DC leakage
    current problem, the ballast resistors actually help by reducing
    the degree of voltage variation across each cap due to variation
    in leakage.
    eg. Design for 450 V across each cap, using 10W resistors.

    R = E^2 / P = 450 x 450 / 7.5 = 27 KOhm. These are available
    in 10W, or use two 12K or 15K 5W in series.
    I = E / R = 17mA approx. This would protect an excess leakage
    fault of 5 - 10 mA.
    Sorry, I don't agree. Every text book and article I have ever seen uses
    resistors. eg. The ARRL handbook represents decades of HAM
    experience, and their HV power supplies for valve transmitters
    always use them.

    Cheers,
    Glenn.
    (If you personally insult me again you will not receive any further
    replies from me,
    effectively halting this interesting objective discussion, which also
    benefits
    other people on the newsgroup)
     
  10. Phil Allison

    Phil Allison Guest

    "Glenn Baddeley ** STOP TOP POSTING !!!!!!!

    ** What does "maybe " mean ???

    Do you see the point or not ??

    I'll post it again: " If neither elector cap is faulty - then nothing bad
    can happen when wired in series and used at around 70% of rated voltage. "

    So far - you have simply not addressed this at all.

    You have yet to post any sort of case.



    ** Totally begs the question.


    ** That is a whole bunch of "ifs" - there.

    The caps in question have tested at less than 10 uA of leakage at 70 % of
    rated voltage.

    You are IGNORING facts and just making up numbers that suit you.



    ** I carefully described the type of fault.

    Go read my post again !!!!!!



    ** Shame how the 47 uF caps in question only have 2 to 10 uA of leakage.

    Shame how 17 mA of extra current is not acceptable in a valve amp or many
    other situations where such electros are used.

    Shame how the 7.5 watts of heat from the resistors would damage the
    electros if placed near them.

    Shame how high value WW resistors are notorious for failing open when
    subjected to continuous, high DC voltages.

    Shame what a load of crapology you post.


    ** Huh - so it is OK by you if one resistor fails open and the other then
    forces an electro overvoltage ???

    Do you realise you just totally contradicted yourself ???


    ** Ok - so you rely on rote learning and the superstitions of others.

    How intelligent - NOT !!



    ** You are posting mindless tripe - Glenn.

    Worse, you INSIST on top posting - the method only used by fools and
    wankers.


    ** Confusing the matter with irrational drivel is not my idea of
    "objective".




    ........... Phil
     
  11. Hi Phil,

    OK, you've tempted me to post again. You are quite entitled to
    your personal opinion, and I to mine. Most of your points are
    quite valid and arguable, but I do see my point of view as
    still plausible. Would anyone else care to throw in their
    personal opinion or experiences?

    Glenn.
     
  12. Geoff C

    Geoff C Guest

    Been involved in designs that used both approaches. Went away from the no
    resistor approach because techs used to get a shock from picking up the
    electronics module, needed to bleed away that charge (700V). Never saw any
    failures of caps in either situation, with a 20 year service life of the
    product. Never paid much attention to this 'problem' before this.
     
  13. Phil Allison

    Phil Allison Guest

    "Geoff C"

    ** The possible need for a bleed R is a separate issue to the ballast
    resistor one.

    The OP wanted to replace a single 500 volt cap with two in series - there
    was no bleed R in his case.



    ** Ditto.




    ............. Phil
     
  14. Terry Given

    Terry Given Guest

    Hi Glenn,

    I have personally seen capacitors explode violently when overvoltaged,
    due to failure of caps in a series/parallel string. Two 2200uF 450V caps
    in series, paralleled 18 times (36 caps). DC bus voltage ranged from
    560Vdc to 850Vdc. The cap bank was made of 6 identical sections, and
    used 450mm x 600mm x 2mm Aluminium sheets as +Vdc and -Vdc, with 1mm
    lexan sheet as insulation/spacing. Each individual section had a
    separate Aluminium plate to join the center points of all 6 caps, and
    each of these 6 plates were connected with 1mm^2 hookup wire. Each
    section also had a pair of (IIRC) 27k "balancing" resistors across the
    series/parallel array.

    The capacitors had been rendered faulty by the injudicious application
    of Methyl Bromide, which ate the cathode terminals. Within a 6-cap
    array, one cap going open circuit shifts the voltage division from 50/50
    to 60/40, which at max bus volts is 510V across one side, enough to
    break the caps. When two paralleled caps go open, one side carries 75%
    of the DC voltage. The resultant explosions tore the 2mm DC bus like
    paper, and in one case blew the drive doors open (mangling the cheesy
    door lock in the process). In all cases the 1mm^2 hookup wire fried.

    The "balancing" resistors had absolutely no effect, as their impedance
    was huge compared with the capacitor impedance when load current was
    being drawn. They only affect the voltage balancing at DC, which with a
    rectifier cap only occurs at NO LOAD. This was confirmed by measurement.
    Cap leakage current specs also showed the 27k resistors could not be
    relied upon to do any real DC balancing either, as the leakage spec was
    quite high - IIRC we needed to throw away many tens of W to get proper
    DC balancing. And the AC balancing is really the important part (hint:
    what does the input current of a rectifier/capacitor filter look like).

    OTOH I normally provide a discharge path of some description, usually
    R's and LEDs to provide a "look out" indicator. To tell the truth its
    more like a "for fucks sake dont stick a scope probe here, its live"
    indicator

    Cheers
    Terry
     
  15. Phil Allison

    Phil Allison Guest

    ( snip drivel)
    ( snip more crazy stuff)


    ** So they don't just do weird things to sheep in NZ ........




    .............. Phil
     
  16. Two 2200uF 450V caps in series, paralleled 18 times (36 caps).

    Wow, that's a pretty big cap bank.
    .... apparently a lot higher than the 10uA example quoted by Phil.
    Some HAM transmitters do throw away 10's watts to getter better
    load / no load regulation from a 1 - 2 KVA tranny. It concerns
    me that all that heat is generated near the caps.
    A good point, I've never seen this aspect mentioned anywhere before.

    Cheers,
    Glenn.
     
  17. Phil Allison

    Phil Allison Guest

    "Glenn Baddeley" = still bloody top posting !!!


    ** STOP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


    ** So how do you know it really is a good point ??





    ........... Phil
     
  18. Terry Given

    Terry Given Guest

    for a 660A 400Vac motor controller. One of the exploding cap banks was
    right beside me - thta was one of three paralleled 660A drives (a 1MW
    motor controller).

    AC motor controller design is all about capacitor lifetime - caps are
    expensive and bulky, and the wear-out mechanism effectively sets the
    operational life of the drive.

    The 660A drive range went from 100A - 1750A. When we designed the
    replacement 140A product, we moved the electrolytics out of the heatsink
    hot air path. This reduced cap temeprature enough to drop one of the 4
    pairs of caps, a significant saving ($50 each).

    Its easy to measure cap core temperature too - just have Hitachi make
    you a few dozen caps with inbuilt thermocouples. Seriously, peel back
    the top of the can (where the score lines are to control rupturing) and
    bung a thermocouple in there.

    That particular little Methyl Bromide problem killed well over
    $1,000,000 worth of product, and required some 1,700 caps to be swapped
    out in-situ, over a 5-day period. oops.
    IMO never use a part whos specs you cannot obtain. All caps are most
    certainly *not* created equal.

    that being said, mass manufacture is a very different beast from low
    volume manufacture, where its perfectly feasible to select-on-test,
    match parts etc.

    If a datasheet gives a min & max range, its often a good idea to allow
    for it. Sometimes the ranges are due to tester limitations (eg opamp
    input bias current), other times due to process/batch variations.

    These particular caps had a +/- 1mm spec on the terminal hole locations.
    And it occurred in practice, too.

    that should be a concern. Look up a decent datasheet for a WW resistor -
    quite often these can have a 200 - 300C temperature rise at full power.

    That sounds like a rather poor approach. I'd rather save the watts, and
    spend the money on a regulator. that being said, if the power
    consumption and temperature can be dealt with, then why not.

    Neither had I, but its pretty obvious really.

    Ultimately it doesnt really matter - cap leakage can pretty much be
    ignored and balancing resistors are pointless. If you make a
    series/parallel array, tie all the center points together. That way the
    (hopefully random) variations tend to cancel each other out. And leave
    yourself plenty of voltage headroom on the caps - Phils 30% spare is a
    good choice.

    But the OP wanted to know how to calculate the balancing resistors.....

    Cheers
    Terry
     
  19. Terry Given

    Terry Given Guest

    nope. a megabuck worth of gear. Of course once we figured out what the
    problem was and why, the actual cost to repair was much lower, but the
    total cost was still pretty high - around $100k.

    thats tiny compared to the potential cost of the problem. It was a
    $3,000,000 order, and the customer (rightly) demanded the problem be
    solved (and proven so) or the equipment removed and replaced (with a
    competitors) at our expense. The follow-on effects could have been
    horrendous.

    But a careful analysis of all (as in the last 30 years) product
    failures, along with dissection of every item of the order, showed no
    correlations at all - size, loading, environment, date manufactured,
    assembler, pcb batch numbers etc. The conclusion drawn from this
    analysis was that something happened to one third of this order. So we
    got the shipping records, and WHAM - every item within one container was
    faulty, all other items were fine. Five minutes later Methyl Bromide
    fumigation was identified as the culprit and the problem was solved to
    the customers satisfaction (although the rework continued for several
    more days).

    electron microscopy was used by the customer to examine the failed caps.
    Surprisingly the metallurgists who looked at them didnt identify
    corrosion or its byproducts, which would have helped.

    despite having sent senior engineers to placate the customer (who was
    spending half a billion dollars with them on another job), several days
    later the cap manufacturer admitted another customer had the exact same
    problem several years earlier. They had forgotten about it.
     
  20. in this case a resistor is good for a bleeder :) thats about all i
    ever use them for
     
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