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Electrolytic caps in series

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

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  1. Phil Allison

    Phil Allison Guest

    "The Real Andy"
    Phil Allison

    ** What type do you use that can stand 250 volt DC indefinitely ??

    High value resistors, of all types, seem to have a very high failure (
    failing open) rate after a few years when subjected to continuous DC above
    about 100 volts.




    ........... Phil
     
  2. Terry Given

    Terry Given Guest

    any ideas as to the mechanism? come across any decent app notes?

    I presume that as high value Rs the peak power dissipation would still
    be pretty low - I've seen lots of low-value resistors fail after being
    abused in this way, eg 0603 10R 12V gatedrive resistor.

    what about electromigration? wouldnt that tend to bridge out the
    serpentine cuts, thus reducing R

    what about VR37s, they are designed for very high voltage pulses. we
    used pairs of 470k VR37 resistors in series for DC bus and line voltage
    sensing, +/-300V - 500V about earth so 150V-250V per resistor. never had
    a single failure over perhaps 10 years and 40,000 products.

    We also used 100k PR02's across each half of the DC bus cap bank, so
    about 0.9W continuous at 600V/2 and 1.6W at 800V/2(short term during
    regen). dont know about failures there though, as power electronics
    failures tend to trash the entire area, so its quite possible. the
    dissipation is a bit high at 800V, but luckily the nearby electrolytics
    will provide some cooling :)

    At one stage I tried damping an EMI filter with 1R PR02s in series with
    the 1uF caps. If turned on at an appropriately high line voltage (which
    one phase always is in 3-phase systems) the PR02 emitted a bright flash
    of light and went open circuit. probably because (400V*1.4)^2/1R = 320kW
    peak pulse power. We found that by observing the flash...wtf?! A carbon
    composition resistor ate the peak pulse though.

    Cheers
    Terry
     
  3. Phil Allison

    Phil Allison Guest

    "Terry Given"

    ** The resistors that fail are operating within published specs.

    Ergo - the "mechanism" is bad manufacture.

    Not much mention of that in any "app notes'.




    ** What - without the right Visa ??



    ** Values start at 1M ohm - useless.





    ............ Phil
     
  4. Terry Given

    Terry Given Guest

    I too suspect this is largely the reason.

    I once had a batch of 0.1% resistors from vishay that were off by up to
    25%. it was a ups DC balancing circuit, and it didnt like it at all.
    Only 20-30 units were affected.

    vishay were great, wrote me a nice report. basically contaminated
    material wrecked part of a batch, but the effect was triggered by
    high(ish) temperatures so they all measured ok at the factory, and
    through our genrad pcb tester, but once the unit heated up to 50C or so,
    they drifted like crazy.

    in general my experience is that nice cheap components are quite often
    unreliable shit, and the only way to tell is to wait a year or two. e.g.
    every company I've ever worked for has had a cheap ceramic bypass
    capacitor horror story to tell.


    lol. other than its existence, I dont know much about electromigration,
    but it appears these guys do:

    http://www.theo-phys.uni-essen.de/tp/forsch/krug.html

    whaddya know, its kind of analogous to crack propagation, so certainly
    could cause open circuits. AIUI its an issue at IC level because the
    small dimensions result in high E field strengths. The same would occur
    in a serpentine resistor at high voltage.
    I have 100pcs 470k sitting in front of me.

    1998 PA08B databook:

    VR25 100k - 15M 1,5,10% E12/E24 1,600Vdc peak
    VR37 100k - 33M 1,5% E24/E96 3,500Vdc peak
    VR68 100k - 68M 1,5% E24/E96 10,000Vdc peak
    PR02 0R33 - 1M 1,5% E24/E96 500Vdc peak

    Cheers
    Terry
     
  5. Phil Allison

    Phil Allison Guest

    "Terry Given"
    Phil Allison wrote:



    ** My sources

    1. Farnell in One cat.

    2. Philips General cat 1978.



    ** I believe you - but that is not the simple answer.


    The simple one is to use a few cheap 0.5 or 0.75 watt MF resistors in
    series - so each cops less than 100 volts and runs cool.




    ............ Phil
     
  6. Terry Given

    Terry Given Guest

    I "inherited" a full set of philips 2000 databooks (many are 1998). I
    regret throwing out so many databooks in the past :(
    absolutely, and with smt its trivial to put down as many as required
    (although per-part power dissipation also drops). The overall footprint
    is not dissimilar, but the inductance is a whole lot lower, as is the
    capacitance. designing to cope with single component failures is a lot
    easier (eg 6 aint much more than 5, but 2 is a lot more than 1) too.

    Cheers
    Terry
     
  7. Did the same thing with a 500V DC distributed supply system. Was much
    easier to simply use a few SMD resistors in series as the ballast
    resistors, rather than having to source high voltage ones. Too bad
    there wasn't the same choice on the caps, volume realestate was at a
    premium (more so than PCB realestate) so we had to run tight margins
    and had little selection available when it came to the cap vs voltage
    vs volume vs height tradeoff.

    Dave :)
     
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