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Quick Capacitor Question?

Discussion in 'Electronic Basics' started by Me, Mar 26, 2007.

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  1. Me

    Me Guest

    Hi,
    just wanted to know:

    If I can't find a ceramic of the value I need, can I place two elects of half value in parallel
    but reverse polls? Ie: + -
    ---¦ ¦---
    ---------¦ ¦---------
    ---¦ ¦---
    - +

    Like that. Or would it be equally ok to do it with like polls, (+ too +)?

    My meter reads it as Double the value either way round.

    Thanks
     
  2. Match the polarities. The parallel-crossed connection has one capacitor
    backwards. Electrolytics connected backward fail, sometimes loudly.
     
  3. ehsjr

    ehsjr Guest

    When you put polarized electrolytics in parallel,
    keep the polarities the same. Whether you can use
    such an arrangement in place of a ceramic is a different
    issue. If a non-polarized capacitor is needed, then
    the two electros in parallel scheme is not the right
    thing to do.

    Ed
     

  4. There are quite a few non polarized electrolytics available.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  5. They are the best solution, but polarized electrolytics can be wired in
    series for half the capacitance value. I think the capacitor that is
    reverse biased just becomes leaky so the correctly biased one charges up.
    It would probably be better to add diodes across the caps so only about 0.7
    volts reverse bias is ever applied.

    Paul
     
  6. I did a quick LTSpice simulation to compare two 10 uF capacitors in series,
    with diodes, to a single 5 uF capacitor, into the same load, and there was
    virtually no difference. I was surprised to see that the diodes only
    conducted during the first few cycles, and then the two capacitors
    alternated charging and discharging, with correct polarity voltages on
    both. The ASCII file follows:

    Paul

    Version 4
    SHEET 1 880 680
    WIRE -112 144 -128 144
    WIRE 64 144 -112 144
    WIRE 144 144 64 144
    WIRE 256 144 144 144
    WIRE 352 144 320 144
    WIRE -128 192 -128 144
    WIRE 352 224 352 144
    WIRE 64 240 64 208
    WIRE 144 240 144 208
    WIRE 144 240 64 240
    WIRE 64 256 64 240
    WIRE 144 256 144 240
    WIRE 144 320 64 320
    WIRE -128 384 -128 272
    WIRE -80 384 -128 384
    WIRE -48 384 -80 384
    WIRE 64 384 64 320
    WIRE 64 384 32 384
    WIRE -80 432 -80 384
    WIRE 352 432 352 304
    WIRE 352 432 -80 432
    FLAG -128 384 0
    FLAG 64 384 Vout
    FLAG -112 144 Vin
    FLAG 352 144 Vout2
    SYMBOL polcap 48 144 R0
    WINDOW 3 -45 37 Left 0
    SYMATTR InstName C1
    SYMATTR Value 10µ
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=35 Irms=44m Rser=2.8 MTBF=2000 Lser=0 mfg="Nichicon"
    pn="UPR1V100MAH" type="Al electrolytic" ppPkg=1
    SYMBOL polcap 80 320 R180
    WINDOW 0 24 57 Left 0
    WINDOW 3 24 8 Left 0
    SYMATTR InstName C2
    SYMATTR Value 10µ
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=16 Irms=35m Rser=2.8 MTBF=1000 Lser=0 ppPkg=1
    SYMBOL diode 128 256 R0
    SYMATTR InstName D1
    SYMATTR Value MURS120
    SYMBOL diode 160 208 R180
    WINDOW 0 -37 37 Left 0
    WINDOW 3 -101 0 Left 0
    SYMATTR InstName D2
    SYMATTR Value MURS120
    SYMBOL voltage -128 176 R0
    WINDOW 3 -151 134 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value SINE(0 12 60 0 0 0 100)
    SYMBOL res 48 368 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 100
    SYMBOL cap 320 128 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C3
    SYMATTR Value 5µ
    SYMATTR SpiceLine V=50 Irms=0 Rser=0.004 MTBF=0 Lser=0 ppPkg=1
    SYMBOL res 336 208 R0
    SYMATTR InstName R2
    SYMATTR Value 100
    TEXT -280 506 Left 0 !.tran 100m startup
     
  7. Eeyore

    Eeyore Guest

    No. You must place them in series, + to + or - to - (doesn't matter which way).

    Electrolytics have *very* different characteristics to ceramics though. That's quite likely to be a
    problem. What's this capacitor for ?

    You'd be far better off using a plastic film capacitor if you can't get the right value ceramic.

    Graham
     
  8. But the question was incomplete. The poster is talking about not having
    a ceramic capacitor of the right value, but by the time you have the need
    for such large capacitances that you find in electrolytics, one would rarely
    be talking ceramic capacitor. Until we have a capacitance value from
    the original poster, his "solution" is irrelevant because there may be
    all kinds of other paths to follow to get to the end game.

    Michael
     

  9. The OP asked about using electrolytics, which implies that the
    capacitance is high enough that he can't use Ceramic, Mylar or other non
    polarized capacitors.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. But he said "If I can't find a ceramic of the value I need..." which
    is ambiguous.

    Michael
     
  11. Rich Grise

    Rich Grise Guest

    I don't get your original point - ceramics and electrolytics are usually
    several orders of mangnitude different in value.

    What value of cap do you need? Great big huge ceramics might go up to
    maybe 1 uF , and I've never seen an electrolytic that small.

    What are your cap values, and what are you trying to accomplish?

    And, yes, to make 'lytics work on AC, they go in series, back-to-back
    (i.e., + to + or - to -).

    Thanks,
    Rich
     
  12. Have you seen any of the high value ceramics? They are made, but are
    not easy to find. I've used a number of them above 1 µf. In that range
    the non polar electrolytic is probably a better choice. We needed 10
    µf, in a unit built to last 20 years, or more so we used SMD ceramics in
    all of the AGC time constant filters.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  13. Eeyore

    Eeyore Guest

    I agree.

    Not at all. Electrolytics are available with values as low as 0.1uF.

    Graham
     
  14. Eeyore

    Eeyore Guest

    Electrolytics go as small as 0.1uF. Quite why you'd want one that small I don't know though.

    Graham
     
  15. Ross Herbert

    Ross Herbert Guest


    Unless I am missing something I would have thought that the reason the
    OP enquired about "ceramic" caps was because of the characteristics
    these possess. His suggestion that using electrolytics might be a
    possible solution doesn't seem right to me since their characteristics
    are nowhere near the same. He appears to want a cap value of fairly
    large value hence the query about using electro's. His drawing shows
    them connected in par'l but with polarities opposing which could only
    be of use where very low voltage was applied, otherwise they would be
    destroyed when polarity was reversed to either of the electro's (with
    higher voltages).

    If he genuinely needs a capacitor with the characteristics of a
    ceramic and can't find one large enough, then why not parallel them to
    get the desired value?
     

  16. It is done in some designs, but some people don't really know what
    family to chose from, so they make wild guesses as to what type of
    component they need.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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