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Why a no-no to use 40/50KHz high V electros for 50/60 Hz use?

Discussion in 'Electronic Repair' started by N Cook, Apr 23, 2007.

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  1. N Cook

    N Cook Guest

    I can understand the other way round being a no-no but what is wrong with
    using switch mode power suply type electrolytics for mains smoothing of high
    voltage rails.?
     
  2. Guest

    for mains or hv rails?

    If you want to use smpsu caps on mains there are 2 problems:
    1. the lytic caps aren't bipolar
    2. the caps also arent X rated (for L-N connection) nor Y rated (For L-
    E connection)

    If you meant using them for hv dc rails, why not?


    NT
     
  3. Guest


    absolutely NO PROBLEM.
    Usually the only time this kind of thing becomes an issue is if the
    electrolytic is expected to handle and extreem amount of ripple such
    as the rather physically big 4.7 uf @ 50V electrolytics used in some
    monitor deflection circuits.... if you try to substitute an equal
    value 4.7uf @ 50V normal size (much smaller) electrolytic it will
    OVERHEAT and burst because the "power factor" rating of the smaller
    cap is not adequate.
    For what you described, I have used those kinds of electrolytics in 60
    Hz filtering applications many times. NO PROBLEMS.
    electricitym
     
  4. N Cook

    N Cook Guest

    For more clarity, I was meaning the 300V or 400V electrolytics used after
    rectifying the mains and before 40/50KHz chopping to drive a smps
    transformer.
    Then using those on the secondary side after rectification, of a totally
    50/60Hz mains transformer, with high voltage secondary. I thought the SMPS
    ones had a different formulation or something to allow for high frequency
    current demand.
     
  5. Arfa Daily

    Arfa Daily Guest

    The high voltage electrolytics in SMPSs are not specced for 40/50kHz. That
    would be the low voltage ones on the secondary, after the high voltage DC
    has been chopped at 50kHz, and transformed down. The high voltage one(s) are
    merely the smoothers for the primary DC rail, derived from the normal line
    frequency (50/60Hz) input. There is absolutely no reason at all that these
    primary smoothers cannot be used for exactly the same purpose on normal line
    frequency transformer-derived supplies, such as might be found in valve
    amplifiers, and indeed, I have used them for such purpose on many occasions.

    Arfa
     
  6. N Cook

    N Cook Guest

    I've never actually weighed or measured any but I always thought that V for
    V and uF for uF that the ones from SMPS were always lighter than the the
    more general application ones. Therefore implying something different about
    their construction.
     
  7. Arfa Daily

    Arfa Daily Guest

    Whether or not they are made lighter ( and I suspect that in a lot of cases
    it's that they are made smaller to better fit switchers and modern consumer
    equipment physically ), if you think about it, they are just working at 100
    / 120Hz fundamentally, as their primary ( ha ! ) job is just to smooth the
    main rail after the bridge, and the ripple from that bridge is just line
    frequency x 2. Whether or not there is a transformer before the bridge, does
    not have any basic effect on this. I can't immediately think of any other
    differences that would cause the caps to *need* to be any different, but I
    would agree that they do tend to be small compared to those found as
    original in amps.

    It might just be a cost thing. As these amps are not generally short of
    internal space, perhaps it is more economical just to use a cheap 'big' one

    Arfa
     
  8. Guest

    Reusing 100Hz reservoirs as 100Hz reservoirs is absolutely fine.


    NT
     
  9. I'm not sure I go along with your ripple argument here.

    The ripple in a SMPS input capacitor has two components. The AC current
    into it at 100/120Hz, typically rather peaky as the input bridge
    conducts for a fairly small angle when the mains exceeds the
    capacitor voltage. The current being taken out of it goes into the
    SMPS transformer, typically a sawtooth (with gaps) at the SMPS frequency.
    As current in equals current out of the capacitor (when averaged),
    the ripple thus has roughly equal components at both 100/120Hz
    and the SMPS 40/50kHz. Thus this capacitor does need to be
    rated for SMPS frequency ripple current.

    However, I agree that a SMPS rated input capacitor should still be
    fine for use generally at 100/120Hz (so long as its ripple current
    rating is adequate for the application).

    Regards,
    Mike.
     
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