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Are there any 24VAC to 3.3VDC/1.5A SMPS?

Discussion in 'Electronic Design' started by Joerg, Jun 5, 2010.

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  1. Joerg

    Joerg Guest

    Gents,

    Does anyone know of a little AC/DC switch mode converter module that can
    take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
    qty would be nice, availability in the US would be even nicer.

    All I see is 24VAC to 12VDC and placing two in a row costs effiency.
    Sure, I could roll my own again but this time we want to avoid NRE and
    agency hassle (EMC et cetera).
     
  2. Guest

    Check out Intersil's new buck regulators. They'll go up to 42V and up to 10A,
    IIRC.
    Regulatory problems, too.
     
  3. Joerg

    Joerg Guest


    Yes, I'd could roll my own buck with some automotive chip. Sorry, forgot
    to mention one minor detail: Must be isolated, or should be (we may be
    able to live without if it really has to be).

    Thing is, with one of those ubiquitous 24VAC -> 12VDC switchers plus
    another buck we'd be at 60% overall efficiency or less and we can't
    quite afford that.
     
  4. Ross Herbert

    Ross Herbert Guest

    :Gents,
    :
    :Does anyone know of a little AC/DC switch mode converter module that can
    :take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
    :qty would be nice, availability in the US would be even nicer.
    :
    :All I see is 24VAC to 12VDC and placing two in a row costs effiency.
    :Sure, I could roll my own again but this time we want to avoid NRE and
    :agency hassle (EMC et cetera).


    I don't think there are any 24Vac input converters off the shelf - almost zero
    demand I would think.

    Why not use one of the 6 - 10W, 18-36V input DC-DC converters (eg. V-Infinity)?
    Provide a bridge rectifier and bulk filter cap in front of it. A few bucks for
    the BR and cap would keep it under $20 per unit in 1K quantity.
     
  5. Hammy

    Hammy Guest

    You could just add a bridge rectifier and a filter cap. This would
    widen your selection to several hundred of the SMPS's that have a max
    input to 36 or 75 Vin. Cheapest one I could find with a quick search
    is

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-3306-ND
     
  6. 24VAC is not a very common voltage (24VDC is much more popular in
    industrial applications).

    I second the suggestion of adding a bridge and filter cap upstream of
    a converter, along with maybe a CM choke and some film or ceramic
    caps.

    You didn't specify whether you needed isolated or not, that's going to
    make a difference.

    When you read the detailed app notes, most of the modules require some
    filtering ahead of them to meet EMC requrements anyway (small caps and
    CM choke maybe), so it's not really "extra" work.



    Best regards,
    Spehro Pefhany
     
  7. Joerg

    Joerg Guest

    Not enough space :-(

    Plus on this one it could get us into regulatory hot water.
     
  8. Joerg

    Joerg Guest


    That's solution B, if I really, really have to. In this case that would
    require some regulatory scrutiny though because the input side would now
    not be "taken care of by module" anymore. If that wasn't an issue I'd
    just roll my own because then I won't have to deal with those dreaded
    50C or 60C derating issues that "professional" switchers often exhibit.
     
  9. Joerg

    Joerg Guest


    That's what Ross suggested and it's a solution I had planned for, but
    only in case there really is no other option.


    We could probably get one from CUI for under $15, offsets the cost for a
    good 105C/5000h or similar electrolytic up front.
     
  10. Joerg

    Joerg Guest

    It used to be very popular in Europe but I haven't seen any industrial
    switch gear there in some 15 years. All the power contactors for huge
    motors and such were run on 24VAC. Up to 30-50A. Larger ones still had
    24VAC "control contactors" but they'd drive huge 230VAC contactors, the
    kind that sounds like a gunshot when they engage.

    While studying for my masters I assembled gear like that because those
    jobs paid quite well, compared to bussing tables :)

    I still have a discarded supply transformer for a cabinet that was sold
    to me at scrap metal price. 24V, one kilowatt (!). That was from an
    older cabinet two stories high where you needed a ladder to get to some
    of the contactors. The secondary of that transformer consists of
    soaked-cloth insulated copper bar that was formed around the primary
    using a wooden mallet.

    Looks like I hafta :-(

    That's why I posted, was hoping for a silver bullet that lets me avoid it.

    Yes, sorry, should be isolated but we could make it work without. AC/DC
    converters are generally isolated though because they use bridge rectifiers.

    Yes, and we'd put one on there anyhow because it affords some surge
    protection.
     
  11. Hammy

    Hammy Guest

    We responded simultaneously. I had my secretary proof read for grammar
    and spelling.This explains the minor discrepancy in time stamp She's a
    bit slow but I keep her around because she's a good lay.:)
    Yea good 105c caps are pricey particularliy for 250VDC or better. I
    just paid 7 bucks for two of these for a off line SMPS.

    http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=P11629-ND

    It's hard being a mad scientist on a budget.;>)
     
  12. Guest

    I assumed 5k hours (@105C), not 5000 henries. ;-)
     
  13. Joerg

    Joerg Guest

    Now, now ... :)

    That must be a SMPS in disguise :)


    500mOhm Rdson can get a bit toasty. But I only need a 63V cap.
     
  14. Joerg

    Joerg Guest


    I always thought it was meggeehoitzes :)
     
  15. Even mutant sea bass run $25/lb.


    Best regards,
    Spehro Pefhany
     
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