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Reason for using A/C power adapters for telco equipment?

Discussion in 'Electronic Repair' started by Thomas Tornblom, May 31, 2007.

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  1. What is the reason that many/most telco related equipment like analog
    modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
    instead of D/C adapters?

    I'm hunting down the mass of inefficient power adapters I have, and
    I'm replacing many of the hot running transformer types with more
    efficient switched ones, and I now have two that are A/C, the SIP
    adapter and the DSL router/modem.

    I have checked around my stash of old stuff, and with the exception of
    an old ZyXEL analog modem, all of the modems have A/C adapters, and
    also the DSL modems and SIP adapters, basically anything that connects
    to the phone company or have analog phones attached.

    Is it perhaps an insulation issue or added safety from feeding high voltage
    to the telco switch?

    Thomas
     
  2. Kind of doubt it's anything that subtle.

    One possibility is that it's easier to generate positive and negative voltages
    using an AC input than a DC input. So, where there is analog circuitry,
    it makes sense to go with an AC input. You get +/-VDC almost for free.

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  3. Price and reliability.



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  4. hr(bob)

    hr(bob) Guest

    Reliability
     
  5. me

    me Guest

    Naw, price. Both are reliable enough that you will most likely replace
    the device before the power supply. So shell out 2 yen/peso/etc for
    transformer times 8 million or 15-20 for same. Charge retail for
    something in the middle and use the cheap one...
     
  6. Franc Zabkar

    Franc Zabkar Guest

    Dialup modems need to generate +/-12V for the RS232 serial interface.
    This can be done with a voltage doubling rectifier and an AC supply.
    The alternative is to use a DC adapter with a +5V RS232 interface IC
    (eg MAX232) which generates its own RS232 supply rails.
    Look inside these devices and I believe you will find switchmode PSUs.
    If so, then you will gain no significant efficiency benefit from an
    external switchmode DC adapter.
    If the device requires an AC adapter, then it can usually tolerate a
    DC adapter of either polarity without damage ... unless there is a
    transformer on the input.

    - Franc Zabkar
     
  7. rebel

    rebel Guest

    No, the isolation comes from the transformer regardless of whether the device
    delivers AC or DC.

    Think price/volume, and then consider that you can't get the connector polarity
    wrong with (isolated) AC.
     
  8. Yes, this was my thought also, but the DSL and SIP stuff have no
    RS-232 type serial connection. The SIP adapter needs a ringer
    generator, which is on the order or 80V AC, and with selectable
    voltage and frequency, so I see no immediate gain from feeding it AC.
    Unless the transformers, as in my case, appears to be undersized and
    runs fairly warm.
    You are correct that many of them use switchmode PSU:s internally. I
    connected a 12V DC switching adapter to the DSL modem before noticing
    that the original adapter was AC, and it worked fine. I guess 12V DC
    may have been on the low side though.

    I was just curious why it appeared that anything telco related used
    AC, while almost everything else is DC.
    Thomas
     
  9. Lionel

    Lionel Guest

    There are several reasons why modems (whether POTS or xDSL) usually
    use transformer-based, AC output plugpacks:

    (1) Modems need multiple power rails for the digital & analog
    circuitry, typically +5V for digital & +/-5V or +/- 12V for the analog
    circuitry. Supplying AC & regulating onboard only requires 2 way
    connectors + cable instead of a 4-6 way cable & connectors, which are
    much more expensive & fragile,

    (1a) Switchers are much noisier than transformer supplies, which is
    bad for analog I/F circuits.

    (2) Onboard regulators are much easier to shield,

    (3) AC plugpacks are cheap, "off the shelf", stock items, but
    multi-rail DC packs are usually custom-order units. (Eg; for some
    Cisco xDSL modem+routers.)
    It's much less relevent than the reasons I've listed, but it /is/
    harder to get telco certification for anything powered by a switcher,
    due to line/power isolation rules - at least in my country,
    (Australia), which is 240VAC mains, & the toughest phone-line
    isolation rules (2.5KV, IIRC) in the world.
     
  10. James Sweet

    James Sweet Guest


    I think Sam's response is probably pretty close. At some point I had an
    external modem which had an AC output adapter, on DC the modem would light
    up and appear to work, but it couldn't communicate with the PC. In that
    case, apparently they were using the AC to get the + and - voltages for
    RS-232.
     
  11. James Sweet

    James Sweet Guest


    I don't see how price is affected one way or another. You either put the
    rectification and filtering in the adapter, or you put it in the device,
    since the two are bundled it's the same either way.
     
  12. me

    me Guest

    The poster asked why they use mains transformer power supplies versus
    switching power supplies.
     
  13. Ross Herbert

    Ross Herbert Guest


    I am convinced that Lionel is on the right track. Any item of telecom
    electronic equipment which is powered from the mains supply must use a
    properly certified and tested power supply. The developer of the
    telecom device prefers not to have to go to the trouble and expense of
    performing this step and therefore leaves that up to the power supply
    developers.
     
  14. Read it again.
     
  15. Gary Tait

    Gary Tait Guest

    My telco provided broadband modem (Elastic Networks) has an electronic
    SMPS supply, as did my old D-link router. My Linksys router and SMC
    switch have iron trasnformers. My NAS has an SMPS.

    Dialup modems would have AC to make it easier to make dual voltages for
    RS232. Otherwise linear supplies tend to produce less "noise" than
    SMPS.
     
  16. Marra

    Marra Guest

    Coz they are cheap.
    Also a seperate adaptor is often easier to get through approvals.
     
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