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Answering Machine - Power Backup?

Discussion in 'Electronic Design' started by Len Krauss, Oct 9, 2005.

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  1. Len Krauss

    Len Krauss Guest

    I have a phone answering machine / cordless phone combo unit which has no
    backup battery. My area has a fair number of short power interruptions. When
    they occur and power is later restored, the unit's day & time-stamp clock
    (for subsequent messages) is inaccurate. There is no function available to
    reset it remotely. So, if I'm away for a month, and there've been several
    power blips, the day&time stamp on messages I retreive remotely is

    The unit is powered by AC to DC converter block. Input: 120v, 60hz, 8.5w.
    Output: 9vdc, 500ma.

    Seeking ideas on possible ways to have backup power to overcome this

  2. Jim Thompson

    Jim Thompson Guest

    APC has some really cheap UPS's. I use their ES-350 to backup my home
    wireless phone system.

    ...Jim Thompson
  3. For something that small, you should be able to hang a rechargeable
    battery across the supply's DC output. Since it will be always
    charging, you want it to supply a low charging current so the
    batteries don't overcharge. That implys a resistor in series with the
    battery would be a good ides. But then you need to parallel the
    resistor with a diode, so that at power failure, the battery can power
    answering machine through the diode with little voltage drop.

    I think an experiment is in order, to determine how low the voltage
    can go and still have the machine operate correctly, to decide if you
    need 6 or 8 cells.

    You might look for some low cost AA rechargables and one of these
    battery holders:
  4. How about buying an uninterruptible power supply...


  5. Pooh Bear

    Pooh Bear Guest

    The most cost effective answer is to buy an answering machine that does what you

  6. Pooh Bear

    Pooh Bear Guest

    At several times the price of a decent answering machine with internal backup ?

  7. Ian Stirling

    Ian Stirling Guest

    Do you have a soldering iron and clue?
    My first attempt would be to take 5 D cells (7.5V) and two diodes.
    Put the diode in series with the 5 D cells and the power supply, with the
    other ends of the diodes going to the plug.

    This should last several hours downtime.
  8. keith

    keith Guest

    If one's worried about it, have the telco take care of it. I've
    disconnected from land-lines, so my cell carrier takes care of such
  9. kell

    kell Guest

    positive from
    power supply
    |_________________positive to machine
    five or six alkaline
    D cells in series
    _______________________|__________________negative to machine
    negative from
    power supply

    the cathode (striped end) of the upper diode in the diagram connects to
    the cathode of the lower diode in the diagram. Sterling suggested five
    cells (7.5 volts) to make sure the battery voltage is lower than the
    power supply, so that the batteries won't direct current into the
    machine during normal operation. But your machine needs to be able to
    run on less than 7 volts in that case. If you find you need six cells
    to get the machine to run, an extra diode or two at the positive
    battery pole might be called for to drop the battery output to below
    that of the power supply.
    For John Popelish's circuit using rechargeable batteries, omit the
    upper diode and put a resistor in parallel with the lower diode.

    If you are using google, hit the show original option to see the
    diagram in a non-proportional font.
  10. If you want to go 'first class', Maxim (and others, I'm sure) make
    controller chips that handle switchover between adapter input and
    battery. They handle battery state monitoring, charger control (if you
    want to make it a NiCad or NiMH). The p/n I have handy is a Maxim
    MAX1538. This might be overkill (it handles two batteries), but they
    might have a similar controller for a single battery.
  11. Al

    Al Guest

    All but one of my UPSs come from my town's recycling center for free. Of
    course I test them before I use them. I even replace the batteries in
    some of them. The latest one I got, is good for 200VA for about 5
    minutes. Should keep a phone happy for a long time.

  12. Rich Grise

    Rich Grise Guest

    A whole UPS, for a telephone? Since the output of the wall wart is
    nominally 9V, I'd just stick an ordinary 9V "transistor" battery
    in parallel with it, with a blocking diode. I'd think an alkaline would
    last almost its shelf life in that kind of application. (after having
    plunked down fifteen bucks for a set of NiMHs, and having to recharge them
    almost weekly, I'm seriously impressed with the lifetime of alkalines!)

    Good Luck!
  13. Jim Thompson

    Jim Thompson Guest

    It's a wireless system, so the base station is of significant power.
    I also run the cell phone chargers at that desk location.

    They're rare, but AZ down-times have been known to be as long as 24
    hours :-(

    ...Jim Thompson
  14. Spajky

    Spajky Guest

    IMHO those AC to DC converter block gives around 16V
    unregulated output w/o load (if not switching type already
    regulated!); when such a device is connected to it, it drops few V &
    appliance has internal voltage stabilizer to 9V. I had a case with
    similar setup @ my neighbour years ago I solved so:

    I fixed a pack of Ni-Cd acu batteries 7x1,2V/800mAh nominal
    (when charged fully gives 10V) to the AC to DC converter block &
    supplied the "pack" thru a 1N4007 & 68ohm/1W resistor in series from
    that AC/DC converter (it was an ordinary one, not switching type!), so
    it was charging the pack with max.80mA long term (half of that took
    the appliance when on hook, so constant charging was only other 40mA)
    The power for apliance than was taken from the battery pack ... it was
    an analog Panasonic answering machine (cassette type) with home
    wireless phone ...

    It was working Ok for years ...
  15. Guest

    How short is short? If you're in the house when one of these happens,
    do the lights go out for less than a second, or do they stay off for
    longer than that? If it's less than a second, you might try hanging a
    big electrolytic capacitor across the power input to the answering
    machine. Use one of a few thousand microfards (uF) with at least a
    25 V voltage rating. The capacitor is polarized and usually the
    negative lead is marked - make sure you hook it up the right way. This
    is the same idea as hanging a rechargeable battery on the power input,
    except the "battery" has much lower capacity.

    If the above doesn't make any sense, buy one of the "fat power strip"
    APC UPSes for about $30 or so and plug the answering machine into that.

    Matt Roberds
  16. Len Krauss

    Len Krauss Guest

    Some great ideas here guys. Thanks!

    I should have been clear that there are some less frequent but longer power
    outages (a few hours) as well as blips. For that reason, I'm leaning towards
    a UPS. I can get an low cost APC BK280 refurb with specs as follows:

    Output power capacity 280 VA, 180 Watts
    Replacement battery cartridge RBC2
    Typical backup time at half load 18.2 minutes
    Typical backup time at full load 6.7 minutes

    On the assumption that the cordless phone's batteries are fully charged when
    a long outage begins, I'm thinking that even less than the indicated power
    (see below) would be drawn to keep the answering base alive until the
    batteries get weak, a which point more power would be drawn for charging.

    What's your guesstimate of backup time given the very light load I've
    indicated ???

  17. Guest

    Since this involves rechargeable batteries, all "marketing" numbers can
    be halved, making the half load number something like 9 minutes. The
    phone's power supply takes 8.5 W, so you'd expect something like (90 W /
    8.5 W) * 9 or about 90 minutes. In reality it will be a bit longer than
    this. Make sure nothing else gets plugged into the battery-backed
    outlets on the UPS.

    Matt Roberds
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