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Need 28 VDC UPS for a phone PBX Control Box

Discussion in 'Electrical Engineering' started by Beachcomber, Oct 12, 2005.

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  1. Beachcomber

    Beachcomber Guest

    I have a PBX phone system that uses the somewhat odd voltage of 28 VDC
    at 1 amp. The current supply is a giant wall wart AC-DC convertor,
    but this has no standby capacity if the AC fails, even for a second.
    It doesn't fail often, but when it does, I spend about an hour
    reprogramming the thing from scratch.

    I was planning on buying a regular 120 VAC in 120 VAC out UPS and
    plugging in the wall wart to the UPS, but then I thought it might be
    possible to simplify the circuit if I had some sort of UPS that puts
    out 28VDC at 1 amp directly and has some sort of standby/rechargeable

    The problem is this seems to be an odd voltage and after numerous
    searches on the Internet, I don't seem to be able to come up with
    anything that closely meets these specs. Budget is a consideration.
    I can afford about $100 or so. I've never seen a 28 V battery
    though. Two 12 Volts in series would give 24 volts, almost, but not
    quite enough.

    Does anyone have any suggestions on the best way to (cheaply) deliver
    28VDC at 1 AM with a UPS backup function? Thanks in advance...

  2. Beachcomber

    Beachcomber Guest

    Thanks to all for the suggestions. I live in the US and am a bit
    confused over the term "certification". I assume it is something
    similar to a UL (Underwriters Labs) listing meaning that it has passed
    certain safety standards. The PBX control unit I have must have some
    DC to AC invertors because it does generate ring voltage (about 90
    VAC) for the phone extensions.

    The Office Max and Office Depots stores are selling these super cheap
    $20 UPS units with a very limited VA rating (I think it was 150 VA or
    so). I bought one of these when they a bit more expensive about two
    years ago and was very disappointed with its performance. Just
    recently, this UPS spontaneously failed and the battery would no
    longer hold a charge. The worst part was that the failure mode
    resulted in a dead output to the protected circuit, even after the
    main AC power came back on. I would expect it to keep supplying
    power, but set off the warning beeper or something. In this case,
    this particular UPS caused more trouble than it was worth.

  3. ehsjr

    ehsjr Guest

    Simplest is to buy a UPS and plug the wall wart into it.

    The idea below requires a little work:
    The PBX probably would run fine from 24 volts. If it will
    run reliably from 24 volts, the solution is simple enough:

    --------- D1 D2 -----
    |Existing |+28--->|--->|---+--------------| PBX |
    |Wall | | | |
    |Wart | ----- | |
    | | | 24V | | |
    | | | Batt| | |
    | | ----- | |
    | | | | |
    | |----------------+--------------| |
    --------- -----

    D1,D2 = 1N5400; Batt = 2 12V Gel Cells

    The two diodes drop the voltage to protect the batteries
    from overcharge, and also prevent them from pushing
    current through the wall wart when mains power drops. The
    wall wart and diodes will keep the batteries charged. You
    would have to measure the wall wart output to verify that
    it produces 28 volts. If it is unregulated, it could produce
    significantly more, which would overcharge the batteries.
    However, if it supplies well over 28 volts, that could open
    up the possibility of using an additional 6 volt battery for
    a total of 30 volts.

  4. TimPerry

    TimPerry Guest

    its possible this UPS sacrificed its life to save your expensive phone
    system :)

    one consideration in rigging up a direct battery back up is it will void any
    warranty. a service tech making a call will go what the heck is this?

    an additional problem that could crop up is hum in the audio if you are
    charging the battery from an unregulated supply.

    one possibility, if warranty is not an issue, is to add a large capacitor to
    the 28 VDC point. this might, depending on capacitor size and load, be
    enough to unplug the unit briefly while swapping UPS units without losing
    your programming.

    you might want to check into the phone system to see if it has an internal
    memory battery that has failed or is missing or something.
  5. The proper float voltage for a '12-volt' lead acid battery is
    about 13.5 to 13.8 volts depending on the exact chemistry
    of the battery. I suggest using only 1 diode.
    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  6. Beachcomber

    Beachcomber Guest

    Again thanks to all contributers. I wound up doing the Office Depot
    route today and bought an APC 200 watt UPS. It seemed like the
    easiest and least expensive solution after considering the discussion

    It cost $39.95 which I think is a pretty good price. In the past two
    years it seems that the commodity UPS technology has advanced. This
    unit appears to have more sensing capacity for the condition of the
    battery, a "soft" on-off switch and more status lights to display if
    the battery is low (or needs to be replaced).

    One reader suggested checking for an internal battery on my phone
    system. Actually, It had a small 6 volt lead-acid battery that is
    supposed to keep the memory alive during power outages. I replaced
    this battery several times but it doesn't seem to hold a charge. I
    tried it with and without a diode Open circuit (charging) voltage on
    the load side is 5 V. I have no schematics to attempt
    troubleshooting, but as long as I keep the main wall-wart power
    supplied with voltage, the system stays happy.

    For those interested in the phone system, this is a Voice Pro 412 PBX
    phone controller that works with plain old telephones and makes it
    sound to your callers like you have a $15000 voice mail system. As
    far as I know, the company is out of business, but sometimes you can
    buy refurbished Voice Pro units on eBay for $450-$500. Except for
    the power issue... It works flawlessly and is a good system for a
    small business.

  7. TimPerry

    TimPerry Guest

    it is easy to make guesses from far away but what if the measured 5 volts is
    what the memory is looking for to stay alive and the battery charge supply
    is just dead?

    a 6.3 volt battery will need a bit more then that (voltage) to keep charged

    installation manual here
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