Need 28 VDC UPS for a phone PBX Control Box

[Beachcomber]

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
capacity.

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...

Beachcomber

 

[Beachcomber]

Very much agree - you could have added that it is quite
possibly going to be the only solution that keeps the phone
company happy!

The "very large" existing power brick is very large probably
because it is type-approved for use with the phone system
and has the necessary fault protection. Any mains-connected
system in parallel with it would need to be similarly certified.

A UPS before the power brick would not need that certification.

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.

Beachcomber

 

[ehsjr]

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
capacity.

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...

Beachcomber

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.

Ed

 

[TimPerry]

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.

Beachcomber

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.

 

[Bill Kaszeta / Photovoltaic Resources]

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.

Ed

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
[email protected]

 

[Beachcomber]

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
[email protected]

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
here.

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.

Beachcomber

 

[TimPerry]

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
here.

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.

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
up.

installation manual here
http://www.lulu.com/items/volume_1/103000/103168/4/preview/Voice_Pro_Preview.pdf