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
How long a power outage do you want to plan for?
Here is one suggestion:
Three 14.4 volt Nicad power packs meant for portable
electrical tools, plus a charger. "Generic" packs can be
bought for ~16GBP in the UK so I guess they are cheap in the US?
Put a rectifier in series with the output from the existing
supply (probably not needed but better to be safe than
sorry). Put five rectifiers in series with two of the power
packs and connect that supply in parallel with the existing
supply (plus its new rectifier).
Fully charge all the packs. Temporarily hook up a
multimeter, on the amps range, in series with the battery
packs. It should read zero. If it doesn't read zero, add
more rectifiers. Now short out one of the five rectifiers.
Repeat until the meter starts reading current, no matter how
small. Then unshort the last rectifier. The meter should
read zero. Turn down the multimeter to a lower amps range
and check that it still reads zero. If not, unshort
rectifiers until it does. Repeat until you reach the mA
range of the meter.
The third battery is so that you can periodically have a
recharge session, putting the two in use packs through the
charger whilst the spare is in their place. Don't forget to
have a recharge session after each outage.
Here is another suggestion:
Buy 3 12 volt sealed lead-acid batteries (sized to suit the
reserve time you want), together with 3 suitable float
chargers. Wire the batteries in series. Buy a 24 volt 3
terminal regulator IC, a suitable heatsink, a couple of
resistors and capacitors and a couple of rectifiers (I can
give you the details if this solution appeals - there is
some electronics-style soldering involved). The resistors
offset the reference voltage of the regulator, so it
produces approx. 28 volts. (This will be adjusted,
monitoring the current, so that the load is met by the main
supply under normal conditions.)
Stick one rectifier in series with each supply and parallel
their outputs.
Note that as the float chargers are connected to the mains
and also to the phone system, via this arrangement, this may
not be a legal solution - as the chargers probably wouldn't
be certified for such use. Laws vary according to where you
live. Most phone companies are (quite rightly) paranoid
about the possibility of mains being shoved back up their
phone lines.. no matter how unlikely the possibility.
Hence the first suggestion - where the charging is done
"off-line". No need for certified mains chargers.
HTH
Sue
Actually two 12 volt batteries is only 24 volts under no load, when
fully charged.
On the other hand, if they are being trickle charged the voltage
will bu just over 26 volts. And if being equalized, 28 volts.
And when being discharged the battery should have a low voltage
cutoff at about 21 volts.
Which suggests that your PBX may or may not run off a pair a 12
volt batteries... and only by looking at the specs will you
know for sure.
It will almost certainly be least expensive to buy a UPS that
puts out 120VAC and plug your existing 28VDC 1 AMP supply into
it. Such an UPS is a commodity item these days, while a 24 volt
system is still a relatively special item not used by consumers.
But one point is also certain: it is a *lot* less complex to
figure out the right 120VAC unit!
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
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
It isn't high voltages appearing on extensions that worry
phone companies, it is high voltage appearing on *their*
lines (it may have an engineer connected to them or someone
from BAT..). But I haven't a clue what is mandatory in the
US, in that respect.
I use UPS and inverters all the time ( I live in the middle
of a National Park and also play with boats). You get what
you pay for. I would rather buy a second hand good quality
one from ebay (and budget on replacing the batteries, if
fitted) than a cheap new one from Staples (a poor copy of
your Office Depot.. doesn't sell *any* guns).
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.
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
snipped-for-privacy@pvri-removethis.biz
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
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
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