Answering Machine - Power Backup?

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 meaningless.

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 problem.

Thanks, Len

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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:

How about buying an uninterruptible power supply...

i

positive from power supply _______________________ | \\/diode - |_________________positive to machine | - /\\diode | 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.

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 trivia.

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  Keith

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

Graham

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

Graham

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.

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.

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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

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|  James E.Thompson, P.E.                           |    mens     |
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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! Rich

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.

Al

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 ...

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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

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 ???

Thanks, Len

When

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