UPS for 12V DC ?

Yes, subject to some conditions.

1) That your answering machine does not require a stable regulated supply that stays very close to 12V.. 2) That you have some form of automatic switchover. Connecting the D batteries and the output of the 12V supply together will not do. Connecting one diode each (say 1N4001) in series with the "12 VDC transformer" and the battery stack should work, provided that the machine does not mind the slight drop in voltage (the diode will drop close to 1V on load). 3) That power does not remain off for too long. D cells won't last very long at a continuous drain of 200mA. OTOH, if 200mA is the nameplate rating on the machine, it's probably the maximum and does not draw that much current all the time.

Get a 12 volt rechargeable battery pack with a charger. Run your device from a battery that is being charged at a greater rate than you are using. In event of a power failure the device doesn't know the difference.

BTW check your power calculation.

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There is no such thing; you must be referring to an AC to DC
wall-wart, yes?

Hi John,

Very cool solution but why do you say, "Assuming your answering machine doesn't draw 200 mA?"

Is that the max those diodes can use?

Dave Google Groups: I do not see postings from GG. I am using an account with

to weed out the bullshit and spam. If you are responding from within Google Groups I will not see your reply. Try usenet4all I think you will like it. It is Usenet the way it used to be. Yes, I am THAT old!

John Fields wrote:

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"... doesn\'t draw 200mA all the time."

By transformer, I mean that it reduces 120V to 12DC. What would a 12V sealed battery run ?

Are the diodes or rectifiers used to smooth out the DC output ?

I was also thinking of using NiMh batteries as an alternative. Making a charger should be too big a deal.

Andy

NiMH batteries require precise charging. They are not very forgiving. ( Look up the laptop battery recall. ) Lead acid or NiCads would be better. As I said in my first post to you, all you need is a battery with a charger.

Tom

Maybe I'm missing something here, but I thought the minimum float voltage for an SLA is 2.25V per cell, to avoid irreversible damage from sulfation. That's 13.5V for a 12V nominal, 6-cell battery. But,

13.5V could cook his answering machine.

???

Michael

Couldn't I use some Zener diodes to keep the voltage at 12.0 on the ouput?

Andy

For the last time. All you need is a battery and a charger.

You didn't answer my question, just complained.

Sorry it sounded like complaining, you received several replies that essentially said the same thing that I did but you didn't seem to care. Best Regards, Tom

On Tue, 05 Aug 2008 18:46:47 -0700, Viel Spass wrote:

Michael is correct, if you use a sealed lead-acid battery, you will need to run your battery charger at a proper float voltage, which has to be regulated. It can be too high or too low and the battery will be damaged. You will have to regulate the voltage at the answering machine to within its rated tolerence. You will have an idle load and a full load, and will need to know both values. If the high load is for short durations only, the charger as set up properly will handle it, you'll be off float for short enough periods. NimH batteries have their own idosyncracies, as has been mentioned.

I know you think that Tom is complaining, but do you really want to design your own UPS, or do you just want to have a way to keep your answering machine from turning off during a power outage? Here's an idea, buy an UPS for your computer gear and plug the answering machine into it!

That is one way. My experience with UPS is the batteries don't last very long for the cost involved. I will go with a SLA and figure the idle and float load in order to make a basic charger for it.

Thanks.

Well, you're right about that, a typical UPS battery will only last two years or so, whereas an SLA could last up to ten years or more, depending on use and care. You should read up on the subject of batteries and battery systems, lead-acid in particular. I notice you said "...figure idle and float...." What you may not know is that idle and float, in the context, are the same thing. When the load is idle, the battery should be at float voltage. There are three basic modes for a UPS battery: float, equalize, and emergency load. Enjoy your reading!

I'm not sure I understand your response. The original poster is discussing a 12VDC supply to an answering machine via a plug-in rectifying transformer. Please explain the danger of ring voltage somehow appearing via the recitified AC voltage from a wall outlet?

And by all means, have the "authorities" contact me if they have questions.

Possibly the 0V DC input terminal of the device is connected directly to one of the phone line wires. I've seen many answering machines bearing a label that reads something like "use only with with [insert phone authority here] approved power supply"

Bye. Jasen