Convert battery powered to battery backup

I just installed one of those cheapo battery powered (two 1.5V AA) thermostats. It occurred to me that I could run a regulator from the

24V supply in the wall to derive the 3V needed to run the unit and use the batteries only as a backup. What would I need to do to safely wire this regulator in parallel with the batteries so that both can supply power to the unit, but so that no current from the regulator runs through the batteries? Any alternate takes on this idea?
Reply to
runderwo
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Don't. The batteries are already a backup - the device is powered off the 24 VAC line except when closed.

Reply to
Homer J Simpson

Use two diodes, say 1N400x or so:

Battery + ------->|-------+ | +-------- equipment | 3V Reg. + ------->|-------+ Hope This Helps! Rich

Reply to
Rich Grise

The manual specifically states that the device is powered only by the batteries...

Reply to
runderwo

Yes, that does help. How should I choose the diodes to ensure that the supply power is drawn from the regulator first?

Reply to
runderwo

Just set the regulator slightly higher voltage (i.e. 3.1 volts) than the batteries and the diodes will pick the highest voltage source.

Reply to
John Popelish

I get two or three years out of a set of batteries. Why make the effort?

Reply to
Homer J Simpson

Make your regulated voltage a little higher, say, 3.3V. Your thermostat won't have a problem with that, and it'll ensure that the diode from the regulator will supply the load unless it's off.

Cheers Chris

Reply to
Chris

I suspect that you cannot do it. It is likely that the

24 volts is not a supply, but rather just a switched loop that the tstat closes when it calls for heat. To find out, connect a meter to measure the 24 volts, then adjust the 'stat until it calls for heat. If the voltage displayed on the meter drops down close to zero, you can't use it in that condition. You might be able to use it when the 'stat is not calling for heat, but that is not worth screwing around with. If the 24 volts is always available regardless of what the tstat is doing, then you can do it as others have indicated. Here's a 3V supply circuit:

----------- | LM317 | Vin----+-----|Vin Vout|----+----->|-------> to tstat (+) | | Adj | | 1N4148 | ----------- / | | \\ R1 240 ohms | | / | | | [.1uF] +----------+ | | | / | \\ | / R2 330 ohms | \\ | | Gnd -----+-----------+------------------------> to tstat (-)

The 24V AC will need to be rectified and filtered before being applied to Vin. There will be about

3.7 volts on the Vout pin of the LM317; the 1N4148 will drop that down to about 3.1 volts and also isolate the circuit from the batteries.

Ed

Reply to
ehsjr

put a slightly higher higher voltage from the regulator than from the battery

--
Bye.
   Jasen
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Reply to
jasen

It isn't. But the batteries last for 2 or 3 years so why bother?

Reply to
Homer J Simpson

As I said in the first two lines of my post: "I suspect that you cannot do it. It is likely that the

24 volts is not a supply, but rather just a switched loop that the tstat closes when it calls for heat. "

The thread went 8 replies and that had not yet been mentioned when I posted. Sorry to see that you snipped it.

But - as to why bother if the 24 vac was available regardless of what the tsat was doing - for the fun and learning. Certainly not a practical project, as you point out, unless it is for the fun and learning.

Ed

Reply to
ehsjr

if it runs for years on batteries, you could probably get enough power from a current transformer to run the unit while the switch is closed.

Bye. Jasen

Reply to
jasen

Possibly. And as an alternative one could charge supercaps while the switch is open, to provide power when it's closed, or modify it at the other end with a lower voltage relay and a resistor in the closed tstat circuit at the tstat end so there would be voltage available, or add a wall wart power supply, etc. It can be Rube Goldberged to get it to work. But that is not the point.

He assumed a 24 volt supply in the wall. He cannot do it based on that assumption if the assumption is wrong, which is highly probable. It is almost certainly a switched loop, not a supply. He cannot simply regulate it to 3V, as he mentioned, and use it in place of the batteries. Additionally, it is almost certainly AC (as it would have to be for your CT idea to work), not DC, so that's another reason I suspect he cannot do it. He would need to not just regulate it, he'd need to add a rectifier and filter first to get DC. His idea makes no mention of converting AC to DC.

Ed

Reply to
ehsjr

Yup, I thought it was DC but it looks like all it goes through is a transformer in the furnace to step 120V down to 24V. This page was quite helpful since the thermostat manual was ambiguous whether the

24V was AC or DC:
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Reply to
runderwo

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