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?
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.
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:
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.
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.
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
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