Re: Convert battery powered to battery backup

runderwo@xxxxxxxxxxxx wrote:
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?


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
.

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