Convert battery powered to battery backup

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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?

 

[Homer J Simpson]

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?

Don't. The batteries are already a backup - the device is powered off the 24
VAC line except when closed.

 

[Rich Grise]

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?

Use two diodes, say 1N400x or so:

Battery + ------->|-------+
|
+-------- equipment
|
3V Reg. + ------->|-------+

Hope This Helps!
Rich

 

[[email protected]]

Don't. The batteries are already a backup - the device is powered off the 24
VAC line except when closed.

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

 

[[email protected]]

Use two diodes, say 1N400x or so:

Battery + ------->|-------+
|
+-------- equipment
|
3V Reg. + ------->|-------+

Hope This Helps!
Rich

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

 

[John Popelish]

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

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

 

[Homer J Simpson]

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

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

 

[Chris]

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

- Show quoted text -

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

 

[ehsjr]

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

 

[jasen]

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

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

 

[Homer J Simpson]

If the 24 volts is always
available regardless of what the tstat is doing, then
you can do it as others have indicated.

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

 

[ehsjr]

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

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

 

[jasen]

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. "

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

 

[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

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

 

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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
24V was AC or DC:
http://xtronics.com/reference/thermostat_wires.html