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Convert battery powered to battery backup

Discussion in 'Electronic Basics' started by [email protected], Apr 19, 2007.

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  1. Guest

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

    Rich Grise Guest

    Use two diodes, say 1N400x or so:

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

    Hope This Helps!
  4. Guest

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

    Yes, that does help. How should I choose the diodes to ensure that
    the supply power is drawn from the regulator first?
  6. Just set the regulator slightly higher voltage (i.e. 3.1
    volts) than the batteries and the diodes will pick the
    highest voltage source.
  7. I get two or three years out of a set of batteries. Why make the effort?
  8. Chris

    Chris Guest

    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.

  9. ehsjr

    ehsjr Guest

    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.

  10. jasen

    jasen Guest

    put a slightly higher higher voltage from the regulator than from
    the battery
  11. It isn't. But the batteries last for 2 or 3 years so why bother?
  12. ehsjr

    ehsjr Guest

    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.

  13. jasen

    jasen Guest

    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.

  14. ehsjr

    ehsjr Guest

    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.

  15. Guest

    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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