Need help on thermostat power supply


I’m in over my head and thought this was the place to start. I’m trying to replicate THIS project and can’t figure out the power supply. The wires coming from the furnace and AC is 26.8 volts A/C and I want to drop this to 5V DC. The current is too low to use as a reliable power source so I am going to use the Adafruit 500 Charger with a 500mA 5V Lipo battery. Can anyone point me to a good solution to something that would fit in the housing? I can’t figure out what they did.

Thanks a lot in advance. I’m definitely a novice.

You should be able to power everything from your nominal 24VAC transformer isolated power supply. You just need to rectify the AC to DC, and then regulate it to 5V with a Buck regulator. Here’s another very nicely documented open source Spark Core powered Thermostat that has a schematic with such a power supply:

Website with full project details here:

The problem with the 500mAh lipo is unless you are putting the Core to sleep most of the time, it’s going to be dead in about 3-4 hours roughly speaking. Try to get the AC lines to give you more juice. You may have to trace the wires all of the way back to the transformer that lives in your furnace to know that you have the correct ones. There should be a schematic of the wiring on your furnace paneling, or service manual.

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Just an FYI: At my old house I tried to put a commercial smart thermostat in and the 24VAC would not supply enough voltage to keep it alive and keep the relay pulled in to call for heat at the same time. We would find the smart thermostat constantly reseting itself and losing its time of day and other settings.

The solution was to add another 24V transformer and relay at the furnace since the existing transformer and relay was a “combo” unit built-in to the furnace. Honeywell sold a kit explicitly for this purpose, so that is what we used.


Okay thanks. This does help quite a bit. I’ll try and figure out the power I can get from the furnace. From what I understand there isn’t enough current to draw from so I’ll have to use the LIPO to power the thermostat when the heater and A/C is running, then the battery will charge when it’s not. I figure the battery can charge every night when hardly anything is running.

Thanks for the feedback. I’ll have to consider this if the battery doesn’t hold charge for long enough.

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