Electronics > Projects, Designs, and Technical Stuff
How does the Venstar Add-a-wire work?
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rea5245:
Hi,

Modern wi-fi thermostats require constant 24V power. That means they need one more wire than old thermostats needed. The Venstar Add-a-wire  and ecobee's Power Extender Kit somehow multiplex two of your existing thermostat wires (e.g. the ones that switch on your heat or fan) into one. That frees up one wire so that you can use it for constant power.

(Amazon link http://a.co/5J41Dzy; install video: )

These devices have a box that installs inside your furnace and a smaller thing - they just call it a "diode Y" - in the thermostat that splits the multiplexed wire into two.

Now, it's easy to multiplex two digital signals onto one wire, but these gadgets are multiplexing two 24VAC signals, and they're doing it with just a so-called "diode" on one end.

Can that really be done with just a diode? What's the likely mechanism they're using?

Thanks,
   Bob
Cupcakus:
If the signals are AC they can be phase shifted and combined onto one wire at the HVAC, and then a diode phase detector circuit at the other end splits it again? Wild guess, this is outside my expertise.
mmagin:
Huh, pretty clever gadget.

Well, in these thermostat connections (before using this device), there is not significant information in the 24 VAC, it's just expressing a binary state by closing a contact and passing the voltage (0 or 24 VAC) from a common wire to another wire (e.g. request heat, request cool, etc).  It looks like the main box is put at the end where the 24 volt transformer supply is (furnace, etc.).  So for example in the attached image, by adding the diode Y, the "add a wire" box at the other end can detect whether it is seeing 0, positive pulses, negative pulses, or AC on the shared line, and then it can close relays on its 'output' side as appropriate.  You could decode this on the receiving end in a number of ways, some even as simple as a couple more diodes and a couple capacitors.  (You might add some active circuitry to reliably drive relay coils, perhaps)
rjparker:
One way...
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