Simple DC-AC inverter

[danbr]

Hi,

Im working on a project where I have 24v DC and I need to invert it to 24v AC in a low power application.

Background, I'm attempting to interface with a Nest thermostat in a standalone application. I do not have access to the normal 24v AC HVAC power/signal wires that are common in North American homes. But I do have 24v DC.

I tried giving the thermostat 24v DC and while it registered that I was powering it, and was getting voltage - the thermostat gave me an error e79 "The common (C) wire isn't working. Power isn't detected." Normally there is "hot" 24v AC going to the R input of the thermostat and C "common" return power back to the HVAC system. There are then various signal wires that complete the circuit (all 24v AC) to switch on/off fans or aircon compressors etc..

My thinking is the nest thermostat is looking for an AC waveform, and when it cant tell there is no power going back on the C line, it throws this error.

So my original question is, are there any simple DC-AC inverting circuits out there that only have to supply less than 5 watts of power? Also if anyone sees any flaws with my idea or line of thinking here, please let me know. Thanks!

[Audioguru]

The 24VAC is probably a sinewave from a transformer that has a peak voltage of (24V x the root of 2=) 34V and expects to see an input of a little higher than 30VDC.
An inverter powered from 24VDC without using a custom made stepup transformer produces a sinewave output of about 17VAC which is too low.

[Zero999]

17VAC might be enough. A squarewave might also work, in which case it would still be 24VRMS.

The simplest way is an off the shelf 24VDC to 120VAC inverter, followed by a 120V to 24V transformer.

Is this running off a battery?

[danbr]

Yes, it would run off a battery. Ideally a li-ion/lipo that could be swapped out for charging. Also trying to make it as small and compact as possible (it will be attached to a wall)

I can try boosting the DC to a little over 30V to see if the thermostat registers. Although my guess would be, it wont.

24VDC to 120VAC then back down to 24VAC seems like a hassle. I was hoping there would be some solid state method of switching?

[romons]

You can build a simple inverter by using a ne555. Use an H bridge configuration to 'double' the effective voltage change. Here is a simple schematic. Put your load instead of R1, which is just there for a test. For an effective 1k ohm load, the RMS voltage is about 20V. You probably want fuses and such, I haven't put anything like that in. Since you didn't specify the load, I couldn't pick the right transistors for you. Let me know if it works. I haven't built it.

I updated the schematic, and improved the range for the output. Also, the cap/resistor pair wasn't set up properly for 60Hz. I've fixed that.

Hmmm. Somebody below pointed out that there is a misconnection in Q2. Happened while I was cleaning up, probably. I fixed that in the new schematic. However, the one presented below by the same guy is simpler and cheaper. I'd go with that one.

[Zero999]

Q2 is connected incorrectly, with its base shorted to the collector.

This circuit will work, but it has a high voltage loss and is relatively complex way of getting a square wave. I'd use four transistors, forming a self-oscillating bridge.

https://www.eevblog.com/forum/projects/dc-to-ac-converter/msg15197/#msg15197