Simple DC-AC inverter - Page 1
Forum section: electronics / beginners

author: danbr
author_rank: Contributor
timestamp: July 01, 2018, 11:42:55 pm
content: 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!

author: Audioguru
author_rank: Super Contributor
timestamp: July 02, 2018, 03:25:39 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2018, 11:29:28 am
content: 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?

author: danbr
author_rank: Contributor
timestamp: July 02, 2018, 10:25:41 pm
content: 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?

author: romons
author_rank: Contributor
timestamp: July 03, 2018, 03:51:09 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2018, 09:00:15 am
content: 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