Heat Pump 24 VDC referenced to 120 VAC mains? - Page 1
Forum section: electronics / projects

author: gdr771
author_rank: Contributor
timestamp: September 09, 2023, 03:25:52 pm
content: So I'm getting a Mitsubishi heat pump installed and I see the outdoor unit sends 240 VAC 60 Hz (two lines of 120 VAC) from the outdoor unit to the indoor air handler. Then on a third wire they have 24 VDC control power, which I think also has a communication signal riding on it. The 24 VDC is referenced to one of the two 120V phases.So one line is 120VAC to ground and the other is 144VAC to ground, meaning neither are of the safer low-voltage category. And only one of two could be current-limited, since the 120VAC line is also used for higher power things like the blower motor. So I don't know what safety classification this gets (in Canada).I've never seen this before, and I suppose it's done to save a a couple of wires. I have some applications that might benefit from this, but are there other reasons this would be done? What other considerations are there when referencing the DC control voltage to AC mains?

author: ajb
author_rank: Super Contributor
timestamp: September 09, 2023, 07:50:27 pm
content: QuoteCurrent limiting on the control line would be largely irrelevant from a safety perspective, because you'd have to limit current to only a few mA, which is probably much too low to run something like air handler controls (maybe if all of the electromechanical parts run at mains voltage and you use SSRs or latching relays for everything you could get away with it?). Any current limit there would be about protecting the low voltage interface/power supply. Hopefully it's quite well protected, given how easy it would be to cross connect with 240VAC if installed improperly.Quote from: gdr771 on September 09, 2023, 03:25:52 pmAs you've noted, the main consideration is that the control line is at a hazardous voltage. More fundamentally, being referenced to mains at all makes it potentially hazardous--even if it were referenced to the neutral in a 120V appliance, it's still referenced to a current-carrying conductor and must be treated as such. This isn't necessarily a big deal in volume manufacturing. Lots of appliance have internal low-voltage supplies that are mains referenced, because it's cheap and easy, and from a safety perspective, pushing a button connected to a tiny MCU running at 120VAC+5VDC isn't much different from pushing a button that directly switches mains. Either way, there must be adequate insulation/isolation between the user and the hazardous voltage, and the DC offset is negligible in terms of hazard level.One place where this becomes more complicated is when electrical connections are required between the mains-referenced circuitry and external equipment. Direct connections would be very problematic, unless the external equipment and all connection points are adequately shielded from the user. The signals could be electrically isolated, but isolating mains-referenced circuitry from user-accessible circuitry moves into reinforced isolation requirements rather than functional isolation, which requires more care (cost). It may make sense to isolate the mains-referenced power+control interface instead, so that the local circuitry is no longer referenced to mains.Another consideration is development, as probing a circuit referenced to mains voltage brings a number of safety and practicality concerns. Digital signals can be isolated fairly easily, but analog signals can be more challenging to measure across an isolation barrier, although some of that can be solved by throwing money at the test equipment vendor of your choice. Using an isolation transformer is an option, although they are not a foolproof solution, and it's fairly easy to make dangerous mistakes. Depending on the specific circuitry/equipment and how the control interface is implemented, it may be possible to disconnect the low voltage circuitry from the mains stuff and run a dedicated DC common wire separate from the mains during development.Bottom line, this solution makes a lot of sense for systems like your heat pump, where suitable 3+Gnd cable is available at every big box home improvement store but 4+Gnd is practically hens' teeth, and they have the volumes to easily amortize the additional development effort. But if the applications you have in mind are low volume or one-offs, I doubt a solution like this is worth while. The additional design effort, safety considerations, and development/testing headaches likely far outweigh the savings of running four conductors instead of five.