Electronics > Beginners
24VAC to DC
joeyjoejoe:
I'd like to build a controller board for automating some HVAC. Unfortunately, I'm not able to find quite a few different components of the "easy route".
High level requirements:
- Open-source support (esphome, tasmota, etc) (Sonoff comes close)
- Powered by 24V AC
- Using reliable components (writes off a few AliExpress)
So, I think I'll build my own - a fun project no doubt! Low voltage and low loads (these are all control signals), but I'll need to design for reliability.
I'd like to use a transformer to go from 24VAC to something a lower for efficiency, but mainly isolation from the power supply which is shared by other components. I'll use a linear regular afterwards, so the output voltage would be dependent on the headroom needed - probably not much with an LDO, so to hit 3v3DC, I could probably head to 4-5VAC, rectify and filter.
I figured this would be an exercise in picking winding ratios on the transformer. But when I head to digikey's Power Transformer section, it's a bit more confusing then that. Mouser's search reveals a few more goodies (Wurth 750313441) but I'd rather know what I'm looking at first to pick something. Component cost is a bit of a factor as well.
james_s:
Why do you need isolation?
Years ago I built a controller to select between my heat pump and gas furnace depending on outdoor temperature. I used a buck regulator made from one of the SimpleSwitcher ICs, I think LM2567. It runs cool and has been working for something like 14 years now. Bridge rectifier, filter, buck regulator, it goes from the 24VAC transformer in the furnace to 5VDC. It's not isolated but that didn't matter for what I was doing.
joeyjoejoe:
I figure, worst case, I build something that fails. I'll do the best to make sure it doesn't, but in the event it does, probably best not to take out other things.
I guess I could also just put a fuse inline as well to do as you suggest. But I'm wondering if an LM2567 would up the BOM costs or complexity significantly? I know I don't want to use those premade LM2567's, as I'm worried about reliability. Rather use genuine components.
T3sl4co1l:
Guessing you don't need three friggin' amperes to run things, a little SOT23-6 part should do, for example.
Why isolate the input? Why not isolate the output? It's just a contact closure (relay or optoisolator), no?
Tim
james_s:
A genuine LM2567 costs a few dollars at most, these days you can get smaller parts but 14 years ago there were a lot fewer easy to use switching regulator ICs on the market and I think I got the part I used as a free sample. Since the existing equipment already uses 24V I'm not sure how you'd damage it, if I remember correctly, I used opto-isolators on the input from the thermostat, I don't recall exactly why I chose that approach but it has worked. BOM cost should be a non-issue unless you're mass producing something. Certainly a switching regulator is going to be much cheaper than a custom 60Hz power transformer.
This is one of those cases though where if you're not certain you know what you're doing, you probably shouldn't be doing it. You don't want your heat to get stuck running for a week solid while you're out of town or something when something malfunctions.
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