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Thermostat control: relays vs TRIACs?

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gxti:
So the thermostat my new place is a decent early digital type but it's too close to the nice, toasty "wiring closet" so the rest of the place is colder and subject to fluctuations throughout the day. So I've got it in my head to make a thermostat with remote sensors :-)

Now the question is, what's the best way to drive those 24VAC control lines? Relays would be the first thing everyone recommends, and they can handle more situations (millivolt systems, which this one is not), but if I'm designing for my specific situation, would TRIACs work well? Specifically, I've got a 24VAC system with hot, common, heat, fan, and cool -- pretty much a standard setup.

Here's my first pass at a TRIAC design: http://partiallystapled.com/~gxti/circuits/2011/11/06-tstat-triacs.png -- not simulated or breadboarded but in my uneducated and inexperienced opinion it looks pretty good. VCC is 3.3 volts, so those resistors should be putting 15mA through the TRIAC, 5mA more than the datasheet calls for. Does the "gate voltage" work line a forward voltage on a diode, where I need to do (Vcc - Vgt) / Igt to get my resistor value? In that case I'd want 120 there.

Uncle Vernon:

--- Quote from: gxti on November 06, 2011, 05:53:14 pm ---So the thermostat my new place is a decent early digital type but it's too close to the nice, toasty "wiring closet" so the rest of the place is colder and subject to fluctuations throughout the day.
--- End quote ---
What this says, is that there is nothing wrong with the thermostat you have other than incorrect placement.


--- Quote ---So I've got it in my head to make a thermostat with remote sensors :-)
--- End quote ---
That may be be a solution but and of the day yourequipment is still binary in operation.On or Off, sensing that one room is colder/warmer than another, wont equalise the conditioned space. What do you intend,manual selection of sensor authority, averaging, hi select, lo select?


--- Quote ---Now the question is, what's the best way to drive those 24VAC control lines? Relays would be the first thing everyone recommends, and they can handle more situations (millivolt systems, which this one is not), but if I'm designing for my specific situation, would TRIACs work well? Specifically, I've got a 24VAC system with hot, common, heat, fan, and cool -- pretty much a standard setup.
--- End quote ---
How sure are you that Triacs will work well? What is the actual load, while it may beturning on a furnace or compressor, odds on those control lines switch a small glass relay or feed into a control PCB. You may well find you need electrical additional burdon to ensure the triacs latch reliably. Your milage may vary but as a universal solution most designers would opt for relay or transformer switching.
The other point I'd note is that most commercial HVAC equipment would use high side switching so you you would be better tying your triacs the the 24VAC rather than the neutral.

gxti:

--- Quote from: Uncle Vernon on November 06, 2011, 09:28:28 pm ---What this says, is that there is nothing wrong with the thermostat you have other than incorrect placement.

--- End quote ---
I can't move the thermostat because the place is rented and I'm not interested in fishing stiff wires through the walls in any case. As for the sensors I'll be either using an average, or perhaps have it switch from one sensor to the other on a timer.


--- Quote from: Uncle Vernon on November 06, 2011, 09:28:28 pm ---How sure are you that Triacs will work well? What is the actual load, while it may beturning on a furnace or compressor, odds on those control lines switch a small glass relay or feed into a control PCB. You may well find you need electrical additional burdon to ensure the triacs latch reliably. Your milage may vary but as a universal solution most designers would opt for relay or transformer switching.

--- End quote ---
I've looked at the internal wiring so I know that the control lines feed into 120VAC relays. I don't think holding current is a concern though as long as the gate is held high, but this might be a misconception on my part. In addition the hold current for these TRIACs is only 10mA which is less than the coil current of even most of the tiny relays I was looking at.


--- Quote ---The other point I'd note is that most commercial HVAC equipment would use high side switching so you you would be better tying your triacs the the 24VAC rather than the neutral.

--- End quote ---
That's what I was aiming for. MT1 on the TRIACs is connected to the hot side, and I also used that as circuit ground so that I can easily hold the gate above that reference point. One benefit relays (or opto-TRIACs) have over this configuration is that I wouldn't have to worry about reference points, but if this configuration works then I'll give it a shot.

Uncle Vernon:

--- Quote from: gxti on November 06, 2011, 09:54:12 pm ---
--- Quote from: Uncle Vernon on November 06, 2011, 09:28:28 pm ---What this says, is that there is nothing wrong with the thermostat you have other than incorrect placement.

--- End quote ---
I can't move the thermostat because the place is rented and I'm not interested in fishing stiff wires through the walls in any case. As for the sensors I'll be either using an average, or perhaps have it switch from one sensor to the other on a timer.
--- End quote ---
Doesn't leave a lot of scope for running additional sensor cables either. You could go RF but expense vs return ratio isn't looking real flash.


--- Quote ---
--- Quote from: Uncle Vernon on November 06, 2011, 09:28:28 pm ---How sure are you that Triacs will work well? What is the actual load, while it may beturning on a furnace or compressor, odds on those control lines switch a small glass relay or feed into a control PCB. You may well find you need electrical additional burdon to ensure the triacs latch reliably. Your milage may vary but as a universal solution most designers would opt for relay or transformer switching.

--- End quote ---
I've looked at the internal wiring so I know that the control lines feed into 120VAC relays. I don't think holding current is a concern though as long as the gate is held high, but this might be a misconception on my part. In addition the hold current for these TRIACs is only 10mA which is less than the coil current of even most of the tiny relays I was looking at.

--- End quote ---
TRIACS may not turn on reliably with non-resistive loads, because due to the phase shift the holding current may not be achievable at trigger time.


--- Quote ---
--- Quote ---The other point I'd note is that most commercial HVAC equipment would use high side switching so you you would be better tying your triacs the the 24VAC rather than the neutral.

--- End quote ---
That's what I was aiming for. MT1 on the TRIACs is connected to the hot side, and I also used that as circuit ground so that I can easily hold the gate above that reference point. One benefit relays (or opto-TRIACs) have over this configuration is that I wouldn't have to worry about reference points, but if this configuration works then I'll give it a shot.

--- End quote ---
It is a common not a ground. Your drawing shows the hot lead at the same potenial as your circuit ground, not a good thing. Opto triacs such as MOC3020 and MOC3021 would be your safest path.  http://www.jaycar.com.au/images_uploaded/optocoup.pdf

gxti:
I'm going to have to get some actual parts and figure this out the old-fashioned way, there's too many conflicting conclusions bouncing around in my head and I won't know for sure unless I try. I don't see any reason that a straight TRIAC shouldn't be able to switch a small inductive load if an opto-TRIAC can.

As for the common/ground/hot/whatever thing, I'm not shorting anything anywhere. I have simply selected the red wire to be common on my PCB, instead of the black one. It's AC so after rectification it's all the same. And since, as you said, the red wire needs to be the one switched into the output, this makes it easy to reference the gate signal to its MT1 terminal. Defining red == 0VDC means that 0VDC turns the TRIAC off and 3.3VDC turns it on.

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