Digital Thermostat - Thanks Chris!!
photomankc
Posts: 943
I'm pretty well along in creating my own digital thermostat using a DS1620. So I now have a prop with LCD and tact-stick that can operate 3 lines through small switching transistors to switch on and off a 5V signal that will then be used to activate three relays and also some indicator LEDs. If I'm understanding the theory here then the relay shorts (R) to either (W) - Heat, or (Y) Cooling, with (G) - Fan being shorted to force the fan on if either heat or cool are not already running.
My code actually runs the fan itself along with the heating and cooling calls since I am not sure looking at the HVAC wiring in the basement if both heat and cool automatically power the fan or not and the relay is required anyway since I plan to have no slide switches on the system. The thermostat is in charge of the mode (Heat, Cool, or Hold) and the fan (On, or Auto).
Heat is obvious as is cool. Hold mode is for the late fall or early spring when it gets hot enough in the day to want A/C but the bottom falls out at night and heat may be needed so it has a range setting (forces a 4 degree spread at minimum) where if the temp goes over X temp cooling is turned on and if it falls under Y heating is turned on. I hate having to flip back and forth , running up the stairs to do so and with a reasonable range setting it should not be flapping between heat and cool. Later I plan to connect a downstairs unit wirelessly so we can control it from downstairs and it can factor in the downstairs temp when turning on and off.
So my questions boil down to the brass tacks of switching the AC voltages. All I have at the thermostat location from the transformer is (R) - 24VAC Hot. They did not bring the common line up which seems to mean I'm stuck setting up an external power supply since I can't rob from the transformer. I was also looking at using SSRs instead of mechanical relays but looking at them it appears they would require hot and common to work with AC voltages where the relay is a simple wire connection and only hot is needed. I'm a bit shaky on my AC terminology as I've rarely worked with it. I'm planning to keep the AC side as isolated as possible with no connections other than the relay points. I'm also a little unsure of the current rating required to operate the HVAC relays in the furnace/cooling system. I don't imagine they draw much current so I thought relays rated to 250 or 500ma load current should be overkill but I'm not having much luck confirming that. Since I need to cram three into the box I need to keep them small if possible.
Will solid state relays work with just hot? Does anyone have pointers on what the general rating used for amperage is in thermostat switching relays? The best I found was a possible mention of them needing to handle 40ma to switch the mains relays.
Post Edited (photomankc) : 10/13/2009 1:42:49 PM GMT
My code actually runs the fan itself along with the heating and cooling calls since I am not sure looking at the HVAC wiring in the basement if both heat and cool automatically power the fan or not and the relay is required anyway since I plan to have no slide switches on the system. The thermostat is in charge of the mode (Heat, Cool, or Hold) and the fan (On, or Auto).
Heat is obvious as is cool. Hold mode is for the late fall or early spring when it gets hot enough in the day to want A/C but the bottom falls out at night and heat may be needed so it has a range setting (forces a 4 degree spread at minimum) where if the temp goes over X temp cooling is turned on and if it falls under Y heating is turned on. I hate having to flip back and forth , running up the stairs to do so and with a reasonable range setting it should not be flapping between heat and cool. Later I plan to connect a downstairs unit wirelessly so we can control it from downstairs and it can factor in the downstairs temp when turning on and off.
So my questions boil down to the brass tacks of switching the AC voltages. All I have at the thermostat location from the transformer is (R) - 24VAC Hot. They did not bring the common line up which seems to mean I'm stuck setting up an external power supply since I can't rob from the transformer. I was also looking at using SSRs instead of mechanical relays but looking at them it appears they would require hot and common to work with AC voltages where the relay is a simple wire connection and only hot is needed. I'm a bit shaky on my AC terminology as I've rarely worked with it. I'm planning to keep the AC side as isolated as possible with no connections other than the relay points. I'm also a little unsure of the current rating required to operate the HVAC relays in the furnace/cooling system. I don't imagine they draw much current so I thought relays rated to 250 or 500ma load current should be overkill but I'm not having much luck confirming that. Since I need to cram three into the box I need to keep them small if possible.
Will solid state relays work with just hot? Does anyone have pointers on what the general rating used for amperage is in thermostat switching relays? The best I found was a possible mention of them needing to handle 40ma to switch the mains relays.
Post Edited (photomankc) : 10/13/2009 1:42:49 PM GMT
Comments
Having tackled this issue before: http://www.savagecircuits.com/digitalthermostat/
I understand your situation. In my case A/C wasn’t available, so I only had to switch the single set of contacts. As for an SSR, it is meant to switch the hot line. You’ll want one rated for the voltage of the transformer (24VAC), which should be easy enough. You could also use a Relay as I did. I did some research on HVAC systems at the time and it seem the fan can be set to be on with the heat/cool (auto) or on all the time, so it should have its own contact, but that was 5 years ago so I’d have to look it up again. Take care.
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Chris Savage
Parallax Engineering
50 72 6F 6A 65 63 74 20 53 69 74 65
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Anyway, just wanted to say thanks for the helpful link and comments. My 100MHz uber-wireless-thermostat is well on the way to becoming a reality.
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Chris Savage
Parallax Engineering
50 72 6F 6A 65 63 74 20 53 69 74 65
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Allowing for "XX.X" in my display would also make me redo all the LCD menuing. I'm looking around for a chip with better than 0.9 steps (9 bit resolution) with that I could at least internally handle 1/2 degrees even if the system didn't offer them as adjustment points. I could still then allow trim adustments of +/- 0.5 degrees and have the A/C system hold a tighter window. It really showed up in the summer electric bills.
Below are some photos of the system, in all it's hideous glory.
It's working well though and aside from power outages has proved very reliable. There is a bug somewhere in the UI code that causes the interface to sometimes go to never-never land when returning from the special functionss menus. The backend keeps working fine but the UI is lost untill a reset. Have not been able to catch it doing it when I'm watching though. I suspect something in my timing code is the culprit.
The "Hold" mode is working great though. It's the reason the wife doesn't want it to go even if it's ugly. The fall and spring we set that mode and it kicks on either heat or A/C for those times when it gets a little hotter or cooler than normal but we don't have to switch it on and off all the time since usually the temp with the windows open will hang in the 70 to 75 degree range and won't trigger either system as we set it. It's really nice to not wake up and find the house at 65 degrees because the night got real cold and you forgot to switch to heat that night.
Thank you!