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HVAC Techs? Digital Thermostat Question — Parallax Forums

HVAC Techs? Digital Thermostat Question

Chris SavageChris Savage Parallax EngineeringPosts: 14,406
edited 2004-11-09 15:05 in General Discussion
If there any HVAC techs on here, or anyone who just knows the answer.· I am building a Programmable Thermostat using a BS2 and I need to control a relay to turn the Furnace on/off.· I know the systems uses·24 Volt AC, as I have wired them before.· But this is a first for me controlling one with my own circuitry, and I was about to arbitrarily select a relay, when I realized that another Digital Thermostat my parents have is rated at .88A to 4A.

This means the 5V Reed Relay I was going to use may not work.· I was wondering if anyone knows what the spec is, as far as how much current these typically use, so that if the system ever changes, I can re-use it.· I know I could just test the current on this one, but if there is a spec for this, I would really like to know.· Thanks!


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Chris Savage

Knight Designs
324 West Main Street
P.O. Box 97
Montour Falls, NY 14865
(607) 535-6777

Business Page:·· http://www.knightdesigns.com
Personal Page:··· http://www.lightlink.com/dream/chris
·

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2004-08-15 17:08
    When you say HVAC thermostat, you are really covering a wide range of technologies from ancient to modern.
    When you say furnace, you are into older technologies (think possibly 4amps in about right)

    In the 1960, the thermostats (on your livingroom wall) used a bimetallic coil with a mercury switch and the power came from the furnace. It might have be an AC bell transformer or even a DC battery.

    Now, everything is energy efficenty and digital.

    If I were you,
    I would go for 125VAC 5AMP (controlled by 5V) rated relay (which rates at about 40VDC). It is not that much bigger and you would have a device that would cover just about any home furnace (and air conditioning ?) system. You may have to use a Darlington or a transistor to drive it from the Stamp. It will use a little more power to close, but DC relays stay closed at about half their closing power, so a slow cycle will save your battery (if that is how it will be powered).

    The big trade off, is speed.
    The little reed relays are much faster.
    But then again -- how much speed do you really need in this situation (not much as that furnace want to cycle in seconds or minutes, not milliseconds).

    Take a look at Nuts and Volts (I think July 2004) for a good article on relay selection.

    Or look into The Art of Electronics if you are a serious learner of electronics. Both have a lot of good points to make.

    Do NOT forget to add a 'flyback' diode (a 1N4004) to prevent that relay coil from damaging the solidstate devices. It is really a must.

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    G. Herzog in Taiwan
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-15 18:02
    ;-) Herzog said...
    When you say HVAC thermostat, you are really covering a wide range of technologies from ancient to modern.
    When you say furnace, you are into older technologies (think possibly 4amps in about right)
    In the 1960, the thermostats (on your livingroom wall) used a bimetallic coil with a mercury switch and the power came from the furnace. It might have be an AC bell transformer or even a DC battery.
    Actually my current Thermostat is a Bi-Metallic Coil type...And the house isn't that old.· I apprenticed for an Electrical Contractor to widen my electrical experience some years ago and installed quite a few of these.· What I am looking for is the general rating for the Digital Thermostats you can buy at your local Wal-Mart store, since these seem to be able to replace just about any type.
    ;-) Herzog said...
    If I were you,
    I would go for 125VAC 5AMP (controlled by 5V) rated relay (which rates at about 40VDC). It is not that much bigger and you would have a device that would cover just about any home furnace (and air conditioning ?) system. You may have to use a Darlington or a transistor to drive it from the Stamp. It will use a little more power to close, but DC relays stay closed at about half their closing power, so a slow cycle will save your battery (if that is how it will be powered).
    The big trade off, is speed.
    The little reed relays are much faster.
    But then again -- how much speed do you really need in this situation (not much as that furnace want to cycle in seconds or minutes, not milliseconds).
    Take a look at Nuts and Volts (I think July 2004) for a good article on relay selection.
    Or look into The Art of Electronics if you are a serious learner of electronics. Both have a lot of good points to make.
    Actually I am an engineer of going on 10 years in electronics...I am just lacking specific information for this particular application.· I guess what I am going to have to do is look up some PDF data sheets on existing commercial models.
    ;-) Herzog said...
    Do NOT forget to add a 'flyback' diode (a 1N4004) to prevent that relay coil from damaging the solidstate devices. It is really a must.
    Yes, I know about the protection diode used in reverse of the polarity of the relay coil, however, before I stopped to think about using an larger relay, I had considered using a Reed Relay, or even a SSR.


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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-15 22:57
    I wanted to add...This is a public project we're posting everything online as we design and build this unit.· This project can be tracked at:

    http://www.lightlink.com/dream/chris/thermostat.htm

    And as I said, we just need to decide on the current capacity of the relay to continue.· I will bring home my DMM tomorrow so I can test and see how much current this unit is drawing when on.· I found one more commercial unit that listed it's contacts as 1.2A per set, 1.5A total for all terminals.· This unit was designed to handle both heating and A/C functions, and had separate fan control.· I don't need all that.· It probably boils down to using a 3A, 5V relay...Or an SSR.


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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    ·
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2004-08-17 17:46
    To clarify,
    The reed relay is not only small, it is really fast (1 micro second), but furnaces are slow.
    Why bother with a marginally rated or under rated device for unnecessary speed?

    The higher voltage rating of 120VAC is very commonly available in subminature, PC mounted relays. And, it can go with DC or AC without any redesign. It is easier to find that a 24 VAC rated subminature relay.

    You could use a 3 amp relay with a fuse, but if the fuse blows regularly, you will want to change to a higher rating. (Testing the circuit for curent and adding a margin of safety is okay- but if it is very low current, the thermostat would always be for that one furnace system.)

    The relay itself is your isolation device from an AC circuit and really your best choice. If you use a SSR, would you put it on the furnace or at the thermostat? If it is at the furnace and powered from the thermostat, someone might try to do a repair without understanding that this is a custom installation.

    ALSO
    I am still curious about the AC. It seems to me that you really need to have two control systems: one for high and one for low (at least a three wire system). If so, one relay would not work because to have a middle 'do nothing state' if you are in your comfortable range.

    And, if you have the fan -- that adds a fourth wire and another relay. This one may be programed for temperature extremes, always on, always off, or timed duration.

    It is a great project for the BasicStamp 24p with the one wire temperature sensor (or sensors) because you can add a real time clock and really program to conserve energy when you are sleeping or not at home.

    The mutiple sensor capability means you can program for the living room during normal use and the bedrooms during sleep hours.

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    G. Herzog in Taiwan

    Post Edited (Herzog) : 8/18/2004 8:52:55 AM GMT
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2004-08-17 17:53
    Excuse me, I accidentally posted the above as I was trying to delete the last portion (which is off the subject because there is no fan or A/C (I misread)).

    Disregard, the ALSO section.

    After you get through with the Furnace thermostat, you might consider a programed controller for you hot water heater (using another BasicStamp). The hot water is America's greatest energy waster. You could program for it to start up just before peak usage periods (with adequate lead in periods).

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    G. Herzog in Taiwan
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-17 19:02
    Herzog,

    ·· The reason I considered using a Reed Relay if I could was for size and voltage requirement more than anything.· I want to try and make this unit as small as possible, and a 3A Relay will require approx 6 times the space, as well as requiring 12V instead of 5V.· I was hoping to keep the power to the unit at 5V and not require parasite power as some of these units employ.

    ·· My house is a converted 4-Car garage (Don't ask) and is a little drafty sometimes...But the place is small enough that the furance heats it up rather quickly...In any event, this unit was installed by my dad, and if I ever move away it will be restored to the "factory" Honeywell Thermostat.

    ·· As for A/C, I have bad Asthma/Allergies and A/C is one of those things that somehow makes it all worse.· So we'll never have a Furnace/AC unit to require the extra hardware.

    ·· In one sense you're correct though...I should just use the 3A Relay and run it off a higher voltage Wall-Wart.· That way I don't have to worry about considerations like future use.· There just happens to be an outlet right below where the Thermostat mounts, so it's not really an issue, I guess...Besides, I really want to put a larger than needed Backlit LCD Display on this that shows me more information than I really need...I was at one point thinking of using the Sensiron Temp/Humidity Sensor that Parallax sells, just to have the extra information (Humidity), but it's back-ordered and I don't know if I want to wait or not.· I will have the DS1620 tomorrow, so I could continue with that.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    ·
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2004-08-18 09:09
    You might be able to use that 24 VAC from the furnace (there may be a door bell transformer somewhere) and just pull a second set of wires for power through the wall to the thermosat.

    I think the 7805 is rated for 30volts DC and after a bridge rectifier and filtering you would have less than 24 VoltsDC.

    If you are lucky that 24VAC transformer might be center-tapped to provide 12VAC which is even better.

    I suppose there is no reason for a battery other than wires (the furnace won't operate in a blackout without the 24VAC control circuit)

    I hate 'wall warts' taking up outlets and dangling wires. This way you would have a clean architectural feel to the whole thing. If the power supply is a size problem create a module that converts the 24AC to regulated DC or use a wall wart at the furnace, then you just have to pull the extra pair of wires to the wall unit.

    I also like the idea that design should take you towards something that is marketable and transferable. If your friends like it, they may ask you to make another one. Or, if it is a good system, Nuts & Volts would like to publish an article.

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    G. Herzog in Taiwan
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-08-18 17:35
    It's more worth it to have the Wall-Wart for this, since pulling wires though without cutting into a wall is not an option.· Besides, our THermostat is located in our kitchen and there's a Microwave cabinet right next to it.· The outlet is behind the cabinet, so I won't see the wire anyway.

    As I understand it, some use parasite power, and have a rechargable battery that it temporarily runs off when the relay is closed.· I was going to do that, but this is just a hobbyist project, and expense isn't an issue, but for anyone copying the project, simplicity is.

    I will consider this in more detail over the next few days...But running more wires is definately out.· It's either Wall-Wart or parasite power.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-11-09 01:55
    Okay, this thread has all but died off, but I have another question for those who can confirm...

    My Digital Thermostat is finally done design and codewise...I am simply waiting to find the right case before I etch a PCB for it.· I finally thought to bring my DMM home and test the current draw on the thermostat at home...It measures only 2.7mA!!!· Does this seem right?· We're talking 24VAC thermostat system for HEAT ONLY Forced-Air Furnace...I was expecting more somehow...Can anyone who knows confirm that this is a correct area for the current draw?· I guess I was expecting around 1A at least...Mostly due to the heating element that's on Analog Thermostats.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
  • BeanBean Posts: 8,129
    edited 2004-11-09 13:35
    Chris just as side note.

    Battery powered digital thermostats use a "latching relay".

    They have two coils, you simply pulse the one coil to close the relay (and it stays closed without any power), then you pulse the other coil to open the relay.

    That is how they can last a year on 2 AA batteries.

    Bean.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-11-09 15:05
    Yeah, I guess I tend to think of latching relays as being bigger...This unit is so small, and I must admit I was surprised to hear the faint metallic "click" of a relay when it kicked on the heat.· I guess I will have to do a little research and see if I can find one of these sub-miniature latching relays.· Thanks.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
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