This instructable shows you how to use an off the shelf digital programmable thermostat to control an inexpensive space heater.
Most cheap space heaters have only an analog knob to coarsely set the temperature; even the fanciest models only allow you to set them to turn off automatically after a preset number of hours.
This project allows you to set the room temperature according to the time of day and day of the week, giving you much needed flexibility to save energy and avoid waking up to a freezing house! You can save energy by programming the thermostat to lower the room temperature at night, but still get out of bed to a toasty room in the mornings.
Most cheap space heaters have only an analog knob to coarsely set the temperature; even the fanciest models only allow you to set them to turn off automatically after a preset number of hours.
This project allows you to set the room temperature according to the time of day and day of the week, giving you much needed flexibility to save energy and avoid waking up to a freezing house! You can save energy by programming the thermostat to lower the room temperature at night, but still get out of bed to a toasty room in the mornings.
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Signing UpStep 1: Materials and safety warning
You will need the following materials for this instructable:
- A digital programmable thermostat. I found a used one on ebay for about $15. It is a Bryant and was originally used in a commercial building. Commercial thermostats typically don't have battery backup, something to keep in mind if you plan on moving the heater around and don't want to reset the clock. Commercial thermostats are also typically cheaper than name brand consumer models. Make sure you get one that is programmable, many digital models are not, shocking considering the minimal effort in adding the feature and the energy savings in a typical home!!
- A relay with a 24VDC coil voltage and around 700 ohms coil resistance. The contacts should be rated to at least 15 or 20A at 110VAC minimum. $3-$5 at your local electronics surplus store.
- A 110V to 24VAC transformer. My transformer was rated at 36VAC, 65mA on the secondary, and maintains about 20VAC under load between one end of the secondary and the center tap. 20VAC seems to be within the input supply range of the thermostat, the exact voltage is not critical. Another electronics store find - $3.
- An enclosure, power cord and AC receptacle. I gutted a fax machine power conditioner and got all three for about $2.
- Some parts you might already have in your junkbox - a 1k resistor, 1n4001 diode, 100uF capacitor. A terminal strip or some perfboard.
- And I almost forgot - a space heater. Mine is a Bionaire MicaThermic Convection Console Heater - about $40 (in store price) at Costco.
Read this!!
Safety warning:
Space heaters typically consume on the order of 1500W, or roughly 15A at 110VAC. All wiring needs to be sized appropriately to handle these currents. Undersizing the wire gauge used or poor connections could lead to a fire! Also, operating a space heater while you are not home is probably a bad idea. I recommend unplugging the space heater before leaving it unattended for an extended period of time. Be safe!
- A digital programmable thermostat. I found a used one on ebay for about $15. It is a Bryant and was originally used in a commercial building. Commercial thermostats typically don't have battery backup, something to keep in mind if you plan on moving the heater around and don't want to reset the clock. Commercial thermostats are also typically cheaper than name brand consumer models. Make sure you get one that is programmable, many digital models are not, shocking considering the minimal effort in adding the feature and the energy savings in a typical home!!
- A relay with a 24VDC coil voltage and around 700 ohms coil resistance. The contacts should be rated to at least 15 or 20A at 110VAC minimum. $3-$5 at your local electronics surplus store.
- A 110V to 24VAC transformer. My transformer was rated at 36VAC, 65mA on the secondary, and maintains about 20VAC under load between one end of the secondary and the center tap. 20VAC seems to be within the input supply range of the thermostat, the exact voltage is not critical. Another electronics store find - $3.
- An enclosure, power cord and AC receptacle. I gutted a fax machine power conditioner and got all three for about $2.
- Some parts you might already have in your junkbox - a 1k resistor, 1n4001 diode, 100uF capacitor. A terminal strip or some perfboard.
- And I almost forgot - a space heater. Mine is a Bionaire MicaThermic Convection Console Heater - about $40 (in store price) at Costco.
Read this!!
Safety warning:
Space heaters typically consume on the order of 1500W, or roughly 15A at 110VAC. All wiring needs to be sized appropriately to handle these currents. Undersizing the wire gauge used or poor connections could lead to a fire! Also, operating a space heater while you are not home is probably a bad idea. I recommend unplugging the space heater before leaving it unattended for an extended period of time. Be safe!
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Why use the external electronic circuit, R1 C1 D1 ?
Usually all you have to use is a 24 volt ac relay, connected to w1 and c .
c= common= black
r= hot= red
y=compressor=yellow
g=fan( airhandler)=green
w=heat=white
Just hook the relay between y=yellow and c=common for cooling.
Thanks for your comment!
- Jeff
You can take advantage of this idea without any modification of the circuit. You only have to make sure that you connected your hot wire to the relay so that the "Normally Closed" terminal has power when the relay is not activated. Then you would run that hot wire out to appliance #2.
Maybe you want a heater running if it's below the set point, and you want a fan running if it's above the same set point. Maybe in your greenhouse you want some bright (and hot) lights on if the temp is below the set point, or some not so bright (but cooler) lights running if the temp is higher than the setpoint.
The capacitor, resistor, and diode are part of a circuit that converts the AC control voltage from the thermostat into a DC voltage used to switch the relay. Most relays need a DC voltage on the coil and won't work with AC.
I know you can convert ac to dc using a bridge rectifier using 4 diodes. I guess just one diode will block 1/2 the voltage so that what comes through is 1/2 strength dc. What do the resistor and the cap in that circuit do?
Thanks for making that more clear for me.
A bridge rectifier would be more efficient but requires more parts. The single diode will output the same voltage but as a series of pulses instead of a constant level. The capacitor smoothes out the pulses into a DC level.
I just measured voltage from line 1 of the transformer, and then to both sides of the resistor. The unmodified reading was 21.4vac, and the reading across the resistor was 21.1vac. Does that sound right to you? If so, why do we need a resistor at all for such a small voltage change? Just wondering cause I thought the voltage change was gonna be a lot more.
Thanks
21VAC should be fine, as long as everything works, don't worry about it.
I've heard of voltage drops with resistors. I looked it up on wikipedia. It gave me a lot of info, but nothing to tell me how a resistor works. Like "What does 1.1 K mean?" "What does 1/2 w mean?" If I use different values how does it change the overal circuit?
You are right. If what I have works, then I shouldn't be asking any questions then, right?
Again, don't mean to be a bother.
Thantks
1/2W is 0.5 watts. A watt is a unit of power. The resistor turns some of the electrical energy flowing through it into heat, and the wattage rating describes how much power the resistor can handle before getting too hot and burning up.
The reason the resistor is there is to allow current to flow in both directions through the triac switch inside the thermostat. If the resistor wasn't there (or was too large), then the diode in the schematic would only allow current to flow in one direction through the triac, keeping it from turning off. If the resistor was too small, the output of the thermostat would get shorted and the relay would never turn on. I chose 1000 ohms because it's a good middle of the road value (a guess), and it seems to work ok so I never changed it.
If you want to learn more (for example, what a triac is), here's a link to my favorite beginners guide to electronics:
http://mightyohm.com/blog/2008/12/the-greatest-electronics-book-ever-written/
I highly recommend that you pick up a copy and start reading. There are many resources for learning electronics online and in print, but I think Forrest Mims' book is a good place to start.
I gave you a 5 Best Instructable Ever.
Just wondering though, if you (mightyohm) is the author, why does it list P914 as the author?
Thank you for the great instructable, and your help with it.
Have you tried this in "Cooling" Mode, and plug in a portable air conditioner instead? I'm sure it is possible, just not sure how the wiring currently controls it all. Does it bypass the Controllers cooling portion, or is it one in the same?
Please advise, as I have a real need for a portable air conditioning controller. Your work is perfect!!
Thanks again!
Good Luck!
A++ Idea
I got: a pack of 1.1k 1/2 watt 2% FP resistors. I have no clue what the 2% or FP means.
I got a 100uf 25volt electrolytic capacitor.
Will these work?
The capacitor should be rated for 50V as I wrote in the instructable. 25V might work fine, but you ideally you want a 2x voltage overhead between the rating and the actual voltage on the capacitor. I would use at least a 35V capacitor.
Please be careful building the circuit, as you are playing with line voltage and high currents thru the relay, so incorrect wiring or shorts can cause fires and personal injury.
I was just guessing at what I might need, since I had just grabbed the info I "Thought" I would need and ran out the door.
Looks like my resistors are fine. I just went out to my garage and pulled a cap off of a circuit board I had laying out there. It's a can type that says 100uF 160volt. I'm guessing that will do :) The leeds are very short, but a little creative soldering should fix that.
I am used to dealing with home electricity, so yes, I will be careful. When I connect my wires to my relay it will be tight. I will use slide on type crimp on connectors that have rubber insulators. If that doesn't do well enough I will use liquid tape and put a good coating of tar on everything. :)
I hope you can see the image I uploaded. I can see that (R) goes straight to the transformer, and (W) goes to the relay, but how do I give my thermostat constant power for it's internal functions?
Thanks for being so helpful.
Robert in TX
It works perfectly except my $5 programmable thermostat I got from Goodwill, the display doesn't work correctly (some of the bars are missing), so I have to guess at what the display is really saying!!! :(
You might be able to take the thermostat apart and fix it. The LCD probably sits on a flexible contact block that could be cleaned with alcohol as well as the PCB pads the strip connects to. But then again sometimes this can make it worse instead. Proceed at your own risk. :)
I think (and I could be wrong) my set up draws too much current (5 watts, according to my Kill-a-watt meter) while idle. When I have it in the "A/C mode" it only draws 86 watts. The only part of the circuit getting hot is the Transformers. I already fried one, lol!
Do you, or anyone else have any ideas why this would happen? Are there any circuits I could use to lower the current draw while plugged in?
Transformer: 18VAC large 10A Radio Shack
Relay: 24VDC coil voltage at 500 ohms coil resistance
Diode, Resistors and Caps are of correct voltage, and rating.
(Picture of Transformer and Project attached)
Thanks in advance for all your help, as in the past!!
What voltage do you see at the output of your transformer? Is it actually 18V or something else? What kind of digital thermostat are you using and how much power is it supposed to consume?
You may want to try a different transformer. Mine is rated for only 65mA and works fine, yours is 10x that. I am not an expert on power transformers, but I know that transformers will dissipate some energy as heat. It's possible that by using an oversized transformer for your supply, it is operating very inefficiently with such a light load and getting hot.
Definitely fix this problem, a transformer fire is not something you want to deal with. None of the components in this circuit should ever get more than warm to the touch.
"What voltage do you see at the output of your transformer? Is it actually 18V or something else? What kind of digital thermostat are you using and how much power is it supposed to consume?"
I am getting 19.3 VAC on the Transformer, going to the controller. My Honeywell RTH221, rated @ 1 Amp, 24 VAC MAX Load, said not to use connection C, so I did not. Also, This transformer (my first, slightly smaller one one got way too hot) I am not using the CT, only the outer 2 connections. Could this cause a problem? Could it be the 24 VAC requirement? I am too low, and it draws too much, trying to get the 24 VAC?
"You may want to try a different transformer. Mine is rated for only 65mA and works fine, yours is 10x that. I am not an expert on power transformers, but I know that transformers will dissipate some energy as heat. It's possible that by using an oversized transformer for your supply, it is operating very inefficiently with such a light load and getting hot."
I will definitely do that, Bigger is NOT always better....:) Time to find a smaller one!
Yes, I expect some heat, so I was carefully monitoring the set up, and it was fine for the first 3 - 4 hours, even days. I would unplug it when I did not need it. When I started leaving it plugged in for long periods, it started to warm up. Placing the fan helped a ton, but one day, even that was not enough. Got too warm, and I had to unplug it all.
I re posted the pics, hope they come thru, Thanks again!!
Sparkyskarts
Your thermostat is battery powered. This is why it doesn't need the C wire connected. It shouldn't be drawing any power from the transformer at all. (I wish it was easier to measure AC current, because then you could actually measure it!)
I'm stumped. Apart from checking your wiring again and switching transformers, I'm not sure what to do. If you disconnect everything from the secondary side of the transformer and leave the primary plugged in for an hour or two, does it still get hot?
Thanks Again,
Happy Holidays to you!
Or use a 120v Thermostat. Something like this http://www.pexsupply.com/Honeywell-T26A-15-120V-SPDT-Thermostat-40-60F
Just some ideas to throw out there.
I wish there was a digistat that simply called for heat if the temp was below the exact set point, and stopped calling for heat exactly at the set point. It doesn't matter if that results in a power cycle every minute or two, as far as I'm concerned. Anyone got any ideas?