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Step 2Schematic
Here is a rough schematic of the circuit (also my first experience with Eagle!).
Notes:
Only the W1 output of the thermostat is used.
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery.
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater.
C1 should be rated ~50VDC. The exact value is not critical.
The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded.
Notes:
Only the W1 output of the thermostat is used.
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery.
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater.
C1 should be rated ~50VDC. The exact value is not critical.
The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded.
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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.
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.