Differential Temperature Ceiling-Fan Control.(temperature-controlled automatic ceiling-fan switches)(Brief Article)

Publication: Poptronics

Publication Date: 01-MAY-01

Author: HUSTER, DEAN

Q My house has cathedral ceilings--excellent heat collectors. I am looking for an automatic switch or circuit to turn on my ceiling fans when the temperature difference between ceiling and floor exceeds a certain value--say, 20[degrees] F--and turn them off when the difference is down to 5[degrees] F. Can this circuit be hooked up in parallel with the normal on/off switch?--P.K., via e-mail

A A pair of thermistors, R2 and R5, and a simple comparator, IC1, should do the trick, as shown in Fig, 3. At 25[degrees] C, the two thermistors have a value of 10,000 ohms, so I chose a 10,000-ohm resistor to go in series with each to create a voltage divider with six volts nominal fed to each comparator input.

A 2000-ohm potentiometer serves to set the temperature differential at which the two circuits will trip the comparator. If you assume the potentiometer to be set so that the R1 = R3 + R4 with both thermistors in the same location, the comparator will be on the verge of either switching on or off. In other words, barely touching R2 will turn the fan on, and barely touching R5 will turn it off.

The ambient temperature will raise the resistance of both thermistors at the same rate, and the comparator will still be on the verge of switching on or off.

If you increase the value of R4, you will increase the differential so that R2 will have to get even warmer than R5 before the fan will turn on. This differential stays pretty much the same whether the room temperature is set warm or cool. You can increase the value of R4 if you want a larger differential. I didn't actually check the maximum differential that this circuit would provide.

The output of IC1 drives a Darlington power transistor, Q2, which turns on RY1. Diode D1 protects Q1 from the reverse voltage created when the RY1 field collapses as it turns off.

Resistors R4 and R5 should be mounted near the floor, away from any sources of heat. I'd suggest running both sensor circuits through a two-conductor shielded cable. Using shielded wire, such as instrumentation or balanced-microphone cable, will minimize stray AC pickup, which would really mess up the comparator. Capacitors C1 and C2 will help by bypassing any stray AC to ground. I used a RadioShack 271-11OA for the two thermistors.

Relay RY1 needs to have a 12-volt coil with contacts rated to handle the load of the fan. The power supply can be made in a similar way to the regulator circuits that were described in last month's "Q&A." The circuit doesn't need a high degree of regulation, but make sure that the relay coil doesn't draw much over 500 mA. You might have to heatsink Q1 if it's expected to pass much relay current.

The relay contacts can be put in parallel with the switch that controls the fan. The closure of either the original switch or the relay will turn the fan on. Turning power off to the control circuit will de-energize the relay so that the original switch has the only fan control.

Be sure to build the circuit into a closed box so it cannot be touched accidentally. There also may be some municipal codes you must comply with when you work on your house wiring.