Electronic Thermostat and Relay Circuit
Here is a simple thermostat circuit that can be used to control a relay and
supply power to a small space heater through the relay contacts. The relay
contacts should be rated above the current requirements for the heater.
Temperature changes are detected by a (1.7K @ 70F) thermistor placed in series
with a 5K potentiometer which produces about 50 millivolts per degree F at the
input of the LM339 voltage comparator. The two 1K resistors connected to pin 7
set the reference voltage at half the supply voltage and the hysteresis range to
about 3 degrees or 150 millivolts. The hysteresis range (temperature range where
the relay engages and disengages) can be adjusted with the 10K resistor between
pins 1 and 7. A higher value will narrow the range.
In operation, the series resistor is adjusted so that the relay just toggles off
at the desired temperature. A three degree drop in temperature should cause the
relay to toggle back on and remain on until the temperature again rises to the
preset level. The relay action can be reversed so it toggles off at the lower
end of the range by reversing the locations of the 5K potentiometer and
thermistor. The 5.1 volt zener diode regulates the circuit voltage so that small
changes in the 12 volt supply will not effect operation. The voltage across the
thermistor should be half the supply or about 2.6 volts when the temperature is
within the 3 degree range set by the potentiometer. Most any thermistor can be
used, but the resistance should be above 1K ohm at the temperature of interest.
The series resistor selected should be about twice the resistance of the
thermistor so the adjustment ends up near the center of the control.
Electronic Thermostat and Relay Circuit
Title: Electronic Thermostat and Relay Circuit
electronic circuit
Source: unknown
Published on: 2007-07-21
Reads: 2388
Print version: 
Other electronic circuits and schematics from Test and measurement
-
Capacitance Meter
-
IR Remote Control Tester
-
Mosfet TESTER
-
Zener Diode Tester
-
A simple Remote control Tester
-
High Resistance Voltmeter
-
Voltage Monitor
-
Oscilloscope testing module (huntron circuit)
-
Latching Continuity Tester
-
Decibel Meter