12 Volt Differential Temperature Controller

12 Volt Differential Temperature Controller

Introduction

Alternative energy projects often require the movement of air or water from a warm place to a cool place for heating, or vice-versa for cooling. A differential temperature controller can be used to automate this process. This device consists of two electronic temperature sensors and some circuitry to detect the difference in temperature between the sensors. When the hot side sensor's temperature rises above the cold side sensor's temperature, the circuit applies power to a circulating device such as a fan or a pump. The circulating device stays on until the two temperatures are equalized. This device can be used to improve the performance of the Self Powered Solar Box Furnace project, it has also been used to circulate hot water through a solar water collector.

Specifications

Supply voltage: 12V (nominal).
Maximum load current: 10A at 12V.
Sensor temperature limits: -40c to +100c
Controls: Off-Auto-On switch
Indicators: High and Low LEDs

Theory

Power to the circuit is switched via half of the On/Off/Auto three way switch (DPDT center-off) and fed to the 78L09 9 volt voltage regulator. The 9 volt regulator powers the LM324 quad op-amp and the 4011 quad NAND gate.

Each of the two LM335A sensors produce a voltage that is proportional to the temperature (in Kelvin). The test points (tp) show the temperature of each sensor. The LM324 amp on the left is wired as a differential amplifier, the output on pin 8 is based on the difference between the two temperatures. The Sensitivity control adjusts the gain of the differential amplifier and the Offset control adjusts the offset between the two temperature sensors. The second half of the On/Off/Auto switch is used to unbalance the differential amplifier when in the On mode, forcing the rest of the circuit to stay in the on mode.

The right two LM324 amp sections are wired as a window comparator. The window comparator outputs feed to the left two sections of the 4011 NAND gate IC, which forms an R-S flip-flop. The window comparator and flip-flop function together to provide a single output that has a hysteresis (dead band) between on and off. If the circuit is on, it tends to stay on and if it is off, it tends to stay off. Note that the sensitivity adjustment affects the effective width of the dead band.

The output of the flip-flop drives the right two 4011 NAND gates, these are wired in parallel and drive the IRLZ44N logic-level input MOSFET. The MOSFET switches the negative side of the load on and off. The 1N4004 diode is used to supress inductive voltage spikes from the pump or fan motor.

Alignment

Adjust the sensitivity control to the middle of its range. With both temperature sensors at the same temperature, adjust the offset control until both LEDs come on. Put the temperature sensors on the hot and cold sides of the controlled heating system and observe the operation. The offset control should then be adjusted so that the circuit doesn't come on until the warm side is significantly warmer than the cold side. The more sensitivity (higher resistance) the circuit has, the smaller the dead zone will be. It may be helpful to monitor the LEDs and the test point on pin 8 of the LM324 IC (temperature difference) when adjusting the circuit. The circuit flip-flops above and below the 4.5V point.

Use

Connect a 12VDC supply to the power terminals and a fan or water pump to the load terminals. Note that if the fan or pump's negative lead is grounded to its case, the case should be electrically isolated from ground so that the motor does not turn on.

The cold side sensor should be thermally connected to the cold side of a heat source and the hot side sensor should be thermally connected to the hot side of the heat source.

If the three way switch is on, the fan or pump will always turn on. If the three ways switch is in the auto position, the fan or pump will go on when the hot side is warmer than the cold side and it will go off when the temperatures equalize. The Red/Green LED will be Red when the fan or pump is running, green when the fan or pump is stopped, or both Red and Green during the transitional hysteresis state. Another way of saying this is that on warming, the LED sequence will be Green, Both, Red and on cooling the LED sequence will be Red, Both Green. If the three way switch is off, the circuit will be disabled.

Resources

This schematic shows a similar circuit that runs directly from a PV panel, it would be a good idea to add a 1000uF 35V capacitor across the panel for stability.

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