Multi-purpose refrigerator electronic thermostat

This thermostat can not only indicate the temperature, but also has quite flexible temperature setting and perfect time-delay protection function. It is used to replace the damaged mechanical thermostat of refrigerator, and the effect is very good.

How it works

The circuit is shown in the figure below. The temperature sensing element Rt is a negative temperature coefficient thermistor, model RC4-B, with a resistance of 3.3kΩ at room temperature of 20℃. According to actual measurements, its resistance is 6kΩ at 2℃, 5kΩ at 8℃, and 4.2kΩ at 14℃. Basically, the resistance changes by about 150Ω when the temperature changes by 1℃.

P1~P4 are LM324 quad op amps. P3 and P4 are connected as voltage followers. When T=2℃, Rt=6kΩ=R5, and R3=R4, so Ua=Ue=10.84V. Ug=Uh=10.84V, then the pointer of thermometer A does not move. When T-140C, Rt=4.2kΩ, Ug=Ua=11.16V, Uh is still 10.84V, so Ug-Uh=0.32V, which is less than the full bias voltage of A 0.45V, so the temperature range that A can indicate is not less than (2~14)℃.

Pl and others form a temperature control circuit. Wl is the lower limit temperature setting potentiometer, and W2 is the temperature difference setting potentiometer. D2, D3, and Q1 form a NAND gate circuit. When both points c and f are high, Q1 is turned on, J1 is pulled in, and the compressor M starts working. D4 lights up to indicate that it is cooling. At this time, Q2 is turned off, J2 is released, and there is no effect on the resistance of W2. Assuming W1+R1+W2=Rw, and because R2=R3, when the temperature drops to Rt>Rw, Ua<Ub, point c flips to a low level, Ql is turned off, and cooling stops. D2 lights up, indicating that the temperature begins to rise. At the same time, Q2 is turned on and J2 is closed, reducing the resistance of W2. Since Rt—Rw when the machine is just stopped, the resistance of W2 after stopping must wait until Rt decreases by more than the same resistance value (temperature rises) to make Ua>Uh, so that point c flips to a high level and M works again.

When Wl=0, Rw=5kn, which is equivalent to the resistance of Rt at 8℃. When W1=1kΩ, Rw=6kΩ, which is equivalent to the resistance of Rt at 2℃, so the actual setting range of W1 is (2-8)℃. Similarly, after J2 is closed, adjust W2. For every decrease of about 150Ω, the temperature must rise by 1℃ (because Rt changes by 1℃, and the resistance changes by 150Ω) to turn on M. Since the maximum value of W2 is about 6.7 times of 150Ω, the temperature adjustment range of W2 is (0-6.7)℃.

P2 and other components form a time-delay protection circuit. Normally, the voltage of C1 is charged to 12V, which is greater than Ue, and point f is a high level. When the power is off, C1 discharges to the circuit through D1. Even if the power is restored soon, since Ud is less than Ue, point f is a low level, D3 is lit, and Q1 is cut off, so that M cannot work. Q2 is turned on. Due to the charging of C2, the contact of J3 is closed for a short time, and the remaining power of C1 is discharged. Subsequently, C1 is charged through R6. After about 6 minutes, Ud>Ue, point f is a high level, providing the conditions for M to start up, and D3 is extinguished, thereby realizing the power-off delay protection function.

K is the delay elimination button. When the refrigerator is powered on for the first time, or after a long power outage, if you want to turn it on as soon as possible, you can press K once to quickly charge C1 and turn it on immediately.

Component selection and fabrication

T can use 220V/6V instrument indicator transformer, A can use 300V small meter head, and remove the internal resistor, diode and scale. 2. JZC-22F can be selected.

The debugging work mainly involves marking the temperature scale on the panel of A and the temperature scale on the position of the knob of W1. Since the above principle description is quite detailed and it is not difficult for readers to complete it by themselves, this part of the description is omitted. The knob of W2 can be marked with 1℃ change for every 150Ω, and (2~6)℃ can be marked.

This thermostat is made into a machine-shaped model, so that the temperature can be easily observed and adjusted. Rt and the lead solder joints are encapsulated in a small metal shell with epoxy resin glue and installed in a suitable position in the cold storage room. The lead wire must be shielded to prevent electromagnetic interference.