Constant temperature incubator using diodes as temperature probes

The circuit is shown in Figure 1. Silicon diodes VD1 and VD2 are used as sensors. When the forward current of the silicon tube PN junction is constant, the forward voltage drop decreases by about 2mV for every 1°C increase in temperature. This sensor has good linearity and can work in the range of -50°C to +150°C.

A1 acts as a buffer and isolates, while A2, R5, RP1, etc. form a differential amplifier circuit. The temperature is controlled by RP1. VD1, VD2, R1, R3, R4, etc. form a temperature measuring bridge, whose output signal is amplified by A2 and then sent to the inverting input terminal of A4. A3, C3, R8, R9, etc. form a triangle wave forming circuit. The triangle wave signal on C2 is sent to the in-phase input terminal of A4 as a reference voltage. Voltage comparator A4 is used as a pulse width modulator, and the pulse width of its output rectangular wave is modulated by the voltage at point A. As shown in Figure 2, when the constant temperature incubator is powered on for heating, the temperature gradually rises, and the forward voltage drop across the temperature sensors VD1 and VD2 decreases linearly with the temperature rise. The linearly rising voltage is amplified by the inverting output of point A by A2, and then sent to the inverting input terminal of the voltage comparator A4 for comparison with the sawtooth voltage at the in-phase input terminal. When the voltage of the inverting input terminal is less than the voltage of the non-inverting input terminal (triangular wave voltage), A4 outputs a high level, S triggers on, and the heater RL is powered on for heating. When the temperature in the constant temperature incubator is higher than the set value, the voltage of the inverting input terminal gradually increases, the pulse width of the rectangular wave output by A4 decreases, the power-on time of the heater also decreases, and the temperature drops. Conversely, the opposite is true, so the purpose of constant temperature can be achieved.

VS is a unidirectional thyristor, and half-wave alternating current passes through the heater RL. Therefore, the heater power is half of the nominal power of the heater, that is, the actual heating power of a 300w heater is 150w, which is beneficial to prolong the life.

The 12V power supply circuit of the device is composed of R10, VD3, DW, C3, etc., among which R10 is a voltage drop resistor and DW is a 12V voltage regulator diode. IC2 provides a 5V stable power supply to the temperature measuring bridge, and the LED is used as a heating indicator.

1. VS uses 2A/400v unidirectional thyristor, such as CR24M or CSM2B, etc. Its trigger current Igt is required to be less than 2mA.

2. RP1 uses a 470Ω solid potentiometer, which is required to be a linear type, that is, its resistance change is in a linear relationship with the rotation angle, so as to facilitate temperature calibration of the temperature indicating dial. RP2 uses a 1kΩ solid fine-tuning resistor.

3. The heater RL can be heated by an ordinary iron with a power of 300W. 4. The volume of the warming box: 300mm long × 250mm wide × 250mm high.