The medical thermostat introduced in this example uses infrared light control technology to heat or cool the semiconductor refrigeration device (with commutation heating function), so that the temperature in the incubator is constant at a set value.
Circuit Operation Principle The medical thermostat circuit consists of a power supply circuit, an infrared control circuit, a temperature detection control circuit, and a temperature abnormality protection alarm circuit, as shown in Figure 9-42.

The power circuit is composed of a power switch S, a power transformer Tl, T2, a rectifier diode VDl-VD8, a three-terminal voltage regulator integrated circuit IC2, and a filter capacitor C4-C7.
Temperature detection control circuit consists of thermistors RT1, RT2, operational amplifier integrated circuit IC3 (NI-N4) internal Nl, N2, resistors R2, R6, potentiometer RPl, RP2, capacitor C9, diode VD14-VD16, transistor V6 and relay K1.
Infrared light control circuit consists of transistors Vl, V5, V7, infrared light-emitting diode VM, infrared light-sensitive transistor V2, resistor Rl, R3-R5, thyristor VT, capacitor Cl-C3, three-terminal voltage regulator integrated circuit ICl, indicator light HL and The control contacts Kl-1, Kl-2, etc. of the relay K1 are composed.
Temperature abnormal protection alarm circuit by potentiometer RP3, RP4, IC3 internal operational amplifier N3, N4, resistor R7-R9, diode VD9-VDll, capacitor C8, LED VLl-VL3, transistor V3, V4, audio integrated circuit lC4 , the speaker BL and the relay K2.
AC 220V voltage all the way through T2 step-down, VD5-VD8 rectification, C4 filter and IC2 voltage regulation, provide +l2V working voltage for IC3 and Kl, K2, etc.; the other way to Tl step-down, VDl-VD4 rectification and then add to infrared Light control circuit.
RT1 and RT2 are used as temperature sensor components to detect the ambient temperature and the temperature inside the incubator.
Adjust the resistance of RPl and RP2, and set the control temperature of Nl and N2 respectively.
When the ambient temperature is lower than the set temperature of RPl, Nl outputs a low level, V6 is in an off state, and Kl does not work. At this time, if the temperature in the incubator is lower than the set temperature of RP2, then N2 outputs a high level, V3 is turned on, VM emits infrared light, and the infrared phototransistor V2 is turned on by light, and then VT is triggered to turn on, VDl - The DC voltage after rectification of VD4 is applied to the voltage regulator filter circuit composed of ICl, Vl and CI-C3 by VT, and then the DC voltage is output from the emitter of V1. The voltage is applied to the semiconductor refrigeration device via the normally closed contact of K1, and the semiconductor refrigeration device is energized to start heating. When the temperature in the incubator reaches the set temperature of RP2, N2 outputs a low level, so that V7, VL4, Vl, V2, and VT are all turned off, and the semiconductor refrigeration device is powered off to stop heating. When the temperature in the incubator drops below the set temperature again, VT and V1, V2, etc. are turned on again. This cycle is repeated so that the temperature in the incubator is constant near the set temperature.
When the ambient temperature is higher than the set temperature of RPl, Nl outputs a high level, V6 saturates and conducts, Kl energizes and pulls, its normally closed contact is opened, and the normally open contact is turned on. When the temperature in the incubator is higher than the set temperature of RP2, N2 outputs a low level, so that V7 is turned on, VL4 emits infrared light V2, VT and Vl are turned on, and Vl emitter output +l2V voltage is normally opened by Kl. The contacts provide operating power for the semiconductor refrigeration device, and the semiconductor refrigerator begins to cool, causing the temperature in the oven to slowly drop. When the temperature in the incubator drops to the set temperature of RP2, N2 outputs a high level, so that V7, VM, V2, VT, and V1 are all turned off, and the semiconductor refrigerator stops cooling. The above work process is carried out repeatedly, so that the temperature in the incubator is kept constant.
When the temperature detection and control circuit works normally, the operational amplifiers N3 and N4 both output low level, V4 is in the off state, K2 does not pull in, IC4 does not work, BL does not sound, VLl and VL2 do not emit light. When the temperature in the incubator is out of control for some reason (beyond the upper limit temperature or lower than the lower limit temperature), N3 or N4 outputs a high level, so that VLl and VL2 are lit, lC4 is energized, and the audio electric signal output by IC4 is amplified by V3. After that, the driving BL emits an alarm sound; at the same time, V4 is turned on, K2 is energized, and its normally closed contact is disconnected, the main power is cut off, and heating or cooling is stopped.
Component selection
Rl-R9 uses 1/4W metal film resistor or carbon film resistor.
Both RTl and RT2 use MF53-1 type negative temperature coefficient thermistor.
Both RPl and RP2 use multi-turn wirewound potentiometers; both RP3 and RP4 use organic solid potentiometers.
Cl-C4 and C7, C8 select aluminum electrolytic capacitors with a withstand voltage of 16V; C5 and C6 use monolithic capacitors or polyester capacitors; C selects tantalum electrolytic capacitors.
VD5-VD9 and VDl2 select 1N4007 type silicon rectifier diode for use; VDlO, VDll and VDl3-VDl6 select 1N4148 type silicon switch diode for use.
VLl-VL selects ordinary light-emitting diodes of φ5mm; VL4 selects PH303 type infrared light-emitting diodes.
Both ICl and IC2 use LM7812 three-terminal regulator integrated circuit; IC3 selects LM324 or D324 type four operational amplifier integrated circuit; IC4 selects KD9561 type sound effect integrated circuit.
V2 selects TLPI07 type infrared phototransistor; V3-V6 selects S8050 type silicon NPN transistor; V7 selects S9012 or 3CG9012 type silicon PNP transistor.
T2 selects 5W, the secondary voltage is 13V power transformer.
Both Kl and K2 use l2V DC relays with two sets of control shaft contacts.
The operating voltage of the semiconductor refrigerator is l2V, and its power should be reasonably selected according to actual needs.
The operating current and power of VD1-VD4, VT, V1 and T1 should be reasonably selected according to the operating current of the semiconductor refrigeration device.
HL selects the indicator bulb of l2V, O.2A.
BL uses 0.25W, 8Ω electric speakers.