Medical thermostat II

The temperature control range of the medical incubator introduced in this example is -50-+150 °C, and the temperature control accuracy is ±0.3 °C.
Circuit Operation Principle The medical thermostat circuit consists of a Power Supply circuit, a temperature detection circuit, and a control execution circuit, as shown in Figure 9-41.

The power circuit is composed of a fuse FU, a power switch S1, a power transformer T, a rectifier bridge stack UR1-UR3, a three-terminal voltage regulator integrated circuit IC1-lC3, and a filter capacitor Cl-C6.
The temperature detection circuit consists of the thermistor RT, the resistor RO-R8, the potentiometer RPl, the R-channel, the integrated voltage regulator IC4, the Nl-N3 inside the operational amplifier integrated circuit lC5 (Nl-N4), and the capacitors C7 and C8. .
The temperature indicating circuit is composed of an IC6, a control switch 52, and a digital meter.
The control execution circuit is composed of resistors R9-Rl5, potentiometer RP3, control switches S3, 1C5 internal operational amplifier N4, diodes VD1, VD2, transistors V1, V2, light-emitting diodes VL1, VL2 and relay K.
After the AC 220V voltage is stepped down by T, an AC 15V voltage is generated on the winding W2 of the T, and two sets of AC 18V voltages are respectively generated on the windings W3 and W4 of the T. The AC l5V voltage on the winding W2 is regulated by URl, Cl filter and ICl voltage, and the +12V working voltage is provided for the relay K and the temperature indicating circuit; the AC 18V voltage on the winding W3 is regulated by UR2, C2 filtering and IC2 voltage regulation. , a voltage of l5V is generated as the negative working power of IC5; the voltage of AC 18V on winding W4 is UR3 rectified, C3 filtered and IC3 regulated, generating +l5V voltage as the positive working power of IC5. The +l2V voltage is also secondarily regulated by IC6 to +5V, which is used as the working power of the digital meter.
Rl and IC4 form a reference voltage circuit that provides a reference voltage for operational amplifiers N1 and N2.
RT is used as a temperature sensor to detect the temperature in the incubator, and its resistance decreases as the temperature increases. S2 is used to select the display state of the digital meter, and has two display states of measured temperature and set temperature. S3 is used for heating/cooling conversion control.
RPl and RP2 are used to adjust the output voltage value of N3. RP3 is used to adjust the preset control temperature value. In use, first place S2 in the "preset" position, adjust the RP3 preset controlled temperature value, then place S2 in the "measurement" position and display the actual temperature in the incubator through the digital meter.
For heating control, place S2 in the "heated" position. At this time, if the temperature in the incubator is lower than the controlled temperature value set by RP3, N4 outputs a high level, so that Vl and V2 are saturated, K is energized, its normally open contact is turned on, and the heating device is energized. At the same time, the work indication LED VLl lights up.
When the temperature in the incubator reaches the controlled temperature value, N4 outputs a low level, Vl and V2 are cut off, K is released, the heating device stops heating, VLl is extinguished, and the heat indicating LED VL2 is lit.
When the temperature in the incubator is lower than the controlled temperature, N4 outputs a high level again, so that Vl and V2 are saturated and the heating device is energized.
For cooling control, place S3 in the "cooling" position. At this time, if the temperature in the incubator is higher than the controlled temperature value set by RP3, N4 outputs a high level, so that Vl and V2 are saturated, K is energized, the refrigerating device is energized, and VLl is lit. When the temperature in the incubator drops to the controlled temperature value, N4 outputs a low level, Vl and V2 are cut off, K is released, the refrigeration device stops cooling, and VLl is extinguished, and VL2 is turned on. When the temperature in the incubator is higher than the controlled temperature value, the refrigerating device is energized again, so that the temperature in the incubator is maintained near the set temperature.
Component selection
R0 uses precision metal film resistors; Rl-Rl5 uses 1/4W metal film resistors or carbon film resistors.
RPl-RP3 selects multi-turn wirewound potentiometers.
RT uses a thermistor with a resistance of 1OOn at 0 °C.
C1 selects aluminum electrolytic capacitors with a withstand voltage of 25V; C2 and C3 select aluminum electrolytic capacitors with a withstand voltage of 35V; C4-C8 selects polyester capacitors or monolithic capacitors.
VDl selects 1N4001 or 1N4007 type silicon rectifier diode; VD2 selects 1N4148 type silicon switch diode.
Both VLl and VL2 use high-brightness light-emitting diodes of φ3mm, VLl is red, and VL2 is green.
Vl and V2 select S8050 or 3DG8050 silicon NPN transistor.
ICl selects M7812 type three-terminal voltage regulator integrated circuit; IC2 selects LM7915 type three-terminal voltage regulator integrated circuit; IC3 selects LM7815 type three-terminal voltage regulator integrated circuit; IC4 selects LM385 type precision voltage-stabilized integrated circuit; IC5 selects D2024 or TLE2024 type high Precision low drift four op amp integrated circuit; lC6 selects LM7805 type three-terminal regulator integrated circuit.
URl-UR3 selects the rectifier bridge stack of lA and 5OV.
K selects JRX-l3F type l2V DC relay. If controlling a higher power load, increase the AC contactor, use the relay K to control the AC contactor, and use the AC contactor to control the load.
T selects a power transformer with a power of 8-1OW.
S1 selects 10A, 220V power switch; S2 selects single-pole two-position switch; S3 selects bipolar double-position switch.
The digital meter uses a 3-work digital voltmeter with a maximum display value of =199.9.