The power circuit is composed of a power switch S, a fuse FU, a power transformer T, a rectifier bridge stack UR, a three-terminal voltage regulator integrated circuit ICl, an IC2, and a filter capacitor Cl-C3.
Temperature detection control circuit consists of thermistor RT, resistor Rl-Rl5, potentiometer RPl-RP5, capacitor C4-C6, diode VDl, VD2, LED VLl, VL2, transistor V, ammeter PA, relay K and operational amplifier The integrated circuit lC3 (Nl, N2) is composed.
When the power switch S is turned on, the AC 220V voltage is stepped down by T, and an AC 15V voltage is generated on the windings W2 and W3 of the T. The AC l5V voltage on the W2 winding is UR rectified, Cl filtered, and IC1 regulated, and a +12V voltage is generated to supply IC3 and relay K. The +l2V voltage is also secondarily regulated by IC2 to +6V and added to the PA. The AC 15V voltage on the W3 winding acts as the operating power source for the heater EH.
RT is used to detect the temperature of the liquid. RPl and RP2 are used to adjust the accuracy of the temperature indication; RP3-RP5 is used to set the control temperature.
When the temperature of the chemical solution is lower than the set temperature, the operational amplifier N2 outputs a high level, so that V is turned on, K is energized, and its normally open contact is turned on, and EH is energized to start heating. As the temperature of the liquid rises, the resistance of the RT decreases. When the temperature of the liquid reaches the set temperature, N2 outputs a low level, so that V is turned off, K is released, and EH is turned off to stop heating. When the temperature of the liquid medicine drops below the set temperature, V is turned on again, K is pulled in, and EH is energized.
The above process is carried out repeatedly, so that the chemical solution can be kept constant near the set temperature.
When EH is energized, VLl goes out and VL2 lights up; when EH stops heating, VL2 goes out and VLl lights up.
PA is used to indicate the temperature value of the drug solution.
Component selection
Rl-Rl5 selects 1/4W metal film resistor for use.
RT uses a negative temperature coefficient thermistor.
RP1, RP2, RP4 and RP5 use small synthetic carbon film potentiometer or variable resistor; RP3 selects multi-turn wirewound potentiometer.
Cl uses aluminum electrolytic capacitors with a withstand voltage of 25V; C2, C3 and C6 select aluminum electrolytic capacitors with a withstand voltage of 16V; C4 and C5 use polyester capacitors or monolithic capacitors.
Both VDl and VD2 use 1N4007 silicon rectifier diodes.
VLl and VL2 select φ5mm light-emitting diodes.
UR selects the rectifier bridge stack of lA and 5OV.
ICl selects LM7812 type three-terminal regulator integrated circuit for IC; IC2 selects LM7806 type three-terminal regulator integrated circuit for IC2; IC3 selects LM324 type operational amplifier integrated circuit for IC3.
PA selects 8002 type ammeter.
EH uses the electric heating wire of the electric blanket, and its DC resistance value is about 100 Ω (power is about 10 W).
T selects the power transformer of lOW.
S selects 5A, 220V power switch.
K selects JRX-I3F type l2V DC relay.
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