boiler circuit

The industrial boiler is a device that integrates temperature indication, temperature control and over-temperature alarm. It can automatically control the starting and stopping of the circulation pump according to the preset temperature
. It is suitable for various industrial low-pressure boilers. Figure 7-4 is a preset temperature low-pressure boiler circulation pump control circuit composed of a thermistor
. ~~ (1) Circuit component composition For the circuit shown in Figure 7-4, the structure, function, characteristics of the integrated circuit and the control relationship between
    the key components thermistor, indicating instrument, alarm device, etc. should first be clarified .
The analysis of components is the key to this circuit.
    1) Three integrated circuits A1NA3: Al (LM324) is a dual-row 14-pin integrated circuit with four independent operational amplifiers.
In the circuit, a current amplifier circuit (composed of Al-4, etc.) and three Voltage comparison circuit (composed of Al-l ~ Al-3, etc.); A2
(NE555) is a time base circuit, which forms the control circuit of the relay KAI coil and indicator lights LED1 and LED2; A3
(7812) is a It is a three-terminal fixed voltage stabilizing integrated circuit with a voltage stabilization value of 12V.
    The power supply of the KAJ relay coil is controlled by the output signal of pin ③ of A2, and the level of the output of pin ③ of A2 is controlled by the
output signals of the stop key switch TA, the run key switch QA and the two comparators Al-2 and Al-3. Controlled by the thermistor RT, the status of the two comparators AB2 and AB3 is
controlled by the temperature of the water in the boiler sensed by the thermistor RT. RT is a temperature sensor sealed in a glass tube with epoxy resin glue, which is used to
directly sense the temperature in the furnace.
    2) Relay: KAJ relay has a set of normally open contacts KAJ1. It is used to control the power supply of the AC contactor KMC coil.
The three sets of normally open contacts KMCI-KMC3 of the KMC control the power supply of the circulation pump motor M.
    3) Indicating instrument: G is a 59L1 ammeter (which can also be modified with a voltmeter), which is used to indicate the temperature value detected by RT, which is
the temperature of the water in the boiler. G is driven by ICl-4, which is also controlled by the temperature detected by RT.
    4) Buzzer: The buzzer BL is a finished component FMQ sounder (>70dB), driven by VT1, and VT1 is
controlled by a comparator composed of Al-l. Al-l is also controlled by the temperature detected by the thermistor RT. SA1 is a switch for automatic and manual temperature control
.
    When we are familiar with the structural principles and various control relationships of components, it will be easy to analyze the working principle of the circuit shown in Figure 7-4
.
    (2) Working principle of the circuit
    1) Power supply circuit: 380V three-phase AC power passes through the FU1-FU3 fuse components and goes all the way to the KMC1-KMC3 main contacts. These three
sets of contacts are closed to provide power for the circulating pump motor M. In addition All the way to the AC contactor and power transformer Tl.
    The primary connection of the Tl power transformer is between Li and L2 phase voltage, which is used to step down the 380V power supply and output 12V AC
low voltage This voltage is filtered by VD1-VD4 bridge rectifier, C, and stabilized by A3 to After 1ZV, it is provided to temperature detection and control circuits as working
power supply.
    2) Turn-on voltage comparison circuit: The turn-on voltage comparison circuit consists of resistor R4, potentiometer RPZ, R3, Al-2, VD6, R8, etc.
    Reference voltage setting circuit: This circuit is composed of Shu, RP2 and MA. It is used to divide the +12V voltage and add it to
the ⑤ pin of the Al-2 in-phase signal input terminal as a comparison reference voltage.
    Comparison voltage circuit: Pin ⑥ of Al-2 is the comparison voltage input terminal. The comparison voltage is the voltage Vn obtained by dividing +12V between RT and Ri.
Since the resistance value of RT will change with the change of temperature, Vo will also Change accordingly.
  Working process: When the temperature rises, the Vo potential also rises. Once the ⑥ pin potential of Al-Z is greater than the ⑤ pin, its ⑦ pin flips and outputs
low level , causing VD6 to conduct t through RH-SA1 (in the automatic position) --Ri;, the ② and @ pins of A2 are triggered due to low level, and the @ pin output
is high level. On the one hand, the green light-emitting diode is turned on and emits green light, indicating that the system has entered the running state; on the other hand, On the one hand, the KAJ relay
coil is electrically closed, and its normally open contact KAJ1 is closed, and the power supply to the AC contactor KMC coil is turned on, so that it is also closed, and its three sets of main contacts
KMC1 ~ KMC3 are all closed and then connected again After the circulation pump motor M is supplied with electricity, the system begins to perform heat exchange (for heating
) and the temperature inside the boiler tank gradually decreases. Adjust the value of RP2 to preset the temperature range for starting the circulation pump.
    3) Stop voltage comparison circuit: The stop voltage comparison circuit consists of 'wind, RP3, Rs. Composed of Al-3, VD7, horse, etc.
    Reference voltage setting circuit: Al-3 rhyme in-phase signal input A terminal @ pin is the reference voltage terminal
. The reference voltage is obtained by dividing the 12V voltage by R6, RP3 and R. ,
    ⑨ pin of the comparison voltage circuit Al-3 is the comparison voltage input terminal, and the comparison voltage is also the voltage Vo obtained by dividing the 12V voltage by RT and R-
. Its working process is: when the temperature in the saw furnace furnace drops, the RT resistance value gradually becomes smaller, causing the potential of the ⑨ pin of Al-3 to drop. Once
the ⑩ pin voltage of A1-3 is greater than the @ pin voltage, its ⑧ pin outputs a high level, which in turn causes A2 to flip to the reset state, and its ③ pin outputs
a low level , which on the one hand makes the LEDZ red light-emitting diode conductive. It lights up, indicating that it is in shutdown state; on the other hand, KAJ is powered
off and released, and its K/Ul contact is disconnected, which cuts off the power supply to the AC contactor KMC coil, and its KMC1-KMC3 contacts are also disconnected, cutting off the circulation pump. power supply to the motor
, causing it to stop working. Adjust the value of RP3 to preset the temperature range in which the circulation pump stops.
    4) Over-temperature alarm circuit: The over-temperature alarm circuit is composed of Rai, RP1, Rz, Al-l, VD5, Pin, VT1 hard buzzer BL, etc.
    Reference voltage setting circuit: The inverting input terminal ② pin of Al-l is the reference voltage terminal. The reference voltage is
obtained by dividing the 12V voltage by Rl, RP1, and finally. According to the comparison voltage circuit : Pin ③ of Al-l is the comparison voltage input terminal, and the comparison voltage is also RT and Shujiang_. -12V voltage
divided voltage U.