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First, the working principle and control process
1. Circuit composition and function of each component
The diagram shows a schematic of a KSW-3 type temperature automatic controller. The circuit includes transistors BG1, inductors L1, L2, L3, and capacitors C1, C2, C3, C5, and C8, which together form a high-frequency inductive three-point oscillator. This oscillator generates a signal that is detected by D1. Transistor BG2, along with resistors R6 and R7, and relay J, forms a DC amplifier circuit. The AC contactor CJ is responsible for controlling the power supply to the high-temperature furnace. The power transformer B not only provides low-voltage AC for the temperature conversion circuit and the red and green indicator lights, but also supplies 12V DC through diodes D3 and D4 to power the oscillator and amplifier circuits. The ammeter A is connected in series with the furnace’s power supply, while indicators XD1 and XD2 indicate whether the furnace is energized or de-energized. The temperature conversion circuit consists of thermocouples R and R9–R13, along with capacitor C9. Diode D5 is connected across the thermocouple to prevent damage from excessive current due to disconnection. Another resonant circuit made up of inductor L3 and capacitor C8, along with a millivoltmeter, forms the indicating and control section.
2. Working principle and control process
When the power is turned on, the oscillator and amplifier circuits start operating. The oscillation signal is output from the emitter of BG1. After being detected by D1, BG2 turns on, activating relay J, which in turn engages the AC contactor CJ, starting the heating process of the high-temperature furnace. The internal temperature of the furnace is converted into an electrical signal (voltage) by the thermocouple and sent to a moving-coil millivoltmeter, causing the pointer with an aluminum plate to deflect to the right. When the pointer enters the gap of the oscillation coil L3 (which corresponds to the preset temperature), the high-frequency eddy current effect on the aluminum plate reduces the total inductance of L3 significantly. This causes the impedance of the resonant circuit formed by L3 and C8 to increase at the oscillation frequency, reducing the oscillation amplitude or even stopping it completely. At this point, the base signal detected by D1 and sent to BG2 becomes weak, causing BG2 to turn off. Relay J then releases its contact, and the AC contactor CJ opens, cutting off the power to the furnace and stopping the heating. As the temperature decreases, the voltage from the thermocouple R weakens, causing the millivoltmeter pointer to move back to the left. Once the pointer exits the gap of L3, the circuit resumes oscillation, BG2 turns on again, relay J holds, CJ closes, and the furnace begins heating once more. This cycle repeats continuously, maintaining the furnace temperature within a set range and achieving automatic temperature control.
Second, troubleshooting
1. Visual inspection and judgment
(1) Diagnosis: The XD1 (red) and XD2 (green) indicators on the control panel show whether the high-temperature furnace is in heating or heat preservation mode (i.e., powered on or off). Under normal conditions, switch K2 should be closed. When the power is on, the oscillator and amplifier circuits activate, the relay J holds, the green light turns on, and then switch K1 is engaged, causing CJ to act and the contacts to close. If the green light is off and the red light is on, and there is no reading on the ammeter and CJ makes no sound, it suggests that the AC/DC circuit and temperature conversion circuit are functioning normally, but the fault may lie in the oscillator, detection, or amplification circuits. If the furnace heats up when powered on, but the green light does not turn off and the red light does not come on when the millivoltmeter pointer reaches the preset temperature, and the furnace continues to heat, it indicates that the control system has failed. In this case, the AC/DC circuit, temperature conversion, and power supply detection parts are likely fine, and the issue may be with the oscillator or amplifier circuits.
(2) Inspection: Remove the casing and carefully inspect relays, AC contactors, transformers, and other components for signs of burning, as well as check if terminals, wiring, or components are disconnected or poorly soldered. Then, power on the system and test each component using a multimeter. Check if the input and output terminals of the connecting wires have voltage.
2. Voltage testing
If the fault remains undetected after the above inspections, use the DC voltage setting on a multimeter to measure the voltages at the collector, base, and emitter of transistors BG1 and BG2. Ensure they match the values listed in the table. If any deviation is found, further investigation into those specific components is necessary.
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