For the dual requirements of anti-interference and galvanic isolation, the pre-circuit of the output current detection or voltage detection of the inverter usually uses the "matching" circuit of the linear optocoupler device and the differential amplifier to complete the transmission mission of the detection signal. The circuit of the differential amplifier takes the current detection circuit as an example, when R1 = R3, R2 = R4, and the differential input signal is zero (shutdown condition), the output is "virtual ground" at the moment, which is 0V, if it is not 0V, it means that there is a problem with the circuit at this stage, and the abnormal alarm after powering on may be here.
Inverter differential amplifier overhaul method
Example 1
The telegraph OC is faulty, and the 5, 6, and 7 pins of N1 are all 2V. Pin 7 is not 0V for the alarm reason.
1. The "imaginary short" rule of the amplifier is still established, and the N1 chip is good to judge, but the fault is in the bias circuit.
2. Further analysis, at this moment, the differential amplifier has been transformed into a voltage follower
It is judged that R3 is broken, virtual welding, or the resistance value is severe. One end of R3 was measured to be soldered, and the recovery was normal after repair welding.
Example 2
The problem is the same as above. Pin 5 is 2V, pin 6 is 2.5V, and pin 7 is -13V.
1. The "imaginary short" rule of the amplifier cannot be established, but it still conforms to the rule of voltage comparators.
2. Further analysis, at this moment, the differential amplifier is transformed into a voltage comparator
The N1 chip is judged to be good, but the fault is R4 open circuit or virtual soldering, which damages the closed-loop condition of the amplifier, so that the amplifier is transformed into a voltage comparator. The resistance value of the online detection R4 has become severely larger, and the circuit is now broken after removing the detection, and the recovery is normal after replacement.
Example 3
The problem is the same as above. Pin 5 is 0V, pin 6 is 0V, and pin 7 is -10V.
1. The "virtual short" of the amplifier and the "virtual ground" rule of the inverting amplifier are still established, and the N1 chip is good.
2. Further analysis, at this moment, the differential amplifier is transformed into an inverting amplifier
The fault is R1 open circuit or virtual welding, which causes the input signal voltage loss at the inverting input terminal, while the 2.5V at the inverting input terminal is amplified by four times the inverting phase. Check that R1 has a virtual welding phenomenon, and the fault is removed after the repair welding.
Example 4
The problem is the same as above. Pin 5, 6 and Pin 7 are all 2.5V.
1. The "virtual short" rule of the amplifier is still established, and the N1 chip is good.
2. Further analysis, at this moment, due to the abnormal voltage divider circuit at the inverting input terminal, the "output virtual ground" condition of the original differential amplifier is damaged. Therefore, the output voltage rises from 0V to 2.5V.
The fault is R2 open circuit or virtual welding, which causes the input voltage of the inverting input terminal to rise. The measurement is as discriminant, and the output of N1 is normal after replacing R2.
Example 5
The problem is the same as above. The pin voltage is 2V, pin 6 is 0.4V, and pin 7 is -8V.
1. The "imaginary short" rule of the amplifier cannot be established.
2. The N1 chip is no longer compatible with the principle of the comparator, and it becomes unreasonable to be completely "undutiful". No, the N1 chip is now broken, and there is no need to check the peripheral components to identify it. Replace the N1 chip and recover to work normally.
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