Talking about the design of switching value acquisition circuit and the solution to the problem of AC interference

The switching value acquisition circuit adopts optocoupler isolation, and the optocoupler is used as the signal transmission between the circuits, so that the front end is completely isolated from the load, which increases safety and reduces circuit interference. The operation quality of the circuit directly affects the accuracy of the protection action, so it is necessary to take some circuit protection measures to increase the safety and reliability of the switch value acquisition circuit. The defect in the design of the optocoupler circuit is the root cause of the protection malfunction. Below we discuss the solution.

The switching value acquisition circuit adopts optocoupler isolation, and the optocoupler is used as the signal transmission between the circuits, so that the front end is completely isolated from the load, which increases safety and reduces circuit interference. The operation quality of the circuit directly affects the accuracy of the protection action, so it is necessary to take some circuit protection measures to increase the safety and reliability of the switch value acquisition circuit. The defect in the design of the optocoupler circuit is the root cause of the protection malfunction. Below we discuss the solution.

1. Typical 24VDC input circuit design

The functions of a typical switch value acquisition loop are mainly filtering, level conversion, isolation and power drive.Take the optocoupler input terminal as 24VDC and the isolated output terminal as the main control CPU system unit as an example, as shown in the figure

Talking about the design of switching value acquisition circuit and the solution to the problem of AC interference

The optocoupler model selected for the circuit in the figure is TLP521-1, which is a linear optocoupler. Since the optocoupler has better anti-interference performance and isolation performance than transistors, the logic circuit formed by it is more reliable, small in size and low in cost. Resistor R1 is selected according to the input voltage, and the selected resistance value according to GB requirements controls the input voltage to operate between 55%-70%. That is (55%-70%) U=(13.2V-16.8V), it is completely cut off below 13.2V, and completely turned on above 16.8V. Assume the on-current of the optocoupler is 4mA-10mA. When the input is above 16.8V, the optocoupler is turned on, so there is 1.7K3.4KΩ, and the resistor R1 can choose a value between 3.4K-4.2KΩ. The minimum conduction current of the optocoupler is 4mA, according to its current transfer ratio CTR is 50%, the collector current IC=IF*50%=4mA*50%=2mA, and in order to make the phototransistor enter the saturation region as soon as possible, choose Pull-up resistor R2 is 4.7KΩ. To prevent the input voltage from changing abruptly, a Schottky diode D1 is connected in parallel at both ends of the optocoupler to protect the optocoupler. The capacitor C1 and R1 form a low-pass filter circuit to filter out various interference signals mixed with the input signal.

2. 220VDC input circuit design

In practical applications, the input voltage provided by the peripheral circuit of the power system to the device is usually DC 220V. The circuit in the above figure cannot meet the demand. Therefore, a 47V/1W Zener diode Z1 and current limiting resistors R3 and R4 are connected in series in the loop. The input 220VDC voltage is first divided by the R4 resistor. When the voltage across R4 reaches more than 100V, Z1 is reversely regulated, and the voltage is limited by R1 and R3 to drive the optocoupler’s on-current and then turn on.In the field application, it was found that the mis-conduction phenomenon occurred in the environment of 50Hz AC interference, so the circuit was improved, as shown in the figure

Talking about the design of switching value acquisition circuit and the solution to the problem of AC interference

In this circuit, capacitor C1 is connected in parallel with both ends of resistor R4, and a Schottky diode D2 is added. And the Zener tube Z1 is replaced with a Zener tube with a withstand voltage of 100V/1W, which improves the circuit breakdown voltage. When the interference pulse is added, the voltage regulator tube Z1 does not conduct the capacitor C1 and the charging process effectively absorbs the pulse. After the interference pulse, the capacitor C1 discharges through R4, thereby preventing the optocoupler element U1 from being disturbed and conducting. When the diode D2 is not added, conduct an electrical disturbance experiment at the input end. The 150VAC voltage is applied to the input end through two serial capacitors, the optocoupler is turned on, and there is signal input at the CPU end. The experiment measured the voltage of the negative end of diode D2 to ground is 150VAC , after adding D2, do the electrical disturbance experiment, add 150VAC to the input end, and the voltage will pass through the diode D2 to reduce the voltage to 20VAC.

The Links:   LS037V7DW05 NL12876BC26-25A IGBT-SUPPLIER

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