PC817 Optocoupler: 4 Simple Electronic Circuits [FAQ]

PC817 optocoupler has been widely used. You can see it in computer terminals, thyristor system equipment, measuring instruments, photocopiers, automatic ticketing, household appliances, such as fans, heaters, etc. Its signal transmission between circuits completely isolates the front end from the load, and its purpose is to increase safety, reduce circuit interference, and simplify circuit design.

PC817 is a commonly used linear optocoupler. It is often used as a coupling device in various functional circuits that require more precision. It can completely isolate the upper and lower circuits without affecting each other.

Main features of PC817:

1. Current transfer ratio (CTR: MIN. 50% at IF=5mA, VCE=5V);

2. High isolation voltage: 5000V effective value;

3. Compact dual-in-line package:

• PC817 is a single-channel optocoupler;
• PC827 is a dual-channel optocoupler;
• PC837 is a three-channel optocoupler;
• PC847 is a four-channel optocoupler;

4. Linear optocoupler components.

Catalog

Optocoupler PC817 application circuit diagram (1)

The PC817 is a commonly used linear optocoupler. It is often used as a coupling device in various functional circuits that require more precision. It has the effect of completely isolating the upper and lower circuits without affecting each other.

Figure 1. Optocoupler PC817 pin diagram and internal circuit

Figure 2. Optocoupler pc817 application circuit

When an electric signal is applied to the input terminal, the light emitter emits light and illuminates the light receiver. The light receiver is turned on after receiving the light, and generates a photocurrent output from the output terminal, thus realizing the "electricity-optical-electricity" conversion.

Ordinary photocouplers can only transmit digital signals (switch signals), and are not suitable for transmitting analog signals. Linear optocoupler is a new type of photoelectric isolation device that can transmit continuously changing analog voltage or current signals. In this way, a corresponding optical signal will be generated as the strength of the input signal changes. Therefore, the conduction degree of the phototransistor is also different, and the output voltage or current is also different.

Optocoupler PC817 Application Circuit (2)

TL431 and PC817 are usually used for the voltage regulation feedback of switching power supply. If the output voltage requirement is not high, then Zener diode and PC817 can also be used. Let me illustrate the coordination of TL431 and PC817 through the following typical application circuit. The circuit diagram is as follows:

Figure 3. TL431& PC817 voltage regulation feedback circuit

The value of R13 is not arbitrarily chosen, because two factors must be considered:

1. TL431 refers to the current at the input terminal. Generally, this current is about 2uA. In order to prevent the current at this end from affecting the voltage divider ratio and avoid the influence of noise, the current flowing through the resistor R13 is generally more than 100 times the reference segment current, so the resistance should be less than 2.5V/200uA=12.5 K.
2. Requirements for standby power consumption. If there is such a requirement, try to take a larger value if it meets <12.5K.

The dead zone current of TL431 is 1mA. That is, when the current of R6 is close to zero, ensure that 431 has 1mA. Therefore, R3≤1.2V/1mA=1.2K.

In addition, it is also a consideration of power consumption. R17 is to ensure the size of the dead zone current, and it may or may not be necessary. But there is one exception: when the output voltage is less than 7.5v, it should be considered must be used. The reason is that since R17 here provides TL431 dead zone current, it is only useful when the LED conduction voltage is insufficient.

If the light-emitting diode can be turned on, it can provide enough dead zone current for TL431. If Vo is very low, the calculation method is changed to R17=(Vo-Vk)/1mA (here Vk=Vr-0.7=1.8v);

When Vo=3.3V, the critical maximum value of R17 from the perspective of dead zone current is R17=(3.3-1.8)/1mA=1.5k, and the critical minimum value from the perspective of TL431 current limiting protection is R17=(3.3-1.8) /100mA=15Ω.

When Vo is high, that is, when Vo is greater than Vk+Vd, that is, almost 7.5v or more, the dead zone current required by TL431 can be provided by the conduction of the light-emitting diode, so R17 is not necessary.

Optocoupler PC817 Application Circuit (3)

TL431 is a precision voltage regulator source, and PC817 is a photoelectric coupling device. In the switching power supply, the design of the voltage regulator feedback circuit usually uses TL431 and PC817 to cooperate. In the design of flyback power supplies, feedback circuits often use them as references.

Figure 4. Circuit diagram of TL431 and PC817 used together

Next, using Figure 4 as a reference, various parameters in the circuit diagram will be analyzed and explained.

To understand the relationship between the two, you must first determine the values of the four parameters R1, R3, R5, and R6 in the TL431 part of Figure 4.

Suppose the output voltage is Vo, and the auxiliary winding rectified output voltage is 12V.

This circuit compares the output voltage with the reference voltage formed by TL431, and controls the C pole of the TOP tube through the current change of the photocoupler PC817, thereby changing the PWM width to achieve the purpose of stabilizing the output voltage.

Because the controlled object is the TOP tube, we must first understand the control characteristics of the TOP tube. According to the technical manual of TOPSwicth, the current Ic flowing into the control pin C is inversely proportional to the duty cycle D, as shown in Figure 5.

Figure 5. Relationship between TOPSwicth duty cycle and control current

The current of Ic should be between 2-6mA, and PWM will change linearly. Therefore, the current Ice of the triode should also change within this range. While Ice is controlled by the diode current If, the PC817 diode forward current If can be correctly determined through the relationship curve of PC817's Vce and If.

Optocoupler PC817 application circuit diagram (4)

TTL control signal input circuit of 12V DC motor composed of PC817.

The figure below is a circuit diagram of the TTL control signal input circuit for turning on or off a 12V DC motor using a commonly used optocoupler PC817.

Figure 6. TTL control signal input circuit

Component Datasheet

PC817 Datasheet

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