The pc817 is a commonly used linear optical coupling. It is often used as a coupling device in various functional circuits that require more precision. It has the function of completely isolating the upper and lower circuits and does not affect each other.

pc817 application circuit

When an electrical 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 a photocurrent is output from the output terminal, thus realizing the "electricity-optical-electricity" conversion.

Ordinary optocouplers 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, which can transmit continuously changing analog voltage or current signal, so that the corresponding optical signal will be generated with the change of the strength of the input signal, so that the conduction degree of the phototransistor is also different , The output voltage or current is also different.

PC817 optocoupler can not only play a feedback role but also play a role in isolation.

Optocoupler measurement:

Test the two sets of pins on both sides with a digital meter to test the diodes. Among them, the one that is conductive only once, the red test lead is connected to the anode, and the black test lead is connected to the cathode (the opposite of the pointer meter). And these two feet are the low voltage end, that is, the feedback signal lead-in end.

When testing the low voltage end in the forward direction, use another multimeter to test the other two legs of the high voltage end. When it is connected, the red test lead is connected to the C pole, and the black test lead is connected to the E pole. When disconnecting the test lead on the low-voltage side, the multimeter on the high-voltage side should read infinity.

The same reason: as long as a certain voltage is applied to the feedback terminal, the high-voltage terminal should be able to conduct, otherwise, the device should be damaged. Whether the optocoupler can be substituted depends mainly on whether its CTR parameter value is close.

The essence of measurement is to measure the quality of light-emitting diodes and triodes respectively.

Another measurement statement:

It can be measured with two multimeters. The photocoupler is composed of a light-emitting diode and a light-receiving transistor package. For example, the photocoupler 4N25 is packaged in DIP-6 and has a total of six pins. Pins ① and ② are anode and cathode respectively, and pin ③ is an empty pin. Pins ④, ⑤ and ⑥ are the e, c, and b poles of the transistor. .

In the past, when using a multimeter to measure an optocoupler, only the quality of the light-emitting diode and the light-receiving triode were detected and judged separately, and the transmission performance of the optocoupler was not judged. Here takes the photocoupler 4N25 as an example to introduce a method of measuring the transmission characteristics of the photocoupler.

1. Judging the quality and polarity of the light-emitting diode: Use a multimeter to measure the positive and negative resistance of the diode with the R×1k block. The forward resistance is generally from several thousand ohms to several tens of kiloohms, and the reverse resistance should generally be ∞. When the measured resistance is low, the red pen is connected to the cathode of the diode.

2. Judge the quality and magnification of the light-receiving transistor: switch the multimeter switch from the electrical barrier to the transistor hFE block, use an NPN socket, connect the E hole to the ④ pin emitter, the C hole to the ⑤ pin collector, and the B hole to connect ⑥ The base of the foot, the displayed value is the current magnification of the transistor. Generally, the hFE value of general-purpose optocoupler is one hundred to several hundred. If the display value is zero or overflow is ∞, it indicates that the transistor is short-circuited or open-circuited and has been damaged.

3. Measurement of optocoupler transmission characteristics: see the figure below for the specific wiring of the test. Set the switch of the digital multimeter to the diode position, connect the black pen to the emitter, connect the red pen to the collector, and ⑥ the base of the pin to float. At this time, the reference voltage of 2.8V in the meter is added between the c and e junctions of the triode through the measuring circuit blocked by the diode in the meter. However, because there is no photoelectric signal at the input diode terminal and it is not turned on, the liquid crystal display shows an overflow symbol. When the input pin ② is inserted into the E hole and the ① pin is inserted into the NPN socket of the C hole, the reference power supply in the meter 2.8V passes through the measuring circuit of the transistor hFE block in the meter to make the light-emitting diode light up, and the light-receiving transistor is turned on due to the light, indicating The value instantaneously changes from the overflow symbol to the indicated value of 188. When disconnecting ① the plug of pin anode and C hole, the display value instantly changes from the 188 display value back to the overflow symbol. Different optocouplers have different transmission characteristics and efficiency. You can choose optocoupler applications with a slightly smaller display value and stable display value.

Since 9V laminated batteries are mostly used in the watch, the time to power up the input diode should not be too long to avoid reducing the battery's service life and measurement accuracy, and the intermittent contact method can be used for measurement.