First, the circuit using optocoupler Why?
Electrical isolation requirements. Between A and B circuit to the signal transmission, but the power level between the two circuits are too poor, all the way to hundreds of volts, the other way as only a few volts; two vastly different power supply system, not the power sharing;
A strong electric circuit associated with the human body contact with an electric shock hazard, need to be isolated. The B circuit board as part of regular contact with the body and should not be mixed together dangerous high voltage. Between the two, it is necessary to complete the signal transmission, but also the need for electrical isolation;
Op amp circuit using high impedance type devices, and circuit voltage of the analog of the weak signal transmission, making the anti-jamming processing circuit to become a relatively difficult task - the noise from all channels mixed with interference, it is possible to become masters , the useful signal "drowned" out;
In addition to considering the safety of human contact, but also must take into account the safety circuit devices, optical coupling device when the input side by the strong voltage (field) of the impact damage, because of the role of opto isolation, the output side of the circuit was able to safe.
  The above four reasons, contributed to optocoupler research, development and practical application. The basic role of the optocoupler is to input and output side of the circuit on the effective electrical isolation; can form optical transmission signal; have better anti-interference effects; the output side of the circuit can be avoided to some extent, the introduction of high voltage and impact.
Second, the optical coupling device of the general attributes:
    1, the structural characteristics: the input side of the commonly used light-emitting diode, the output side of the use of photosensitive transistors, integrated circuits and other means to signal the implementation of the electro - optical - electrical conversion and transmission.
    2, input and output side of the transmission of light between, without electricity directly. The availability and strength of the input signal controls the LED luminous intensity, while the output side of the receiving light signal, according to photographic intensity, the output voltage or current signals.
    3, input and output side of a high degree of electrical isolation, isolation voltage of 2000V or more generally. Can the AC and DC signal for transmission, the output side of the current output capacity of some, and some can directly drag the small relay. Special type of optocoupler can mV, even microvolt AC and DC linear signal transmission.
    4, due to the structural characteristics of the optocoupler, input and output side of the need to separate isolated power supply, it will take two no "common ground" point of the power supply. Following a second class optocoupler input side of the signal voltage provided by the input current path, it is essentially the input signal circuit, also has a power supply branch; and the linear coupler, the input side and output side, is There are two isolated directly connected the power supply.
Third, in the inverter circuit, often used in optoelectronic coupled devices, there are three types:
  1, one for the transistor type optical coupler, such as PC816, PC817, 4N35, etc., commonly used in switching power supply circuit of the output voltage sampling and the error voltage amplifier circuit, the inverter control terminals are used in digital signal input circuit. The most simple structure, a light-emitting diode by the input side, output side by the form of a phototransistor, is mainly used for the isolation of the switch signal and transmission;
  2, the second for the integrated circuit type optical coupler, such as the 6N137, HCPL2601, etc., enter the side of the LED using a new type of delayed effect of low emitting materials, the output side of the gates and Schottky transistor, the performance greatly increased. The frequency response faster than transistor-based optocoupler greatly improved, the drive fault detection circuit and switching power supply circuit also has application;
  3, the third linear optocouplers, such as A7840. Structure and performance is very different from the first two optocoupler. Mainly used in the circuit level of mV linear weak analog signal transmission, the drive circuit, often used to amplify the output current of the sampling and processing, the main circuit DC voltage of the sample processing and amplification.
     The following picture shows the three types of optocoupler pins, function diagram:

Three optical coupler circuit
Fourth, the measurement first optocoupler and online testing :
  The first type of optical coupler, the work input voltage drop of approximately 1.2V, the maximum input current of 50mA, typical application is 10 mA; 1A maximum current output is about, which can directly drive small relays, the output saturation voltage drop small at 0.4V. Tens of kHz can be used for low frequency signals and DC signals. Input voltage / current polarity requirements. When the formation of forward current path, the output side of the channel state of two pins present, the forward current is less than certain value, or under certain reverse voltage, the output side between the two pins as open.
Measuring method:
Diode file number table, measuring the input side of the forward voltage drop as 1.2V, the reverse infinity. Output side of the positive and negative voltage drop or resistance values are close to infinity;
Pointer table x10k resistance profile, measured 1,2 feet, with a clear positive and negative resistance of different forward resistance of about tens of kO, infinite reverse resistance; 3,4 foot forward and reverse resistance of infinity;
Measurement of two tables. Analog Multimeter with x10k resistance profile (to provide 15V or 9V, tens of µA current output), a positive turn 1,2 feet (black ink to take a foot), with another form of resistance profile measurement with x1k 3 , the resistance value of 4 feet, when the 1,2-foot table pen access, the 3,4-feet around 20kO resistor between the present value of the foot of the table pen disengaged 1,2, 3,4 feet between the resistance is infinite.
A DC power supply available in series resistance limit the input current 10mA or less. Input circuit is turned on, the state channel resistance of 3,4 feet, the input circuit is open, the 3,4-foot infinity resistance.
  3,4 species more accurate measurement methods, such as optocoupler devices using the same model comparison, and even failure can be detected devices (such as the output side of the resistance is too large.)
The measurement is necessary before installing the new device process. Inconvenience of on-line measurement of the case, if necessary, the device can also be removed from the circuit, off-line measurements, to further determine the device's good or bad.
  In the actual maintenance, the off-line resistance measurement is not very convenient, power detection is more convenient and accurate. To take measures to look at the input side of the circuit changes, based on the output side of the corresponding changes produced by (or no change), measured to determine the device's good or bad. Breaking the faulty circuit in the "balance" so that a "transient imbalances", which exposed the cause of the malfunction. Optocoupler input and output side of the circuit current limiting resistor in series, the power detection, can reduce the (parallel) resistance and increase the resistance of the method (to open) and so on, with the output side of voltage detection, determine the optocoupler's good or bad. Part of the circuit, or even shorted or open circuit can be directly input side and output side, to detect and observe the dynamic changes of the circuit, which will help determine the fault zone and repair work.
  Measurements note: the side of optocoupler devices may be "strong power" is directly linked to risk of electric shock hit, the proposed maintenance process to provide isolated power for the machine!
  The figure below shows the application of common transistor optocoupler circuit.

Schematic diagram of optocoupler line detection
  The figure above (1) circuit, the inverter control circuit of the digital signal input terminal circuit, when the forward COM terminals FWD and public terminals shorted, PC817 between 1,2 feet into a 1.2V voltage from 0V , 4-pin voltage is 5V becomes 0V. Similarly, when the control terminal was open state, PC817 voltage between 1,2 feet 0V, the voltage between 3,4 pin 5V. Figure (1) circuit can be seen optocoupler voltage value of each foot, fault or normal state in the input and output pin voltage can be derived judgments.
  The figure (2) circuit, measuring between 1,2 to 0.7V (AC signal average), between pin 3,4 is 3V, that there optocoupler input signals, but the optocoupler itself is normal? Shorted with a metal forceps 1,2 PC817 feet, measuring 4 feet up the voltage to the original 3V 5V (or a marked increase), indicating optocoupler is good. If the voltage is unchanged, indicating optocoupler damage.
Five, the second optocoupler measurement and line detection :
  The second type of optical couplers (6N137), the pressure drop of about 1.5V input about the work, but the input and output only mA maximum current level, only played on the high frequency signal transmission function, the circuit itself does not have current drive capability that can be used effectively MHz level signal transmission. As with the first optocoupler, input voltage / current polarity requirements. When the formation of forward current path, the output side of the channel state of two pins present, the forward current is less than certain value, or under certain reverse voltage, the output side between the two pins as open.
The composition of this type of optocoupler circuits, with the first optocoupler circuit composed of similar form, but the high frequency transmission circuits. The measurement and inspection methods are basically similar. If the first class and ordinary low-speed optocoupler optocoupler is, then the second optical coupler, to be called high-speed optocoupler, the difference between the two, but the different response of the signal, the circuit is the same form The.
  Line measurement, 1, 2,3 can be shorted or open circuit input pin, while measuring the output voltage change of 6.5 feet; 2, decrease or increase the input for an external resistor, measuring the output voltage without the corresponding changes in the foot; 3 from the +5 V power supply or other limiting resistor string is introduced into the input pin to detect whether the corresponding output pin voltage changes. To determine whether the device is normal;