Why are optocouplers used in circuits?
Electrical isolation is required. Signal transmission is needed between circuits A and B, but the power supply levels of the two circuits are too different—one is hundreds of volts while the other is only a few volts; these two vastly different power supply systems cannot share power.
Circuit A is connected to high-voltage electricity, posing a risk of electric shock to humans upon contact, and therefore needs to be isolated. Circuit B, on the other hand, is a part that humans frequently come into contact with, and dangerous high voltages should not be mixed in with it. Both circuits must achieve signal transmission while maintaining electrical isolation.
The use of high-impedance devices such as operational amplifiers and the transmission of weak analog voltage signals by the circuit make the anti-interference processing of the circuit a rather troublesome matter—noise interference from various sources may take the initiative and “dredge up” the useful signal.
In addition to considering the safety of human contact, the safety of circuit devices must also be taken into account. When the input side of the optocoupler is damaged by a strong voltage (field) impact, the output side circuit can remain safe and sound due to the isolation effect of the optocoupler.
The above four factors have contributed to the research, development, and practical application of optocouplers. The basic functions of an optocoupler are to effectively isolate the input and output circuits electrically; to transmit signals in optical form; to provide good anti-interference; and to a certain extent, to prevent the introduction and impact of strong voltages on the output circuit.
II. General properties of optocouplers:
1. Structural features: The input side generally uses light-emitting diodes, while the output side uses phototransistors, integrated circuits, and other forms to perform electrical-optical-electrical conversion and transmission of signals.
2. There is light transmission between the input and output sides, but no direct electrical connection. The presence and strength of the input signal control the light intensity of the LED, while the output side receives the light signal and outputs a voltage or current signal based on the light intensity.
3. The input and output sides have high electrical isolation, with isolation voltage generally exceeding 2000V. It can transmit AC and DC signals, and the output side has a certain current output capability; some can directly drive small relays. Special types of optocouplers can linearly transmit millivolt or even microvolt-level AC and DC signals.
4. Due to the structural characteristics of optocouplers, the input and output sides require independent power supplies that are mutually isolated, i.e., two power supplies without a common ground point are needed. The input side of the first and second type optocouplers described below provides the input current path through the signal voltage, but in reality, the input signal circuit also has a power supply branch; while for linear optocouplers, the input side and output side are the same, directly connected to two phase-isolated power supplies.
III. In frequency converter circuits, there are three types of optocouplers commonly used:
1. One type is the transistor-type optocoupler, such as PC816, PC817, 4N35, etc., which is commonly used in the output voltage sampling and error voltage amplification circuits of switching power supply circuits, and is also applied in the digital signal input circuit of the inverter control terminal. It has the simplest structure, consisting of a light-emitting diode on the input side and a phototransistor on the output side, mainly used for the isolation and transmission of switching signals;
2. The second type is the integrated circuit optocoupler, such as 6N137 and HCPL2601. The input-side LED uses a new type of light-emitting material with low delay effect, and the output side consists of gate circuits and Schottky transistors, greatly improving performance. Its frequency response speed is significantly faster than that of transistor-type optocouplers, and it is also used in fault detection circuits of frequency converters and switching power supply circuits.
3. The third type is the linear optocoupler, such as the A7840. Its structure and performance differ significantly from the first two types of optocouplers. In circuits, it is mainly used for the linear transmission of weak analog signals in the mV range. In frequency converter circuits, it is often used for sampling and amplifying the output current and the DC voltage of the main circuit.
The following diagram shows the pinout and functional schematics of three types of optocouplers:
Three types of optocoupler circuit diagrams
IV. Measurement and Online Testing of Type I Optocouplers:
The first type of optocoupler has an input operating voltage drop of approximately 1.2V , a maximum input current of 50mA (typically 10mA), and a maximum output current of around 1A. Therefore, it can directly drive small relays, and its output saturation voltage drop is less than 0.4V . It can be used for transmitting low-frequency signals of tens of kHz and DC signals. It has polarity requirements for the input voltage/current. When a forward current path is formed, the two output pins are in a closed circuit state; when the forward current is less than a certain value or a certain reverse voltage is applied, the two output pins are in an open circuit state.
Measurement method:
Using the diode setting on a digital multimeter, the forward voltage drop on the input side is 1.2V , and the reverse voltage drop is infinite. The forward and reverse voltage drops or resistance values on the output side are both close to infinity.
When measuring the resistance at pins 1 and 2 on an analog multimeter using the x10k resistance range, there is a significant difference between the forward and reverse resistance. The forward resistance is approximately tens of kΩ, while the reverse resistance is infinite. Similarly, the forward and reverse resistances at pins 3 and 4 are infinite.
Two-meter measurement method. Use the x10k resistance range of an analog multimeter (which can provide 15V or 9V and tens of μA current output), connect pins 1 and 2 in the forward direction (black probe to pin 1), and use the resistance range of another multimeter to measure the resistance between pins 3 and 4 at x1k. When the probes of pins 1 and 2 are connected, there should be a resistance of about 20kΩ between pins 3 and 4. When the probes of pins 1 and 2 are disconnected, the resistance between pins 3 and 4 should be infinite.
A DC power supply with a resistor in series can be used to limit the input current to within 10mA. When the input circuit is on, the resistors at pins 3 and 4 are in a closed circuit state; when the input circuit is open, the resistance at pins 3 and 4 is infinite.
The third and fourth measurement methods are relatively accurate. If compared with the same type of optocoupler, they can even detect faulty components (such as excessive output resistance).
The above measurements are a necessary process before installing new devices. If it is inconvenient to measure online, the device can be removed from the circuit for offline measurement to further determine its condition.
In actual maintenance, offline resistance measurement is not very convenient, while power-on testing is more convenient and accurate. Measures should be taken to slightly modify the input circuit, and the corresponding changes (or lack thereof) on the output side should be measured to determine the condition of the device. This involves disrupting the "balance" in the faulty circuit, causing a "transient imbalance," thereby exposing the cause of the fault. Optocouplers have current-limiting resistors connected in series on both the input and output sides of the circuit. During power-on testing, methods such as reducing (parallel) the resistance or increasing the resistance (opening it) can be used, combined with voltage detection on the output side, to determine the condition of the optocoupler. In some circuits, even directly shorting or opening the input and output sides can be used to detect and observe dynamic changes in the circuit, facilitating the identification of the fault area and the commencement of maintenance work.
Precautions during measurement: One side of the optocoupler may be directly connected to "high voltage", and touching it may cause electric shock. It is recommended to provide an isolated power supply to the machine during maintenance!
The following diagram shows the application circuit of a common transistor optocoupler.
Schematic diagram of online testing of optocouplers
The circuit in (1) above is the digital signal input circuit of the inverter control terminal circuit. When the forward terminal FWD is shorted to the common terminal COM, the voltage between pins 1 and 2 of PC817 changes from 0V to 1.2V , and the voltage at pin 4 changes from 5V to 0V. Similarly, when the control terminal is open, the voltage between pins 1 and 2 of PC817 is 0V, while the voltage between pins 3 and 4 is 5V. The circuit in Figure (1) shows the voltage values of each pin of the optocoupler. The fault or normal state can be determined by measuring the voltage of the input and output pins.
In the circuit shown in Figure (2), the voltage between pins 1 and 2 is 0.7V (average AC signal), and the voltage between pins 3 and 4 is 3V, indicating that the optocoupler has an input signal. However, is the optocoupler itself functioning properly? Short-circuit pins 1 and 2 of the PC817 with metal tweezers and measure the voltage at pin 4. If the voltage rises from the original 3V to 5V (or shows a significant increase), the optocoupler is good. If the voltage remains unchanged, the optocoupler is damaged.
V. Measurement and Online Testing of Type II Optocouplers:
The second type of optocoupler (6N137) has an input voltage drop of approximately 1.5V , but its maximum input and output current is only in the mA range. It is only used for transmitting higher frequency signals and does not have current-driven capability. It can be used for effective transmission of MHz-level signals. Like the first type of optocoupler, it has polarity requirements for the input voltage/current. When a forward current path is formed, the two output pins are in a closed state. When the forward current is less than a certain value or a certain reverse voltage is applied, the two output pins are in an open-circuit state.
The circuit configuration of this type of optocoupler is similar to that of the first type, but the signal frequency transmitted is higher. The measurement and inspection methods are also basically similar. If the first type of optocoupler is considered low-speed and ordinary, then the second type can be called a high-speed optocoupler. The difference between the two lies only in their signal response speed; their circuit configurations are identical.
For online measurement, 1. Short-circuit or open-circuit input pins 2 and 3, and simultaneously measure the voltage change at output pins 6 and 5; 2. Decrease or increase the external resistor connected to the input pins, and measure whether there is a corresponding change in the output pin voltage; 3. Connect a current-limiting resistor from the +5V power supply or other power supply to the input pins, and check whether there is a corresponding change in the output pin voltage. These methods can be used to determine if the device is functioning correctly.
VI. Third type of optocoupler device – linear optocoupler:
Linear optocouplers are a relatively special type of optocoupler.
1. Characteristics of linear optocouplers:
(1) Structural characteristics: Its input and output circuits are no longer like those of the first type of optocoupler, which are simply
It is not a simple circuit of diodes/transistors, but contains an amplifier and has its own independent power supply circuit; there is no requirement for signal input polarity, and it only linearly amplifies the amplitude of the input signal.
(2) The input side signal input terminal no longer exhibits the forward and reverse characteristics of the light-emitting diode. Perhaps we can regard the two signal input terminals as the two input terminals of the operational amplifier. The input impedance is very high, and it no longer draws the signal source current. It can be used as the input and amplification of weak voltage signals. It has extremely high amplification capability for differential signals and a certain suppression capability for common-mode signals.
(3) The output side circuit is a differential signal output mode, which is convenient to connect with the subsequent amplifier to further process the signal.
2. Pin function diagram of linear optocoupler A7840:
A7840 (HCPL-7840) Functional Block Diagram
The A7840 (HCPL-7840) operates as follows: Typical input and output power supplies are 5V; input resistance is 480kΩ; maximum input voltage is 320mV; it features differential signal output. The internal input circuitry amplifies the signal and provides high impedance, enabling distortion-free transmission of mV-level AC and DC signals. The output signal serves as the differential input signal for the subsequent operational amplifier. It offers approximately 1000 times voltage amplification. Typical applications include using it in conjunction with subsequent operational amplifiers to amplify and process weak (AC and DC) voltage signals.
Pins 2 and 3 are signal input pins, pins 1 and 4 are input-side power supply pins; pins 6 and 7 are differential signal output pins, and pins 8 and 5 are output-side power supply pins.
Online testing method: The internal circuit can be viewed as a "comprehensive operational amplifier," with pins 2 and 3 being the non-inverting and inverting input terminals, and pins 7 and 6 being the signal output terminals. When pins 2 and 3 are shorted (making the input signal zero), the output voltage between pins 6 and 7 is also zero. When there is a voltage input in the mV range at pins 2 and 3, there is an "amplified" proportional voltage output between pins 6 and 7.
3. Current signal detection circuit composed of A7840:
Output current sampling circuit of INVT G9/P9 low-power frequency inverter
Some low-power frequency converter models omit the current transformer for output current sampling. A current sampling resistor in the mΩ range is directly connected in series in the U and V output circuits, converting the output current signal into a mV-level voltage signal. This voltage signal is then introduced to the signal input terminals of U3 and U4 (A7840) R via R1 and R2. U3 and U4 perform opto-isolation and linear transmission, and after amplification (impedance transformation) by U5 (TL082), it is sent to the subsequent current detection and protection circuit for further processing before being sent to the CPU. The input sides of U4 and U3 are powered by the driver circuit (isolated power supply) and then regulated to 5V by U1 and U2 (L7805 voltage regulator). This power supply must be isolated from the control circuit. The output sides of U4 and U5 are powered by the +5V power supply from the CPU motherboard. The A7840 amplifies the input voltage signal (in the hundreds of mV range) into a differential voltage signal (in the V range) representing the output current. This differential voltage signal is then inverted by the subsequent operational amplifier U5 and sent to the subsequent current signal processing circuit. It is then processed into analog signals of a certain amplitude and sent to the CPU for output current display and control; or processed into switching signals for fault alarms, shutdown protection, etc.
These two current detection signal outputs are marked with IU and IV on the circuit board, which are the detection points.
4. DC loop voltage signal detection circuit composed of A7840:
Alpha 200018 5kW Inverter DC Circuit Voltage Detection Circuit
The Alpha 200018 5kW frequency converter DC circuit voltage detection circuit directly samples the 530V DC voltage from the P and N terminals of the DC circuit. After voltage reduction and division by a resistor network, the resulting mV-level voltage signal is applied to pins 2 and 3 of the small-signal processing optocoupler A7840 (U14). After strong and weak current isolation by U14, a differential signal is input to pins 2 and 3 of the LF353 operational amplifier. This stage of the circuit is configured as a voltage follower. The output signal is output from the potentiometer center connector (test point VPN marked by the manufacturer on the circuit board) to pin 8 of the CNN1 connector on the CPU motherboard and power/drive board. When the three-phase input voltage is 380V, the DC voltage sampled at pin 8 is 3V.
The A7840's input power supply is provided by the AC voltage from an independent winding of the switching transformer, which is rectified and filtered by D41, C46, etc., and then regulated to 5V by the integrated voltage regulator 78L05; the output power supply is provided by the CPU motherboard power supply +5V.
The DC circuit voltage detection signal enters the CPU mainboard through pin 8 of the CNN1 and CNM terminals, and is directly input to pin 53 of the CPU via R174. This signal is an analog voltage signal, and its functions are: 1. To display the DC voltage value on the operation panel. Some inverter models display the input AC voltage value after program conversion; 2. Some models use it to control the output U/F ratio, making the output voltage value proportional to the input voltage value; 3. A few models use it as a sampling reference for over- and under-voltage protection.
Another path sends the signal via R155 to the inverting input of a voltage comparator formed by the open-collector output operational amplifier of the LF393. This output signal, along with overcurrent (OL), OC, and OH signals, is mixed into a "fault summary signal," which is further processed by the CPU's peripheral circuitry and then sent to the CPU pins for shutdown protection and drive pulse cutoff control. The non-inverting input of the LF393 can be considered a "programmable reference voltage terminal." The amplitude of its reference voltage is controlled by the output voltages of pins 42 and 51 of the CPU, providing different reference voltage values during startup and operation, which are compared with the input voltage detection signal. The protection action threshold also varies depending on the different operating stages of the inverter.
When the voltage detection circuit itself malfunctions, the troubleshooting method is as follows:
a. After the inverter is powered on, it reports an overvoltage or undervoltage fault, as shown in the voltage detection circuit diagram above. Measure the voltage at terminals 8 of CN1; the normal value should be around 3V. If the measured voltage at this point is too high or too low, it indicates a fault in the voltage detection circuit. First, check if the 5V power supply to the input and output sides of the A7840 is normal, and if the ±15V power supply to the LF353 is normal. If not, repair the relevant power supply branches. If normal, proceed to the next step of troubleshooting.
b. Measure an input voltage of over 100mV between pins 2 and 3 of A7840. Short-circuit pins 2 and 3 of A7840 with metal-tipped tweezers. Measure a significant drop in the voltage at pin 1 of the LF353 output. This indicates that the voltage signal transmission links are normal, and the fault lies in a faulty or misaligned external potentiometer connected to the LF353. Replace and readjust it. Adjust the relevant parameters of the inverter so that the operation display panel shows the DC circuit voltage value. When the input three-phase voltage is 380V, adjust the potentiometer so that the DC voltage display value is 530V.
c. Short-circuit pins 2 and 3 of A7840 with metal-tipped tweezers. If the voltage at pin 1 of LF353 does not change, further check the voltage at the input pin of LF353 (the normal value is around 3V, which becomes 0V when the tweezers are used to short-circuit the input pin of A7840). If the value does not change, A7840 or an external circuit component is damaged. If the voltage at the input pin of LF353 is normal, LF353 is damaged and needs to be replaced.
d. When shorting pins 2 and 3 of A7840 with tweezers, the input voltage of LF353 changes, but the value is too low, such as changing from 1V to 0V. Check that the external components of A7840 are normal. The fault is that A7840 is inefficient. Replace A7840.
