Unit circuit device learning optocoupler device
The principle of optocoupler device is very simple, that is:
When an electric signal is applied to the input end, the light emitter emits light, and after the light receiver receives the light, it generates a photocurrent, which flows out from the output end, thus realizing the "electricity-optical-electricity" conversion.
Its advantages are also outstanding:
The signal is transmitted in one direction, and the input terminal and the output terminal are completely electrically isolated. The output signal has no effect on the input terminal. It has strong anti-interference ability, stable operation, no contact, long service life, and high transmission efficiency.
Safety guarantee for electrical isolation
For example, taking the optocoupler in the switching power supply as an example, using optocoupler to increase the photoelectric conversion is unnecessary. In fact, the reason is very simple for safety.
There have been news reports that the mobile phone was plugged into the charging cable to make a phone call and died because of the poor isolation of the high-voltage part and the low-voltage part of the charger. The charging cable transmits the high-voltage power directly to the phone, so the charger is the most Good choice of good workmanship.
Another function of the optocoupler device is the level conversion in the circuit,
For example, in the drive circuit of the motor, we know that the general system voltage is 3.3V and 5V. In the figure below, the light-emitting diode is driven under a small current, and then the phototransistor is amplified by weak light. Voltage signal to drive related devices.
Sometimes in order to drive larger devices or higher voltages, only optocouplers will not achieve the purpose, and often need to cooperate with relays to re-amplify to control the high voltage.
Optocoupler devices on the market
There are many optocoupler devices on the market, so how to choose the corresponding model, so it is necessary to understand what parameters the optocoupler device has and their respective influence ranges.
The above figure shows the photoelectric characteristics of optocoupler devices. Among them, we found that in the characteristic of CTR (current transfer ratio), each device is different, so this should be an important differentiating parameter of optocoupler devices.
So what is the CTR parameter?
First of all, we usually express the CTR as the DC current transfer ratio.
That is, when the output voltage remains constant, it is equal to the percentage of the DC output current IC to the DC input current IF.
The device manual in the above figure shows that the CTR range of EL817 is 50% to 600%, and the CTR range of EL817D is 300% to 600%. This indicates that if you want to obtain the same output current, the latter only needs a smaller current. See CTR The parameters are similar to the hFE of the transistor.
You can go to station B to watch the experiment using optocoupler devices to intuitively feel how the current at the input and output ends of the optocoupler device changes, and directly read the current values at both ends.
Optocoupler circuit application design
In the above figure, when the Uin input is a positive signal, and UGS is greater than the turn-on voltage, the left side of the optocoupler will be turned on to cause the internal light-emitting diode to emit light, and the right side will be turned on accordingly, and the load will be energized.
However, it should be noted here that the circuit not only realizes the level conversion, but also realizes the isolation between the signals. In the above figure, the power supply of the input end and the output end of the optocoupler are different, and the ground of the input end and the output end are also different.
And in high-frequency signals, such as the 115200 baud rate of the serial port, we need to consider the switching rate of the optocoupler.
When designing the circuit of an optocoupler device, it needs to be divided into input circuit and output circuit. How to design
In the input circuit, the key is the value of the current-limiting resistor, and the value of the current-limiting resistor is determined by the rated current of the light-emitting diode inside the device.
According to the above figure, R3=(VCC-diode forward voltage drop)/diode rated current can be calculated.
In the output circuit, it can be divided into two circuit designs
These two circuit designs can also be obtained according to the basic Ohm's law:
A circuit design:
Uout=VCC-IC*R4；
B circuit design:
Uout=VCC-IE*R5。