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Optocoupler in parallel transmission of long-term

In Electronic Infomation Category: R | on July   10, 2011

Optocoupler (hereinafter referred to as the coupler) is a light-emitting devices and SN75154N datasheet and optoelectronic devices consisting of photosensitive devices. It can realize the electrical → light → electrical signal conversion, and SN75154N price and the input signal and SN75154N suppliers and output signal isolation. Most of the optocoupler input current gallium arsenide infrared emitting diode, the output silicon photodiode, and optical silicon photo-transistor trigger the SCR. Because the peak wavelength of 900 ~ 940nm GaAs infrared LED optoelectronic devices with silicon peak wavelength response is consistent, can obtain higher signal transmission efficiency.

Turn electrical isolation. Meanwhile, in the electric light → → conversion of electrical signals to optical coupling devices, as long as certain of its current input, the output terminal can output a corresponding digital signal. Therefore, the logic level of signal transmission into a fixed current loop in the current status of any transfer. Suitably increasing current (low resistance transmission), so inclusion of electrical noise in the signal is completely limited to the selected switch current range, the relatively weak interference signal current can not change the availability of a useful signal current state, can be effective suppress interference and increase reliability of information transmission. And increased data transmission distance. Coupler generally light emission, light reception and signal amplification components. Input signal to drive the LED diode (LED), so that given a certain wavelength of light. Generated by the light detector receives light current, and then after further amplified output. To complete the electrical power conversion of a light one, and thus play input, output, isolation effect. As mutual isolation between input and output optical coupler, with one-way signal transmission characteristics, which has good electrical insulation and anti-jamming capability. Also, because the optocoupler input terminal is low resistance current-work component, which has a strong common mode suppression. Therefore, it is information transmission in long-term isolation as a terminal device can greatly enhance the signal to noise ratio. Real-time computer control of digital communication and signal isolation as the interface device, which can greatly increase the reliability of computer work.

2 Optocoupler Features

The main advantage of a one-way coupler transmission signal, the input and output side fully realized the electrical isolation, anti-interference ability, long life, high transmission efficiency. It is widely used for level shifting, signal isolation, isolation level, switching circuits, long-distance signal transmission, pulse amplification, solid state relays (SSR), instrumentation, communications equipment and computer interfaces. In the switching power supply, may constitute a linear optocoupler optocoupler feedback circuit, by adjusting the control side to change the current duty cycle to achieve precise regulation purposes.

Main technical parameters of optocoupler emitting diode forward voltage drop VF, forward current IF, the current transfer ratio CTR, the input stage and output stage of the insulation resistance between the collector-emitter breakdown voltage of a V (BR) CEO, a collector-emitter saturation voltage VCE (sat), In addition, transmission of digital signals need to consider the rise time, fall, asked, delay time and storage time and other parameters. Optocoupler current transfer ratio CTR is an important parameter, usually expressed by DC current transfer ratio. When the output voltage remains constant, the mean IC and DC output current DC input current, IF the percentage of: CTR = IC / IF × 100%.

Using a phototransistor optocouplers, CTR in the range of 20% to 300% (eg, 4N35), the PC817 is 80% ~ 160%, Darlington-type coupler (such as 430) 100 % ~ 5000%. This shows that the desire to obtain the same output current, which only a smaller input current.

3 principles Optocoupler select

Optical isolation in the design of opto circuits optocouplers must be the right choice model and parameters, select the following principles:

(1) optical coupler for the signal as one-way transmission device, and circuit data transfer is bidirectional, circuit board size requirements will, combined with the practical requirements of circuit design, we must choose a single-chip integrated optical multi- coupling devices;

(2) optocoupler current transfer ratio (CTR) of the allowed range of not less than 500%. Because when CTR 5.0mA), in order to ensure signal transmission in long-term error does not occur, this will increase the power coupler;

(3) the transmission speed optocoupler optocoupler also select one of the principles must be followed, the slow switching speed optocoupler, input levels can not make a correct response, it will affect the normal operation of the circuit.

(4) recommended using a linear optocoupler. CTR value is characterized by a certain range can be adjusted linearly. The design of the circuit is a high input and output are low-level signals, therefore, the circuit operating in nonlinear state. The line of applications, because the signal is transmitted without distortion, so the requirements should be based on dynamic work, set the appropriate quiescent point, the circuit in the linear state.

Typically, single-chip devices integrated multi-channel optocoupler slower than the speed, and faster devices are mostly single, and a large number of isolation devices need to occupy a large layout area, but also makes the design cost greatly increased. In the design, by the board size, transmission speed, design costs and other constraints, can not use the speed is the dominant single optocoupler, in this selection of TOSHIBA Corporation TLP521-4.

4TLP521-4 Introduction

Optical isolation module TLP521-4 (GB) is a base with a complete performance of a good emitter fixed delay optical coupler, it has the best conversion rate, high temperature performance characteristics. Key features of the device: the current conversion rate was 100% ~ 500%; isolation voltage of 2500Vrms (min); launch a receiving voltage of 55V (min); leakage current lOμA (max) (Ta = 85 ℃); minimum conversion time is 42μs.

TLP52l-4 (GB) of a typical circuit shown in Figure 1, the specific conversion time parameters in Table 1. Table 1 shows, TLP521_4 (GB) maximum transmission delay time of 42μs, 1ms system needs to be completed in 8 byte read or write, the maximum transmission delay time is to meet the level of the circuit propagation delay time, which is completely in the transmission speed to meet the long-term transmission requirements. Through its control input, optocoupler can turn on or off according to operational needs. When the increase in the high-side input control, the optocoupler work. The input signal is coupled to the output terminal when the input control plus low, the open collector output transistor cut-off, external high-impedance state.

5 circuit design

In the long transfer, it is precisely because the ground of the AC impedance characteristics of the circuit makes the ground has become the de facto largest noise source. The main reason is the interference caused by ground there ground impedance, when the current flows through the ground, it will produce a voltage in the ground line, which is ground noise. Driven by this voltage will result in ground loop current, the formation of ground loop interference. South in the sending and receiving devices share a ground, will form a common impedance coupling. Using optical isolator TLP521-4 devices for sending and receiving electrical isolation, for reducing the role of impedance is very obvious, thus increasing the transport current, effectively suppress ground noise; the same time as the application of 74LS244N. Bus drive capability are protected. Figure 2 shows the circuit diagram to send and receive optocoupler. Figure 2, the upper half of the two optocouplers transmit signals from left to right, bottom half of the two optocouplers transmit signals from right to left, left 74LS244N IDT7132 SRAM with the processor through the data exchange, right through 8255 and the processor data exchange.

Debugging, enter the period of 100μs, duty cycle 1 / 2 +5 V square wave input to the coupler all the way through the 20m long wave and, after receiving the optocoupler and the output waveform after plastic surgery after 74LS244 waveform record. Record the results shown in Figure 3.

From the input and output waveform comparisons, the circuit can be superimposed on the input noise waveform significantly inhibited, so that the output waveform becomes smooth and stable, increase the output SNR. Although the timing of the conversion of the optocoupler, the duty cycle waveform trace transformation occurred, but because the circuit is a high input and output are low-level signals, it does not affect the correct signal transmission.

6 Conclusion Parallel using the above method is applied

long transmission circuits, can maintain the speed advantage of parallel transmission, and system structure constant, to ensure the accuracy of signal transmission. Parallel in the long-distance transmission, as long as the short-distance parallel to the original circuit is modified to ensure that communications can be isolated and both high-speed communication, and thus has great practical value.

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