The working principle of coupler
Apr 13, 2018

⒈ working characteristics (with photosensitive transistor as an example)

The internal common mode rejection ratio of optocoupler is very high. 。

Because the coupling capacitance between the light-emitting tube and the receiver is very small (less than 2pF), the common-mode input voltage has a very small effect on the output current through the coupling capacitance between the poles, thus the common-mode rejection ratio is very high.

⒉ Output characteristics The output characteristic of the photoelectric coupler refers to the relationship between the bias voltage VCE and the output current IC in a certain light current if, when If=0, the light-emitting diode does not glow, at this time the photosensitive transistor collector output current is called dark Current, generally very small. When if>0, under certain if action, the corresponding IC basically has nothing to do with VCE.

The relationship between IC and if is linear, and the output characteristics of the photoelectric coupler measured by the semiconductor tube characteristic diagram are similar to the ordinary transistor output characteristics.

⒊ Optocoupler can be used as a linear coupler A bias current is provided on the light-emitting diode, then the signal voltage is coupled to the light-emitting diode by a resistor, so that the photoelectric transistor receives the light signal which increases and decreases in the bias current, and its output current will be linearly changed with the input signal voltage. The photoelectric coupler can also work in the switch state and transmit the pulse signal.

When the pulse signal is transmitted, there is a certain delay time between the input signal and the output signal, and the input and output delay time of the photoelectric coupler varies greatly.

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