Talking about the low current input circuit of 6N139 series optocoupler

This article discusses the idea of ​​a low current input circuit using the 6N139 series optocouplers. Some of these circuit designs are signal detection, transient detection, matrix and unloaded line reception. An example of the 6N139 output circuit and calculation method are also presented.

This article discusses the idea of ​​a low current input circuit using the 6N139 series optocouplers. Some of these circuit designs are signal detection, transient detection, matrix and unloaded line reception. An example of the 6N139 output circuit and calculation method are also presented.

Introduction

Advances in optical coupling and LED technology have given us the 6N139. This unique optocoupler has an input LED current specification of 500 µA, opening some interesting design doors. In addition to the obvious and well-known ability to be directly driven by CMOS circuits, the 6N139 can also be considered for signal detection, transient detection, matrix and unloaded line reception. Here are some circuit ideas that excite designers.

Signal Detection

Detection of noise, spikes or oscillations can be easily and directly detected by the input of the 6N139, as shown in the circuit of Figure 1.

Talking about the low current input circuit of 6N139 series optocoupler

6N139 Input circuit for signal detection.

In order to detect bad signals on the DC power supply use:

Talking about the low current input circuit of 6N139 series optocoupler

C = inject 500 microamps into the LED
X = follower latched or non-latched output circuit
LED = input diode of 6N139

A forward current of 50µA is supplied to the LED to charge the LED capacity to the VF level. In this way, the LED does not induce conduction in its output circuit, but is ready to turn on very quickly. Any noise or oscillations on the “DC power supply” will couple through the “C”, resulting in a signal on the LED. Even small unwanted signals can cause large changes in LED forward current. Once the forward current of the LED equals or exceeds 500 microamps, the output circuit turns on, indicating the presence of a harmful signal.

Transient detection

The presence or absence of a waveform can be easily detected by the circuit in Figure 2.

Talking about the low current input circuit of 6N139 series optocoupler

Pulse or waveform detection circuit.

In order to detect the presence of the desired signal, pulse or waveform, use:

Talking about the low current input circuit of 6N139 series optocoupler

X = follower non-latching output circuit
LED = input diode of 6N139

f = frequency

Matrix optocoupler

Taking advantage of the 6N139’s low input LED current, the matrix can now be driven with a TTL output, as shown in Figure 3.

Talking about the low current input circuit of 6N139 series optocoupler

Optical coupling outside the matrix.

unloaded line receiver

For dummy loads, the 6N139 is compatible with the differential amplifier circuit of Figure 4.

Talking about the low current input circuit of 6N139 series optocoupler

Differential amplifier driver.

For a dummy no-load opto-isolator circuit, use:

X = follower non-latching output circuit
LED = input diode for 6N139
The current requirement at “in” will be less than 20μA.

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