# Designing Optocoupler circuit

I'm trying to learn how to design an optocoupler using 6N137.

There is no CTR (current transfer ratio) value or graph in the 6N137 datasheet so I cannot estimate how much current (Ic) will flow through the transistor in the optocoupler.

However, in the output section of the datasheet, there are two values: low level output voltage and high level output voltage. what I understand is these are 2 different outputs like logic outputs. Am I thinking correct? Because there is no well defined graph in which we can see what parameter changes based on another parameter.

Another thing, datasheet says low level output voltage based on the conditions is maximum 0.6 V. This means Vce of optocoupler is 0.6 V which is close to saturation isn't it?

In normal BJT, we can use many parameter, graphs like hfe etc. to approximate the behaviour but in optocoupler I don't know how to design for different Ic or voltage levels because there is only specific output conditions? How should I think of it?

The 6N137 is built for logic signals, so both input and output have only two states, high and low.

The recommended operating conditions show that the input is guaranteed to be read as high when the LED current is at least 5 mA, and that it is guaranteed low for at most 250 µA:

So for practical purposes, you can use any LED current of at least 5 mA.

(The actual threshold where the device toggles between these states is somewhere between these two values. Figure 5 shows typical values for this threshold.)

The electrical characteristics show that in the low state, the output is guaranteed to be at most 0.6 V when the output current is no more than 13 mA:

So for practical purposes, you can use any IC not larger than 13 mA.

Figures 6 and 7 shows some typical values for the output.

Some 6N137 datasheets show the relationship between input and output (below for the original 6N137 from HP). But Vishay's 6N137 is a purely digital device with hysteresis, and the input threshold does not depend on the output current, so they think a graph is not needed.

The only analog parameter that you have to care about is the relationshiop between LED current and forward voltage (shown in figure 4); this is similar to plain phototransistor optocouplers. (The forward voltage of high-speed LEDs tends to be higher.)

The 6N137 is an optocoupler with logic-level outputs. If you want a plain optocoupler, perhaps look at 4N25 and similar.

You can see the internal functions in the 6N137 datasheet, and the current transfer ratio applies to the optical circuit which is not accessible in this part.

From datasheet

• Does that mean ı can only use the conditions stated in the output section of 62137 datasheet? like logic output @jonathanjo
– Mhan
Commented Jun 25 at 12:17
• @Mhan yes, it's a part designed to simplify use of optocouplers in logic circuits. If you want it for something else, it's probably the wrong part. Commented Jun 25 at 12:42