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How can I calculate the proper resistence for a optocoupler

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Full Member level 4
Aug 10, 2011
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How can I calculate the proper resistence for an optocoupler

Greatings! I have a problem with adjustment of 6N137 optocoupler. Here is my chart:View attachment forum.pdf
R1 is 560 ohms(5V max input voltage). The input resistence of the integrated circuit connected to the output of the optocoupler is high. My problem is in the optocoupler`s R2 resistence. How can I calculate this value?? In the datasheet the example is for 5V(350ohms). In my case I have 20V. What should be its value??

R2 is a pull-up for when the opto is off. The value is dependent on switching speed and what you are driving with Vo. Lower values will give better edges for high speed; higher values will draw less current when the opto is on, sacrificing faster operation. Your load on Vo may have a min. current requirement that needs to be met, or if it's driving something like a LED or relay coil, you don't even need it. In short, there isn't enough info to give a range on the value.

You show C1 connected to Vo. That will limit the switching speed as well. Did you mean to connect that to Vcc as a bypass cap and not Vo?

6N137 is a fast (10 MBaud) logic optocoupler. It's open collector output is intended to drive 5V logic, the specified supply voltage range is 4.5 to 5.5 V. It can't work from a 20 V supply and needs a level converter to drive 20V logic.

I saught this C1 connected in the datasheet so I put it. I want to pass 25-30kHz signal. So when my R2 was around 330 ohms I have a significant lost of pulses on the output. Now my R2 is 560 ohm and it works better. Could you suggest value for R2? Sould I remove the C1 cap?

You have a much more fundamental problem as FvM pointed out above - The device only works to 5V. 20V supply will destroy the part. You say you changed R2 to 560 and it works better - how are you getting the device to work at all if your voltage is that high?

Usually the voltage drop is not critical for all electronic devices. This which destroys them is the current. As I said I`m not draining any(very small current). The 6N137 have 50mA output current.This means power of 250mW. So if I drean smaller current on bigger voltage drop everything is fine. I thing in my case I just don`t have to pass the limit of 10mA, which is impossible because I have a logic with big input impedance as I said.
Best regards!

The datsheet says "7V supply voltage for 1 min maximum". I don't believe that the device will tolerate 20 V, not even for seconds.

First regulate the 20V to about 5V using 7805 so that the IC (opto-isolator) does not burn out and then use the stipulated value of R for 5V.

An alternate approach to using an optocoupler would be a digital isolator. Digital Isolators are a more modern alternative to optocouplers, offering improved performance with lower power dissipation. They’re also much easier to implement in a design since the inputs and outputs use standard digital logic. There would be no need to calculate bias resistor values.

Standard digital isolators have different numbers of input and output channel combinations and can operate up to 150Mbps. Specialized products also exist for specific serial interfaces, such as USB and I2C.

You can see the entire digital isolator product line available from Analog Devices at our web site:

Digital isolators generally run from standard logic supply voltages in the 1.8 V to 5 V range, so you'd still need to regulate the 20 V supply down to a lower value as mentioned by several other posters.

Please I have been hearing of digital isolators. I want to know how they operate.

Digital Isolators use a coupling element such as a transformer to pass signals across a high-impedance barrier. An input signal (digital or analog) is typically encoded and passed across the coupling element, then decoded to recreate the original signal. An in-depth description of the design used by many ADI digital isolators can be found on our web site:

High Speed Digital Isolators Using Microscale On-Chip Transformers

This article is a few years old so some of the performance metrics may be out of date, but the fundamentals still apply.

ADI uses a polyimide insulation process that enables high levels of integration. This has allowed us to create a large portfolio of isolated products including: standard digital isolators, dc/dc converters, gate drivers, A/D converters and communications products (USB, CAN, I2C, RS-485, RS-232).

I invite you to review our technical and video libraries for more info, or ask more questions here!

I thank you for the wonderful introduction you gave to me on digital isolators. I will get back to you if I read the PDF material.

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