Linear Optocoupler Circuit

This linear optocoupler is built with MOC5010 and can be used to isolate a circuit from main grid, audio interface, in medical electronics and many other applications. Because of its high isolation resistance (1011 Ω), MOC5010 is used in applications where a circuit is powered directly from main power. With a bandwidth from 5 Hz to 100 kHz you have no reason to complain about audio signal response.

MOC5010 transforms an input current variation into an output voltage variation. The linear optocoupler circuit presented here has an amplification factor of 0.75. The input must not overcome 2 Vef while bandwidth is 118 kHz at -3 dB.

Amplifier A has a transfer resistance of 200 mV/mA resulting in a total amplification of 0.6 … 0.8 (-4.5 … -2 dB). The output impedance is not higher than 200 Ω so you can connect an external amplifier at pin 4. If the input voltages are higher than 2 Vef then connect a potentiometer as voltage divider like showing below optocoupler circuit diagram.

Linear Optocoupler Circuit Diagram

If the global amplification is too small use a regular transistor instead of FET T1. It is important to mention that 2 separate power supplies are required: both the +12 V terminals as well the 0V (ground) must be isolated from one another. In many cases it is possible to use a 12V voltage for the transmitter part from the connected device.

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• Frank

Sounds good. What opto isolator did you use?

Just got hold of a few 4n25’s. I am assuming they won’t be linear, but may work.

• Frank

Does not have to be accurate because I can define the values obtained to represent the conditions they represent. In other words, rather than to directly rely on a reading, I can indirectly interpret a reading. It would be ideal to obtain an output that would range from 0 to 10 vdc. The input range to the opto isolator can be something like 0 to 14 vdc.

Something else I have discovered is that I cannot find a source for the MOC5010.

Thank you for the support Jim,

Frank

• Jim Keith

Watch for the next post: Linear DC Signal Opto Isolator. I did not test it for 10V, but with 15V power supplies it should do this.

• Frank

The response is stated to be good down to 5 Hz. I need to monitor a weather condition that varies on a cycle of 24 hours or more. If the output capacitor was not used, could this IC be used to provide such an output?

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