Optocouplers:

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Dear all,

I am looking at a circuit that contains both a CNY17-3 optocoupler, and a 6N136 opto-coupled logic gate. The specific circuit is from the OpenDCC project (model railways).

http://www.opendcc.org/elektroni...

I am rather confused as to the reason why there needs to be two different types. I understand that the 6N136 supports 1MBit/s, and the CNY17-3 is much slower (about 6us rise time when saturated).

Why not, for example, choose a faster optotransistor (say, Fairchild's 6M135M and use that for the data reception in this circuit?

I'm probably missing something :) I haven't dealt much with opto-isolation since I used a TLP2200 waaaay back at uni so any and all advice is appreciated....

-- Damien

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The two optos are used in quite different situations.

One is used where the output is floating in a conventional application driving a transistor inside a bridge to provide signalling on the line ( line gets shorted via the bridge to signal its state)

The other is used in a situation where input is floating with respect to the rest of the circuit and the output is at a logic level.

CNY-17 could have been used in both instances with additional buffering in case of logic level output.

It is debatable whether 6N136 could have been used instead of CNY17 since 6N136 requires a power supply and ground for it's operation.

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ignoramus wrote:
CNY-17 could have been used in both instances with additional buffering in case of logic level output.

That's more or less what I was thinking. Any suggested modern parts to do this sort of job?

-- Damien

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Speed would be the parameter the opto would need to have.

Speed and the base terminal of the opto transistor.

The reason for base connection is to stabilise the operating point and provide a composite output stage which would use external device to improve gain -bandwidth product of the opto coupler.

Consider the opto transistor within a cascode combination.