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[SOLVED] 4N25 model in LTSpice, CTR too high?

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DanyR

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I discovered that the 4N25 model delivered with LTSpice IV (v4.23g) gives a CTR of 74% (measured in a LTSpice simulation), while the typical CTR (acc the datasheet) is 50%, and the minimum CTR is 20%.

So, why did they choose such a high value in the model?

Reference project: https://www.edaboard.com/threads/345069/#post1473941
 

Hi,

there are several manufacturers for the 4N25.

Some just specify a minimum CTR. So 74 is within spec.

Toshiba datasheet says ctr= 100typical.

***
Maybe this is thae cause for the confusion.

Anyway, it is a optocoupler for digital informations. Here you typically use a lot of overdrive causing saturation and don´t go to the ctr limit.

There are special analog (linear) optocouplers where you operate it in the linear region. Non saturating. Here the CTR is more detailed specified.

Klaus
 
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    DanyR

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The minimum CTR is the only parameter which we should consider for use in applications working saturated. Considering that manufacturers specify CTR within a wide range, a simulation would not give a reliable result for a linear region.
 
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    DanyR

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Somebody had to pick a value, and they did. Might have
come from the mfr's SPICE modeling folks or been a WAG.
In any case, margining / worst casing is on you.
 
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dick_freebird said:
Somebody had to pick a value, and they did. Might have
come from the mfr's SPICE modeling folks or been a WAG.
Click to expand...
Thanks.

dick_freebird said:
In any case, margining / worst casing is on you.
Click to expand...
Yes, indeed.
To be able to do my tests I had to make a model of a (very) generic (very rudimentary) optocoupler with transistor output and settable CTR. Perhaps it can be usefull for other users.

This is the circuit diagram of OptoC:
OptoC.jpg

This is the symbol of OptoC:
OptoC_Symbol.jpg

The files of the OptoC symbol:
View attachment OptoC.zip

An example of its usage (zero crossing detector here):
ZeroCrossingExample.jpg
View attachment Draft5.zip

As you can see you can set the CTR value (1 means 100%) in the diagram that uses OptoC. In the example the CTR is set to 3 different values: 20%, 50% and 100%.
 
Good job. Consider submitting it to the Yahoo! LTSpice
user group for their contrib models archive.
 

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