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So I am working with an optocoupler at the moment, and having trouble understanding what CTR value to use in my calculations.

The datasheet states 50% Min and 600% Max, which is a huge difference. Do we always take the minimum for our IC/IF equation? Or do we take it from a graph? Its not clear on this datasheet

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You need to be sure your circuit works for any value of CTR between 50 and 600%.

If you're verifying your circuit in a simulator, you should test it with both extreme values, and maybe an in between value too.

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    \$\begingroup\$ When it comes to ordering the actual parts, you can choose binned parts with a tighter CTR spec. So if you're prepared to spend more you could verify your design with less extreme values. \$\endgroup\$ – brhans Oct 8 at 15:37
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Looking at the datasheet, it looks like the current transfer ratio is also dependent on the forward current as well as the voltage.

Figure 13 (Current Transfer Ratio vs. Forward Current) shows that.

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It'll be be necessary to use the minimum CTR given (50%) for proper circuit operation. Ensure that the upper limit doesn't give you a problem.

It's important to note that many semiconductors have rather broad specs. Current gain for a small signal transistor, for example, may vary as widely as 150 to 900. You can either buy the broad spec one or in many cases a selected part may prove beneficial to you. Loads of Pro-Electron transistors e.g. BC548 for example have A. B and C gain ranks and many Japanese devices have 'colour' gain ranks, e.g ORG (orange), GRN (green), BLU (blue!). Similarly some power devices have A, B, C, even D voltage ranks.

In the case of an optocoupler you're subject to 2 variables. The luminous flux of the emitter diode and the sensitivity of the receptor diode (junction). So the apparently wide CTR of 12:1 is only a difference of 3.5:1 for each part of the device.

In volume manufacture it's often possible (if you need) to get parts selected to your spec although it's inadvisable to rely on this.

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