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I am having trouble calculating the power consumption of this optocoupler (4n25). In the datasheet the input power dissipation stated as \$ P_{diss} = 100mW \$ and forward current of \$I_F = 60mA\$. In the figure below are two setups with different resistor values.

opto

changing the resistor value of R3 results in:

\$R3 = 100\Omega\$: \$U_{in} = 1.24V, I_{in}=37.22mA \$ => \$ P_{R3} \approx 47mW\$

\$R3 = 10k\Omega\$: \$U_{in} = 1.01V, I_{in}=0.398uA \$ => \$ P_{R3} \approx 0.402\mu W\$

Looking at Fig3 and inserting the \$U_{in}@25°C\$ results in

\$R3 = 100\Omega\$: \$I_F\approx 50mA \rightarrow P_{opto} = 62mW\$

\$R3 = 10k\Omega\$: \$I_F\approx 1mA \rightarrow P_{opto} = 1.01mW\$

(The graph only goes to 1.2V but for my thought experiment I extrapolated the values).

The consumed power of the input circuit would be $$ P_{tot} = P_{R3} + P_{opto}$$

Is this approach correct? If this is the case it would be potentially dangerous to remove the resistor. Also: Is it best practice to choose a the highest resistor which still guarantees that the opto functions as intended?

EDIT: This post is based on this question

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You're quoting the ABSOLUTE MAXIMUM RATINGS for \$P_{diss}\$ at the input and \$I_F\$. You should not use these parameters as design parameters or actual values, but consider them as values which should not to be exceeded. Like the footnote says:

Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. Functional operation of the device is not implied at these or any other conditions in excess of those given in the operational sections of this document. Exposure to absolute maximum ratings for extended periods of the time can adversely affect reliability.

Regarding your calculations: \$I_{in} = I_F\$ for all situations, so, you should use the same value for the current to calculate the \$P_{R3}\$ and \$P_{opto}\$.

Then, yes,

The consumed power of the input circuit would be $$ P_{tot} = P_{R3} + P_{opto}$$

Is it best practice to choose a the highest resistor which still guarantees that the opto functions as intended?

Yes, like shown in Current limit resistors for Optocoupler controlled Mosfet
When you use a resistor value that just works for the absolute worst situation, then it works for all situations.
For linked question, a minimum current of 2.5 mA was required. It cannot hurt to increase this to e.g. 5 mA as the maximum allowed forward current is 60 mA. But still, there is no need for as the worst situation is already covered.

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  • \$\begingroup\$ Thank you. I am still getting turned around when calculating my circuit. Almost any change on the output side influences the whole circuit and maybe some parts need to be replaced, which have different conditions, which change some other parts and so on \$\endgroup\$
    – v3xX
    Jan 30, 2020 at 10:28
  • \$\begingroup\$ Welcome to the joy of "proper" engineering. :) You're doing fine by the way, keep it up. \$\endgroup\$
    – rdtsc
    Jan 30, 2020 at 12:24
  • \$\begingroup\$ "Is it best practice to choose a the highest resistor which still guarantees that the opto functions as intended?" Like Huisman said, the answer is YES. But for a robust design you to need to consider all of the corner cases - variations in various components such as min CTR in the opto, resistor tolerance, voltage tolerance, temperature and aging effects, etc. You just don't want to design with typical or nominal values, unless this is just a one-off lab or hobbyist project, \$\endgroup\$
    – SteveSh
    Jan 30, 2020 at 12:24

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