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I have an optocoupler and I connect it to 230V, the datasheet tell me the optocoupler has a forward voltage: IF = ± 5 mA VF - 1.5 V, so, for limiting the current I need an resistor of 4.7K - 11wats, but I don't have this type of resistor so I use a Capacitor and a Resistor in series, but my problem is that I don't know how to calculate the value of the capacitor resistance (Xc=?), and if i know the capacitor value, what value resistor should have?

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The reactance of the capacitor \$C\$ is (magnitude): $$X_c = \frac{1} {Cw} $$ Where \$w = 2\pi f_{Line} \$

Net series impedance is therefore: $$ |Z|= \sqrt{R^2+X_c^2} $$

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