I have a circuit like below.
I wanted to know how these resistance values were calculated. I wanted to use 12V/24V in place of 9V.
Let me know selection of opto-coupler and resistance value.
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\$\begingroup\$ Do also check possible duplicate: Resistor value for Opto-coupler input \$\endgroup\$– HuismanCommented Jun 20, 2019 at 6:18
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\$\begingroup\$ @CL. What's the additional value of repeating my comment? \$\endgroup\$– HuismanCommented Jun 20, 2019 at 6:27
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\$\begingroup\$ @CL Ah, thanks for clarification \$\endgroup\$– HuismanCommented Jun 20, 2019 at 6:33
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1 Answer
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Follow the next steps:
- Calculate the value of resistor R1 (for LED) --> U = I x R --> R = U / I
We know the following things:
- Uin = 12 V
- Uled (opto) = 1,2 V (take a look into your datasheet)
- ILed (opto) = 4 mA (take a look into your datasheet)
So, the total voltage = 12 - 1,2 = 11,8. R = U / I --> R = (11,8 / 4x10^-3) = 2950 Ω = 3 kΩ
- Calculate the value of resistor R2 --> U = I x R --> R = U / I
we know the following things:
- 0.3 V – Saturation voltage of photo transistor (voltage across photo transistor when current flows through it)
- Ic= Collector current (Current flowing through photo transistor when light falls on it) see datasheet. Let take 1 mA in this case.
So, the total voltage = 5 - 0,3 = 4,7 V . R = (4,7 / 1 x 10^-3) = 4700 Ω = 4,7 kΩ
