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IC MPN- FODM2701R2

I am giving If=5 mA current through diode.

CTR is 300%.

Ic current through transistor will be 15 mA.

R1=0 Ω.

R3=1500 Ω.

Therefore I am limiting the max Ic current to 3.33 mA.

Question is if CTR becomes 300% and Ic will be 3.3 mA then will the opto be saturated or be in linear region? If it will be in linear region it will give some resistance right? So my Vout won be properly grounded.

Or it will be saturated but only the current will be externally limited? It will be able to sink 15 mA but due to 1500 Ω resistance it will only sink 3.33 mA. Is my understanding right?

Basically i am trying to make a isolated pwm switch.

What i want to understand is when does opto Ic transistor get into saturation? Let’s say If=5 mA and CTR is 100% then Ic should be 5 mA. If Ic is 5 mA does it mean that opto Ic is in saturation?

enter image description here

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  • \$\begingroup\$ Welcome! Is your circuit a test bench to get to understand how an optocoupler behaves, or part of a larger circuit? What’s Vcc and voltage on your Zener diode? \$\endgroup\$ Commented Sep 8 at 9:01
  • \$\begingroup\$ That opto has 3us rise and 3us fall time. It’s too much for PWM. Use 6N136 with 0.5us slews. \$\endgroup\$ Commented Sep 8 at 9:15
  • \$\begingroup\$ @winny Vcc=5v and vdd=3.3v. Zener will be of 5.1v. \$\endgroup\$ Commented Sep 8 at 9:38
  • \$\begingroup\$ You LED have a Vf you need to subtract for in your equation. Your primary current is closer to 2.3 mA. Also, you didn't answer my question if this is just a test bench or part of a bigger circuit. \$\endgroup\$ Commented Sep 8 at 9:48
  • \$\begingroup\$ @winny no,i am giving 3.3v , r2=660 If=3.3/660 =5mA. right? if not how should i calculate? vf=1.3v. THis is part of a bigger board but this circuit is isolated. Vdd is connected to fpga io pin. \$\endgroup\$ Commented Sep 8 at 9:57

1 Answer 1

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If R1 is 0Ω, then regardless of anything else collector current \$I_C\$ is limited by R3 and \$V_{CC}\$ alone. That limit is \$I_C=\frac{V_{CC}}{R_3}\$ (actually very slightly less, since collector-to-emitter potential difference \$V_{CE}\$ is not quite zero, leaving slightly less than \$V_{CC}\$ across R3), and when that happens, the transistor is fully saturated.

This maximum \$I_C\$ cannot be exceeded, even if LED current far exceeds the quantity necessary to produce that \$I_C\$, because the transistor cannot be more "on" than this, it is saturated.

CTR will vary slightly from device to device, and with temperature and collector current itself. Check the datasheet for information about CTR based on your intended operating temperatures and collector current in saturation.

Even then, it would be foolish to rely on that figure to calculate the exact LED current necessary to guarantee any particular collector current. If you determine that CTR is 300%, and \$I_C=15{\rm mA}\$ in saturation, then \$I_{LED}=\frac{15{\rm mA}}{3}=5{\rm mA}\$ is the absolute minimum required to saturate the transistor. To be sure of saturation (the full 15mA of collector current), you should aim for a good deal more LED current than that, say 7.5mA.

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