open-collector configuration

You could calculate it that way but since you don't know Ic it is a but hard. The optopcoupler is essentially a switch so its transistor is either hard on (saturated) or off. The choice of resistor value depends on the circuit the opto coupler is driving. It needs to be small enough to source sufficient current when the transistor is off and high enough to ensure the is not excessive dissipation in the opto coupler when its transistor is hard on.

Ian

You'll find a parameter in the data sheet called CTR (current transfer ratio). That basically tells you how the output current relates to the drive. So calculate your diode current, multiply by the CTR to get the collector current, and set the resistor such that the output (the collector voltage) is low enough for your logic (+noise margin) when the diode is driven.

You weren't listening at school! It's just the bare collector of a transistor, with the emitter grounded. You use it when you have to (as in your optoisolator, or say driving an interrupt line), and it gives the advantage that the outputs of several devices connected together, with one shared pullup resistor, are ANDed (positive logic) or ORed (negative logic- this is often used for shared interrupts, arbitration schemes etc). Usually called "wired-OR".

Paul Burke

In the linear region, the static current is Ic = If * CTR, so the output voltage is Vdd - (If * CTR * Rc), but that falls apart when the Vce gets low. Also CTR is variable from unit-to-unit, with temperature, and it decreases with time.

Most applications also have to consider speed- the higher the load resistor, the slower the optocoupler. Most are spec'd with an unrealistically low collector resistance (eg. 100R) and will be far slower in a practical circuit.

Best regards, Spehro Pefhany