Physics unit 3 electronics basic

[Shad]

Doing some unit 3 electronics work and i know how to use  formulas and numerical answers.
But i can't understand the concept of diode in photoconductive mode (reverse bias with power supply)
When we do calculations why do use I (current) that is produce by the diode ? There is another power supply why not use that ?

[availn]

Diodes do not produce power. Diodes in forward bias act similarly to diodes in reverse, they can only drop a certain maximum amount of voltage. Too much current through them will burn them out, so you usually have a resistor. You calculate the current the resistor allows through (with the resistor's voltage drop being the power source's voltage less the diode's breakdown voltage in V = IR) and then you use that current in any other calculations.

The power supply does not give out a fixed current, so you can't really use that in calculation. They usually give out a fixed voltage or power. The current in the circuit depends on how much the circuitry "draws" from the source. Less resistance in the circuitry will draw a greater current.

[paulsterio]

The diode does not produce energy, what it simply does is allow specific amounts of current through depending on the light shining upon it.

Think of it as a freeway, you can vary the speed limit on the freeway based on another factor.