Do diodes obey Ohms law

Diodes do not obey Ohm's Law, at least when viewed as an individual component. When doing circuit analysis, however, you can make the case that, for a specific situation, they do. In fact, using this technique, everything "obeys" Ohm's law.

In the simple case of a resistor, operated within its limits, Ohm's Law works just fine. Voltage is current times resistance. As voltage goes up, current goes up proportionately. You can predict the voltage or current knowing the other along with resistance.

A diode, on the other hand, is non linear. As you apply voltage across it in the forward direction, it initially has high resistance. All of a sudden it has low resistance as it begins to conduct. As you increase the current, the resistance decreases, so the voltage drop is relatively (though not exactly) constant. At a certain point, around when you exceed the maximum forward current, the diode fails.

In the reverse direction, an ordinary diode seems to have high resistance, until you reach about max reverse voltage, at which point the diode fails. In the case of a zener diode, reverse behavior is similar to forward behavior, except at a different voltage.

Even though the diode does not obey Ohm's law, you can still use Ohm's Law to analyze the circuit. You just have to remember that the dynamic resistance of the diode changes as a function of the applied voltage. The value of Ohm's Law becomes evident when you consider that, at each static condition of the circuit, you do know the voltage and current through the diode and, as a result, you can use Ohm's law to calculate its dynamic resistance. This will allow you to extend the analysis by back substitution and simplification until you know more and more about the circuit.