# Determining the state of a diode

#### awesommee

Joined Nov 18, 2019
13
Can someone explain to me how to determine state of these diodes. I understand why D1 and D2 are ON, but I don't understand how to determine which one will turn off first and at which value of Ug it will happen.

#### WBahn

Joined Mar 31, 2012
26,074
The surest approach is to do the circuit analysis two ways. In each way pick one of the diodes and assume that is is just at the point of turning on/off and then see what state the other diode has to be in.

#### ci139

Joined Jul 11, 2016
1,680
by simple reasoning -- the $$D_1$$ allows only the one way current - thus - it basically jumps conducting when the voltage exceeds the 0.6V downwards
after what you subtract the $$D_1$$ drop from the supply and the circuit current is defined by $$R_1+R_2$$ until the voltage drop on $$R_2$$ is greater than 600mV . . .
. . . then the $$D_2$$ holds it conducting at 0.6V) and the further increase of voltage occurs at $$R_1$$ ← this one might be tricky to reason as what current passes through the $$D_2\ ||\ R_2$$ -- but it's fairly simple for ideal diode -- as the increasing current won't increase the drop on $$D_2$$ nor on $$D_1$$ the $$I_{R_2}$$ remains constant and the additional 600mV is extracted from the supply . . .

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