# Very Simple Diode Question

#### jegues

Joined Sep 13, 2010
733
I'm confused as to how I can check why the diodes are non-conducting/cut off when vi is postive. (See figure)

For the diodes to be cut off the voltage across the diodes must be negative.

I don't see how I can check this.

I'm looking at the first diode and trying to find vd1, and it looks like the voltage across it is,

$$v_{D1} = \text{floating voltage?} - v_{I}$$

Can someone clarify this for me?

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#### Georacer

Joined Nov 25, 2009
5,182
If you want to be sure that the diode is not conducting, you can assume that it is, and replace it with a voltage source of 0.7V. This clearly shows you that it is not forward biased.

If you wonder about the actual voltage, maybe you could replace the open-circuit with a very large resistor and say that you have half the Vi dropped in each diode. A simulation would be enlightening. A second opinion too.