Question

Trouble understanding how to do diode problems over a range of voltage.

In circuits below with Si diodes (VD0 = 0.7 V) and VZ = 5 V for Zener diodes, A) Find v and/or i for vs = 10 V, B) Find v and/or i for vs ranging from −20 to
+20 V.

Answer 1

For Part A, my answer for V = 9.3. i = 9.3/5 = 1.86. For B, i don't quite understand some aspects of it.

Answer 2

First I assume it's off. Then the given is, id= 0, Vd < Vdo. So then I derive that V = 0 and i = 0. But should there be more to the answer, such as a parametric equation? The professor also wrote Vs < Vd as part of the answer, but i'm not sure how he derived that. (This is all just for assuming it's off). The same type of questions apply when I assume it's on. So clarifications for both cases will be very helpful. Thank yoU!

Answer 3

@ganeshie8 @ParthKohli @Michele_Laino

Answer 4

@dan815

Answer 5

My understanding is that Vs = Rid + Vd, and when we assume it's off, Vs = Vd. Since Vd < Vdo, the following is also true: Vs < Vdo. Am I understanding this right?

Answer 6

For part b :

\[V = \left\{\begin{array}{}
V_s-(-5) &: -20 \le V_s \le -5\\~\\
0 &: -5 \le V_s \le 0.7\\~\\
V_s-0.7 &: 0.7\le V_s \le 20
\end{array}\right. \]

Answer 7

Hey @ganeshie8, I don't quite understand the above solution you mentioned. Can you please explain further?

Answer 8

What do you know about zener diode ?

Answer 9

That you can turn on in both directions.

Answer 10

1. off: -Vz < Vid < Vo; 2. on: Vd = Vdo, id > 0; 3. Zener: Vd = -Vz, id < 0

Answer 11

Exactly!

We're given that VD0 = 0.7V and Vz = 5V

That means :
In forward direction, the diode turns on at 0.7V
In reverse direction, the diode turns on at 5V

yes ?

Answer 12

Right, but is this a zener diode? I thought zener diodes are denoted by a squigly line in front of the triangle.

Answer 13

<-- Like so.

Answer 14

Right, they forgot to put that squigly symbol in the circuit diagram

Answer 15

The question clearly says that it is a zener diode and they gave you the zener volatage, so it must be a zener diode.

Answer 16

Oh wait a second, do you have more circuits ?

Answer 17

Looks you're correct, we should not interpret it as a zener diode

Answer 18

Hmm, sorry, i must have not given the full picture.

Answer 19

Sorry about that confusion! But if it was evaluated as a zener diode, can you please explain that to me? I haven't learned how ot solve zener diode circuits well.