Help with a circuit problem (Zener Diode)

[november1992]

Homework Statement

In the circuit below, a zener diode (VON = 0.7V, VZ = -5.7 V) is in series with two
resistors. Determine the current (I) if Vs is:

a) 6V b) 4V c) 0.2V d) – 0.2V e) – 4V f) – 6V

[PLAIN]http://img809.imageshack.us/img809/6995/zenerdiodecircuit.png

Homework Equations

I was thinking that I=V-Von/Req

The Attempt at a Solution

not sure if i have the correct formula.I'm not asking for the answer i Just want to know whether I'm using the correct formula. I was wondering what exactly is Vz

 

[rude man]

You almost have the right formula, although I think you left out a parenthesis, and it's actually I = (Vs - Vd)/Req where Vd is the drop across the zener.

Vd = Von, I > 0
Vd = -Vz, I < 0

Note - the Zener is NOT as source of emf!

 

[november1992]

So, when you say that:

Vd = Von, I > 0
Vd = -Vz, I < 0

Do you measure the current excluding the diode first?

Problem 1.c)
The voltage is 0.2V so would the current be measured excluding the diode to determine whether the current is negative and then, after figuring out the current, would you use that value to determine the value of Vd?

I=V/Req
I=0.2V/10ohm
I=0.02 Amps
Vd = Von, I > 0
then,
I=Vs-Vd/Req
I=0.2V-0.7V/10ohm
I=-0.05

 

[rude man]

So, when you say that:

Vd = Von, I > 0
Vd = -Vz, I < 0

Do you measure the current excluding the diode first?

Problem 1.c)
The voltage is 0.2V so would the current be measured excluding the diode to determine whether the current is negative and then, after figuring out the current, would you use that value to determine the value of Vd?

I=V/Req
I=0.2V/10ohm
I=0.02 Amps
Vd = Von, I > 0
then,
I=Vs-Vd/Req
I=0.2V-0.7V/10ohm
I=-0.05

No. The zener conducts no current if Vz < Vs < Von. This is your golden rule. Remember that Vz is negative. No zener current means I = 0.

So when Vs > 0 you need Vs > Von to get any current. What you do is subtract Von from Vs and then compute the current. If it computes < 0 there is no current.

Likewise for Vs < 0. Subtract Vz from Vs. Remeber now Vs < 0. If current computes > 0 there is no current.

 

[november1992]

So I have to subtract the Von from the Vs?

So for a voltage of 6V, would the current be 0 since 0.7V-6V/10ohms=-0.53Amps

 

[rude man]

You contradicted yourself in those two sentences. Think about it. (Hint: the first one is correct).

 

[november1992]

No. The zener conducts no current if Vz < Vs < Von. This is your golden rule. Remember that Vz is negative. No zener current means I = 0.

So when Vs > 0 you need Vs > Von to get any current. What you do is subtract Von from Vs and then compute the current. If it computes < 0 there is no current.

Likewise for Vs < 0. Subtract Vz from Vs. Remeber now Vs < 0. If current computes > 0 there is no current.

So does this mean that if the voltage is 0.2V that I=0Amps because Vs<Von,
and if the voltage is -0.2V I=-0.09 Amps because [(-5.7V)-(-0.2V)]/10=0.09A?

 

[rude man]

Yes to the first part, no to the second.
To repeat: "Likewise for Vs < 0. Subtract Vz from Vs. Remeber now Vs < 0. If current computes > 0 there is no current. "

What is Vs - Vz? answer: -0.2 - (-5.7) = + 5.5V. Current computes to +5.5V/R > 0 so no current.

Again you wrote Vz - Vs when I said to compute Vs - Vz. Tsk tsk! :-)