# Zener Diode Circuit Analysis

#### SamR

Joined Mar 19, 2019
3,400
I can't get this into my thick skull. If this is correct please enlighten me.

If you measure across the power supply you get 30V so the 100Ω resistor should be seeing 30V? Except that the Zener is dropping 20V across it. Does the power supply never reach 30V? Does that 10V magically disappear?

I get the load part but having a hard time understanding only 10V across the 100Ω resistor...

#### bertus

Joined Apr 5, 2008
21,362
Hello,

The supply voltage is 30 Volts.
The zener clamp voltage is 20 Volts.
The 100 Ohm resistor is between them.
So there is a voltage drop of (30 - 20) volts accross the resistor.

Bertus

#### SamR

Joined Mar 19, 2019
3,400
K, got it. I kept thinking the full 30V was across it... Ahha it's like a solved voltage divider.

#### Hymie

Joined Mar 30, 2018
976
Other practical things you should be aware of in relation to the circuit;

The power dissipation in the zener diode will be the zener voltage x the zener current (20V x 60mA = 1.2W).

In reality there will be a small resistance within the zener diode; when in reverse bias the equivalent circuit of the zener diode is a battery (of the zener voltage) in series with the resistance value. Normally the series resistance value is of the order of a few ohms (and for practical purposes can be ignored). But if a high power zener diode is used that results in a significant voltage drop across this series resistor due to the zener current – it may lead to an increase in the effective zener voltage.

When the reverse bias voltage drops below the zener voltage, the component (the zener diode) effectively becomes open circuit. And remember if forward biased the zener diode will conduct current with a small voltage drop (much like a normal diode).

#### SamR

Joined Mar 19, 2019
3,400
Just doing the 1st approximation. Something about Zeners screws up my thinking. Rarely used them...

#### Wolframore

Joined Jan 21, 2019
2,273
There’s a dam analog for zener diodes, the zener voltage being the height of the dam. It’s rated for a certain current so too high and the dam bursts, when the voltage is high enough it overflows over the dam.

#### SamR

Joined Mar 19, 2019
3,400
Zeners are in the same box as Negative Voltage for me. One of these days it'll gel.

#### Hymie

Joined Mar 30, 2018
976
Zeners really are very simple, consider them as a battery of the zener voltage (clamping the circuit at that voltage), only you can’t get any current out of the ‘battery’.

#### ericgibbs

Joined Jan 29, 2010
12,884
hi Sam,
I know you use LTS.
These two files may help clear the zener fog.

E

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#### MrAl

Joined Jun 17, 2014
8,234
I can't get this into my thick skull. If this is correct please enlighten me.
View attachment 227852
If you measure across the power supply you get 30V so the 100Ω resistor should be seeing 30V? Except that the Zener is dropping 20V across it. Does the power supply never reach 30V? Does that 10V magically disappear?
View attachment 227853
I get the load part but having a hard time understanding only 10V across the 100Ω resistor...
Hi,

Step 1: Remove the zener diode and calculate the resistive voltage divider voltage across RL call it Vd.
Step 2: If the voltage Vd is greater than the zener voltage, the zener is conducting, if it is less than the zener voltage, the zener is not conducting and the voltage across it is the voltage divider voltage and we are almost done. You can now calculate the current through the resistors if you have not done that already.
Step 3: If the voltage divider voltage was greater than the zener, then the zener is conducting so replace it with a constant DC voltage source equal to the zener voltage and with the proper polarity. Now you can calculate the current through the resistors easily because all the resistor voltages are known.

So first you open the zener, then decide if it is conducting or not, then either leave it open or replace it with a voltage source, then proceed with the current calculations if needed.

Watch out for your power calculations though as a zener diode never puts out any voltage or current except for times when the voltage changes due to its small capacitance but you probably dont have to worry about that capacitance here it's a static circuit problem.

#### SamR

Joined Mar 19, 2019
3,400
These two files may help clear the zener fog
I guess I'm just being lazy about using LTS. It is a tool that is very good for looking at circuits...

#### Wolframore

Joined Jan 21, 2019
2,273
Simpler ones like Falstad and EveryCircuit are fine for just simple sanity check for diodes and resistor dividers, I wouldn’t trust them for more complex analysis.