Voltage regulating with a Zener Diode

Bordodynov

Joined May 20, 2015
3,177
By the way, how do you explain this value of the dynamic resistance at a current equal to 1 mA?
Zzk@ 1mA = 564 Ohms (I assume this is the minimum current needed for the zener diode)
With a larger current through the zener diode, the dynamic resistance is less!
 

Thread Starter

uhf737

Joined Dec 9, 2017
14
I assume it has something to do with the I/V characteristics of the diode at breakdown. For very slight changes in voltage, you get small changes in current, since the dynamic resistance is calculated as delta V/delta I and the slope is very steep after breakdown, the resistance should be lower for higher currents
 

MrChips

Joined Oct 2, 2009
30,707
I assume it has something to do with the I/V characteristics of the diode at breakdown. For very slight changes in voltage, you get small changes in current, since the dynamic resistance is calculated as delta V/delta I and the slope is very steep after breakdown, the resistance should be lower for higher currents
Bingo!

Dynamic Resistance = ΔV / ΔI

For an ideal zener breakdown, Dynamic Resistance = 0
For non-ideal zener diode, Dynamice Resistance is finite. This results in a slight increase in breakdown voltage as current increases.
 

Bordodynov

Joined May 20, 2015
3,177
Bingo!

Dynamic Resistance = ΔV / ΔI

For an ideal zener breakdown, Dynamic Resistance = 0
For non-ideal zener diode, Dynamice Resistance is finite. This results in a slight increase in breakdown voltage as current increases.
I did so in my calculations. See # 17.
.meas V5mA find v(X) at 5mA
.meas V5.1mA find v(X) at 5.1mA
.meas Zzt param (V5.1mA- V5mA)/0.1mA
and
.meas V1mA find v(X) at 1mA
.meas V1.1mA find v(X) at 1.1mA
.meas Zzk param (V1.1mA- V1mA)/0.1mA
result of calculations:
v5ma: v(x)=3.00022 at 0.005
v5.1ma: v(x)=3.00912 at 0.0051
zzt: (v5.1ma- v5ma)/0.1ma=89.016 ==> Zzt=89Ohm
v1ma: v(x)=2.18432 at 0.001
v1.1ma: v(x)=2.24164 at 0.0011
zzk: (v1.1ma- v1ma)/0.1ma=573.22 ==> Zzk=573Ohm
Compare the dynamic resistances calculated by me and the initial data for the zener diode. I have fairly well chosen the approximation.
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

Not sure if this helps but we can use a simple model for the zener to study the effects of different bias and/or stimulation.

The model of a 10v zener is then simple:
Vd=(507*ln(1e12*Id+1))/1250

The static resistance is then:
Rs=Vd/Id

and the dynamic resistance is:
Rd=(4.056*1e11)/(1e12*Id+1)

At 50ma Vd=10v and the static resistance is 200 Ohms and the dynamic resistance is 8.11 Ohms.
 

MrAl

Joined Jun 17, 2014
11,389
Simple models:
Rd=8+13*25mV/Iz
Iz=25mA ==> Rd=21Ohm
Iz=50mA ==> Rd=14.5Ohm
Hi,

That's a good idea too, but i was shooting for something a little more like a real diode where we have an actual diode curve not just a straight line. Still I think the straight line model is informative too though.

Here is the curve for the diode i mentioned in both linear and log x coordinates.
 

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