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Why does zener voltage decrease as series resistor increases?

Sidecar Bob

Dec 19, 2021
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It need to supply 6mA @ 5.1V (regulated) to a circuit so I've breadboarded the circuit below to try to determine the value required for the series resistor (Rz) required for a C5V1 Zener diode.

6mA/5.1V = 850Ω so I'm using an 820Ω resistor for test purposes.

According to the datasheets for various C5V 1 zeners I looked at Izt could be 11mA but I am not certain so I thought it would be good to use 20mA for the calculation.

26mA/6.9V = 265Ω so I set my resistor substitution box to 270, connected it as Rz and measured 5.35V across the zener.

From everything I've read about zeners in the last few days that indicated that not enough current was flowing through the zener for it to regulate so I needed to decrease Rz to increase the Iz but when I decreased Rz the voltage across the zener increased.

A bit of experimenting found that the zener voltage is 5.1V when Rz is 960Ω, at which point there is less than 1mA through the zener.
If I increase the Rz above that the voltage decreases and if I decrease Rz the voltage increases.

What am I doing wrong?
Zener test circuit.jpg
 

Harald Kapp

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Nov 17, 2011
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A zener diode is not suitable to "regulate" the voltage to a load directly. Since the load current passes through Rz, the voltage across Rz depends on the load current. This is not what one wants.
In your example Rl and Rz form a voltage divider. With
Rl = 820 Ω
Rz = 960 Ω
and an input voltage of 9 V, the output voltage across the load will be
Vout = 9 V × 820/(820+960) = 4.2 V -> the zener diode has no effect at all.

The classic zener diode based voltage regulator uses a transistor to decouple load current from zener current. See e.g. figure 4 here (the article could interest you per se).
 

ramussons

Jun 10, 2014
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Look at it this way.
Your setup behaves like a voltage divider made of Rz and 820 Ohms resistors.
The output voltage gets clamped to 5.1 volts due to the zener.
 

crutschow

May 7, 2021
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A Zener is not a perfect regulator.
Its voltage will change some with a change in current through it.
Look at the value for the Zener impedance to determine what the change with current will be.

A much better regulator is an IC that mimics a Zener, such as the TL431 shunt reference.
It can be programed with two resistors to give any desired voltage within its voltage limits, and its voltage varies very little with current.

Below it a simulation of a 5.1V Zener and the TL431 configured for 5V, with a current variation through both of 1mA to 20mA.:
As you can see, the Zener (yellow trace) varies about 220mV whereas the TL431 (green trace) varies only about 16mV.

upload_2022-2-14_0-54-28.png
 
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cjdelphi

Oct 26, 2011
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Sidecar Bob

Dec 19, 2021
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Thanks for your replies
Vout = 9 4.2 V -> the zener diode has no effect at all.
Look at it this way.
Your setup behaves like a voltage divider made of Rz and 820 Ohms resistors.
The output voltage gets clamped to 5.1 volts due to the zener.
This is running at such low current that the "9V" supply's output is actually putting out something like 11.7V at that load so Vout = 11.7V × 820/(820+960) =5.4V, which means that it is just the voltage divider producing that output. I don't know why I didn't see that last night.

But still, when Rz was set to 240Ω Vout would be about 9V if the zener wasn't doing anything.

I wonder if I am just overthinking this.
The project is to power one of the infamous "red" variable power supply kits from eBay with a switching power supply instead of a transformer.
Project log discussion: https://www.electronicspoint.com/forums/threads/bench-power-supply-upgrade.296479/

The Chinese products tested blog says:
"To be able to control from an output voltage of 0 V, the system needs a negative supply voltage. A simple pump circuit was chosen, consisting of the diodes VD4 and VD5 and the two electrolytic capacitors C6 and C7. The negative voltage across C6 is stabilised at about -5.1 V via R16 and VD3. This voltage is used to supply the two op-amps OP1 and OP2 and is also necessary for the operation of the circuit around transistor VT3."
https://chinese-electronics-products-tested.blogspot.com/p/hesai-30v-3a-power-supply-kit-tested.html
Kit schematic.jpg
Audioguru has been coaching me with the changes required to power it from the switching supply (& make it more reliable), including replacing the pump circuit with a small DC supply as in the drawing below. I am at the point where I need to finalize the value for Rz. He told me that the circuit needs about 6mA and suggested that I use a value of about 20mA for the zener current to ensure that it is able to regulate and I decided to test it to make sure.
Perhaps I should just assume that if the zener would provide a suitable regulation in the original circuit it would be OK with a value of 270Ω in this circuit
Schematic with two DC supplies (3rd version).jpg
 
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