How can a Zener diode maintain a constant voltage?

Question

A Zener diode in reverse bias more than breakdown starts conducting. If you increase the reverse bias:

1. It conducts more i.e current in it increases
2. The voltage across it remains same

I think I get all except 2 above. Why does the voltage across it remain constant even when the reverse bias is increased?

Answer 1

Why does the voltage across it remains constant even when reverse bias is increased.?

It doesn't. The voltage increases very slightly as the current is increased.

To use a hydraulic analogy, it's like a weir across a river. While the water is below the crest of the weir, no water flows over. Once the water exceeds the crest, water flows. If you send a much greater flow than that down the river, the water height above the weir will increase only slightly.

The rate at which the voltage increases for increasing current is variously called the slope resistance, or the dynamic resistance, or the incremental resistance. This is usually listed at a particular current in zener diode data sheets, and is often in the ohm range. The 1N4278A 6.2 V diode has for instance typically 2 ohms slope resistance at 41 mA, and warranted less than 700 ohms at 1 mA.

Answer 2

OP's question

You asked the question:

Why does the voltage across it remain constant even when the reverse bias is increased?

... and then you almost (2 of 3) answered it:

1. It conducts more i.e current in it increases

2. The voltage across it remains same...

You just have to mention what the resistance does to make it so:

3. ... since the resistance decreases to the same extent as the current increases.

I am sure you have already guessed that.

Electrical explanation

So, the closest and natural analogy to a Zener diode is a variable resistor (rheostat) or more precisely, a "self-varying" or "dynamic" resistor that has the property to decrease its instant resistance R when the current I increases and v.v. so that to keep the voltage V constant:

V = Iinc.Rdec = Idec.Rinc = const

(an extremely simple arithmetic trick where we change one multiplier in the opposite direction of the other to keep the product constant).

This electrical analogy is not only simple and obvious but can also be easily implemented - we just need to move the rheostat's viper adequately. Only, we have to keep some considerations in mind:

Current supply. You have said "when the reverse bias is increased", i.e. you mean a voltage is applied across the Zener diode and then the voltage is increased. However, the diode behaves like a voltage stabilizer that does everything possible to maintain its voltage... and, as we saw above, in the name of this, it can enormously decrease its resistance finally short connecting the voltage source. The conclusion is that, to avoid the conflict, we have to supply the diode through a current source and not a voltage source... or at least through a resistor.

Diode "resistance". It is embarrassing for some people to consider a diode as a "resistor" but this is just an analogy. It has "resistance" in the broadest sense of the word as something that prevents current from flowing through it when voltage is applied. More precisely, it has "nonlinear resistance".

How to explain it

The second part of my answer is aimed at my colleagues who are trying to answer the question in the best way, in particular @Neil_UK.

General idea. The art of explaining is to first reveal the most general idea (concept) and then consider its specific implementations. Thus, people will be able to understand not only the specific (Zener diode) circuit but also all related (diode) circuits based on the same idea (dynamic resistance).

Ideal device. Concepts should be explained by ideal devices. At this stage, details are unnecessary; they only distract from the idea. So the "ideal" Zener diode has a vertical IV curve and it makes no sense to specify that it is not so vertical but has a slight slope because this does nothing to explain the basic idea.

Analogies. Any analogies with well-known phenomena (electrical, mechanical, pneumatic, hydraulic, biological, social, etc.) help to understand the idea but the closest in nature is the electrical analogy. Electronic circuits are based on more elementary electrical circuits and can therefore be naturally explained through them.