What does a zener do?

[boogyman19946]

Hello all!

I've been trying to hone my understand of electronics more in the direction of intuitive thought rather than the trite way of balancing equations and I've come across a book called Art of Electronics. Thus far, (a good 24 pages or so into it) it's proving to be a challenging boot camp for me, so I kind of plow through using whatever electronic components I have to test if what I'm getting out of the book is correct or not; that is, I'm trying to supplement the theory and various rules by experiment to kind of see how it all works out.

Now, I've come a section about zener diodes. From prior-knowledge, I understood that zeners act as diodes until a certain voltage after which they allow current to flow both ways. However, in the book, the diode is described via the following:

Zeners are used to create a constant voltage inside a circuit somewhere, simply by providing them with a (roughly constant) current derived from a higher voltage within the circuit.

I'm not exactly sure of what this means. Can anyone elaborate more on what is going on here?

 

[(*steve*)]

Let's assign directions as forward and backward through a diode.

As the voltage across the diode reaches a threshold voltage, current starts to flow.

In one direction, the voltage will be less than the other. That we will call the forward direction. The voltage drop varies for different diode types, but may be between 0.1V and 0.7V (Assume 0.7V unless you know otherwise).

In the backward direction things get interesting.

For most "normal" diodes, the reverse voltage is quite high. In fact it is probably significantly higher than the voltage rating of the diode.

But as you increase the voltage past the reverse breakdown voltage, the current rises in a similar manner to the way it does in the forward direction as the forward voltage increases.

If we allow the current to rise to too high a value, the diode will be destroyed (this applies in either direction, but clearly the device is dissipating more power in the reverse direction, so thermal effects are greater there at a given current).

Now it just so happens that we almost never apply a constant voltage across a diode that is in a conducting state.

So we have this zener diode. And a Zener diode is special because it has a known reverse breakdown voltage. It is also optimised so that it works well (i.e. in a predictable and stable manner) when reverse biased.

Now from the preceding statement, we try to control the current through the zener diode, and this may be by means of a constant current source. When used this way, the zener exhibits a near-constant voltage across it.

As I said, there is some symmetry. if you place a zener diode in a circuit back to front, you get a constant voltage across it, but one of only... you guessed it 0.7V.

 

[boogyman19946]

So, for a normal diode, it basically does it's usual job of stopping the current from flowing in reverse until the voltage applied exceeds ts threshold voltage where it actually starts to conduct in reverse right? And that's bad for the diode (the normal kind).

The zener is designed to work well in reverse bias and in such state tries to maintain its voltage drop to be as constant as possible (or I guess however it suits the situation). And when current is applied in forward bias, the zener acts like a normal diode. That is insane! I knew there was something about conducting in the backwards direction but I didn't know it had such cool properties XD

So wait, if you were to build a voltage divider with a zener, does that mean it would provide a (within limitations of input voltage and all other factors) stable voltage? Its kind of like... an automatic variable resistor then no?

 

[(*steve*)]

So, for a normal diode, it basically does it's usual job of stopping the current from flowing in reverse until the voltage applied exceeds ts threshold voltage where it actually starts to conduct in reverse right? And that's bad for the diode (the normal kind).

Yes. In some broad sense, that behaviour describes the function of ALL diodes.

The zener is designed to work well in reverse bias and in such state tries to maintain its voltage drop to be as constant as possible (or I guess however it suits the situation). And when current is applied in forward bias, the zener acts like a normal diode. That is insane! I knew there was something about conducting in the backwards direction but I didn't know it had such cool properties XD

Yes. The main thing is that it is designed to break down at a predictable (and generally low) voltage and to exhibit a reverse voltage that varies as little as possible with current.

In the forward direction it will work just like any other diode, but maybe it won't be as good. (it may have a higher forward voltage drop, or it may exhibit other characteristics that aren't useful because it hasn't been optimised for that use.

There are occasions where we use a zener diode for both its forward and backward properties. One of those is in protection circuits where we connect the anode to earth. This will clamp an incoming voltage to the zener voltage (reverse) and also not allow it to exceed one diode drop below ground (forward).

So wait, if you were to build a voltage divider with a zener, does that mean it would provide a (within limitations of input voltage and all other factors) stable voltage? Its kind of like... an automatic variable resistor then no?

*EXACTLY* A resistor is not a great current source, but within certain limitations it may be acceptable as one. What you have made is a shunt regulator using a zener diode as the regulating element. This is a very common use of the zener diode.

 

[boogyman19946]

Thanks a whole bunch, that clears a whole lot! I have a hard time following Mr.Horowitz's book. I guess I kind of understand how people can miss the "obvious" relationships between concepts, as I'm finding out just now. I mean, it's been three days and I'm only on page 21... what a boot camp!

I have another question about capacitors, but I'll just make a new thread for that. Thanks for the help (*steve*)!

 

[(*steve*)]

The Art Of Electronics is a great book. Consider yourself lucky it doesn't try to cover the mathematics

Keep with it. You may find you need to cover some more basics first, but if you can do that then your return to this book should be much more rewarding.

 

[boogyman19946]

I'm not sure how lucky I should consider myself without the math XD I take math as a hobby (just imagine reading a math book in your spare time) so the more advanced concepts, mathematically speaking, aren't all too difficult. The only problem I'm finding is that I'm not quite sure how to connect the math that is presented with the reality.

 

[(*steve*)]

I like math too, but I find that a good general understanding is possible without it.

If the math makes it easier for you then there are a large number of electronic engineering books that will dump you in it. TAOE is an almost lone example of one that doesn't.

 

[boogyman19946]

Mathless examples are perfectly fine of course. I'm just saying, math or no math, I don't mind learning either of the two ways.