Q About Zener Diodes In Series

[Paul Marossy]

Can you use them in series to get a certain voltage? I don't know why, but my intuition tells me no. I need some confirmation on that, though.  :?

[MartyMart]

When you use them "reverse-biased" ( as always ) they are a "voltage regulator" so, if you had say 3v across your zener from within a 9v circuit, when you double the circuit voltage to 18v, there is still 3v across the zener ...  correct ?
If you had two is series, the second zener would only ever see the "fixed voltage" from the first, so things could get wierd  !!
I dont think they work in the same way as say a pair of 1N4148's in series ....
Am I right on this ?  thats how I understand it  :shock:

Marty.

[Paul Marossy]

A zener diode is bascially a voltage regulating diode. It will hold at a certain voltage regardless of changes in current supplied to the diode.

It seems to me that if the "first" zener was set at a high enough voltage, then it would supply the "second" zener with a set voltage, which would in turn be the maximum voltage possible. In other words, the second zener voltage is all that you would get. So, I guess I answered my own question!

EDIT: I guess MartyMart beat me to it...  :oops:

OK, but here's a another question: What if you were to place them in parallel? It seems to me like it would be somethin akin to parallel resistors, you would get some kind of ratio of the original voltage, correct?

Darn, I guess there's no way to rig a DIY zener diode combination.  :x
I guess I have to go downtown this weekend.  

[tauscj55]

For ohmic devices, isn't the definition of being in parallel that they have the same Voltage, rather the current is what changes?  Then again I suppose diodes aren't exactly ohmic in nature... I don't know.

[Paul Perry (Frostwave)]

If you put zeners in series in the same direction, the voltages add together.
It's as simple as that!

[Paul Marossy]

If you put zeners in series in the same direction, the voltages add together.
It's as simple as that!

Hmm... that's what I was thinking at first, before I posted, but that seemed too simple.  :shock:

[loscha]

I found this illustration, which clears up the matter for me.
Sometimes, a visual on the concept makes a difference, although, the descriptions above were fine!



the page I foudn it on, http://www.electronics-tutorials.com/basics/diodes.htm recommends, however, that you don't actually do that with your Zener collection in a practical circuit, due to current mis matches.

Am I correct in my assumption that Zeners in parallel work the same as normal diodes, increasing the current handling ability?

[jjs]

My understanding of zener diodes is, that they start conducting when the voltage across them is higher than their zenervoltage.
So if you would wire two zener diodes with different voltage ratings in parallel, the one with the higher rating would do nothing more than a regular reverse biases diode (only leakage current passing through), whereas the lower voltage one would pull the voltage to its rated voltage (which turns the zener effect in the other diode off, because the voltage is lowered below its threshold level). Probably this would be true even with two zener diodes rated at the same voltage because of their 10% variance.

I got a question about zeners: If you assume you have an ideal voltage source (with internal resistance of 0Ohm), say with 12 V output and you would try to use a 10 V zener diode to bring the voltage down, then this wouldn't be possible, would it? I mean the ideal voltage source can source as much current as needed, so it's voltage wouldn't drop by increasing the current drawn by the load.
The same should be true for a source that is almost ideal (can source large amounts of current like a car battery).
So does a zener diode use the voltage drop across the internal resistance of the source to regulate the voltage?

[Paul Perry (Frostwave)]

Am I correct in my assumption that Zeners in parallel work the same as normal diodes, increasing the current handling ability?

I don't think so. Because, the diodes won't be perfectly matched, so one will draw current before the other, with unfortunate results.
And I wouldn't do it for 'ordinary' diodes either, for the same reason.

Incidentally, that 'stacked' zener arrangement to give different outputs is perfectly OK by me, except that as the current drawn by one output changes, the other voltages vary slightly also (zeners having a certain impedance). You could hang electrolytics on the outputs though to get the AC impedance down.

[Paul Marossy]

Thanks all, I have a much better understanding about the previously mysterious zener diode now!  

[Johnny G]

paul, i thought that just so long as the diodes had a minimum current going through them then they would keep regulating the voltage. so as long as there is, say, 5ma going through all three diodes. even if you tap off at the point above the second from bottom diode and have extra current running through the top one, just so long as theres satill enough current flowing through the others it should be fine

[R.G.]

My understanding of zener diodes is, that they start conducting when the voltage across them is higher than their zenervoltage.

So if you would wire two zener diodes with different voltage ratings in parallel, the one with the higher rating would do nothing more than a regular reverse biases diode (only leakage current passing through), whereas the lower voltage one would pull the voltage to its rated voltage (which turns the zener effect in the other diode off, because the voltage is lowered below its threshold level). Probably this would be true even with two zener diodes rated at the same voltage because of their 10% variance.

That is 100% correct.

So does a zener diode use the voltage drop across the internal resistance of the source to regulate the voltage?

No. The zener just conducts when the voltage "pressure" across it is enough to let electrons spill over. It's like a spring loaded water pressure valve. No water gets through until the water pressure is just larger than the spring pressure. Then water comes through and only slight increases in pressure let vastly more water through. The pressure dropped across the spring valve is equal to the spring pressure.

The voltage is regulated entirely by the zener breakover voltage. Of course, Mother Nature insists that power is still voltage times current, so unless you limit the current with an *external* resistor, an ideal voltage source with a voltage larger than the zener voltage will instantly fry the zener because the zener does not limit current to any significant extent. A zener must always have some external current limiting mechanism to survive.

Zeners in series do nothing until the applied (current limited!) voltage is greater than the sum of the two zener voltages, then they both conduct.

You may freely pull different currents out of the junction of zeners in series. The zeners conduct the differences in currents. The voltage at a junction will vary only by the amount of the internal zener resistance, and that is a few ohms for most zeners. Caps to lower AC impedance are a good idea, especially since zeners themselves generate noise (they were once used for noise diodes.)

The way you design zener circuits is to first know your load and raw DC supply - you need to know how much current you need, both maximum and minimum, as well as the min and max supply voltage. You then put a resistor in series with the raw DC supply that is low enough to supply the maximum load current when the raw supply is minimum. Under those conditions, the zener would have its current just barely drop to zero, and the resistor would be supplying all the load current. Then you calculate the current that would flow  with that resistor when the load was minimum current and the raw supply was maximum. That is a slug more current, and the zener "regulates" by eating all that extra current.

Once you know the max zener current, you multiply that by the zener voltage to get the zener power dissipation, and you buy a zener that will get rid of at least that much heat.

[Johnny G]

oh and jjs, for the zener to do its job there does have to be some resistance. hence if you look at the picture there is a resistor inline with the diodes

only thing you have to make sure of is that you use a large enough resistor that it wont dissipate too much power but a small enough one so you can source all the current you need. other problem is that any current that doesnt go to whatever load you put on the circuit will pass through the zener instead. so you have to make sure that the zener can take the power

ah, i remember having to design regulated power supplies for my GCSE electronics. back then it was all so difficult lol

[Paul Marossy]

A zener must always have some external current limiting mechanism to survive.

That explains a few things that I thought were strange the first time I tried using a zener diode!  (Like why it was getting so hot  :shock: :oops:)