zener diode regulators.

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Here we go. Consider a resistor and a zener connected as a shunt regulator
between +ve and -ve supply rail. The resistor is sized so that a safe
no-load current of, say, 25mA passes through zener. Naturally, we measure
the nominal zener voltage or thereabouts at the point where resistor and
zener connect. If we apply a load at this point, how much current can the
load draw before we lose regulation? Surely not the full 25mA?  There must
be a minimum current we must leave for the zener so that it continues to
operate as a zener.

 If this vital information can be gleaned from the zener data-sheet, what's
the term I should be looking for?

I did look up my old ARRL manual and it says that the feed resistor should
be calculated by  SupplyV minus ZenerV  divided by 1.1 Load current. That
sounds like a rule of thumb of "add another 10% for the zener".

Does that sound like a wise rule to follow?  I can think of situations where
the load current varies widely and there's a fine line between losing
regulation at high loads and burning the zener up at low loads. In such a
case, the amount of current I must leave for the zener will affect the
maximum load current, given a zener of a particular wattage. On the other
hand, it's dumb to design a power-gobbling zener current of 80mA when the
load will never exceed 10mA.

Apart from that, I've looked at zener data sheets and Googled around the
electronics education websites and am no wiser.

Maybe I'm just tired and dense after a hard day so I'm asking you
knowledgeable gentlemen to think for me again.

PH



Re: zener diode regulators.



"Peter Howard"
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** Where the load current varies over a wide range  -  better use a zener
plus transistor or an IC regulator !!!

Simple zener diode regulators are fine for modest variations  ( +/- 20%) in
the DC  *SUPPLY* voltage to a more or less fixed load.

BTW

The performance of zeners varies greatly with the actual voltage -  those
under about 5 volts are as soggy as wet toast.

Using a zener with more than adequate power rating helps with regulation.




.........   Phil



Re: zener diode regulators.


On Mon, 10 Oct 2005 20:48:47 +1000, "Phil Allison"

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I was going to suggest having a peek at a datasheet to give a visual
of what phil said, but after looking at a few data sheets i cant find
one that publishes a graph on current v voltage. Been a whiles since I
looked at a zener datasheet :) I notice that the fairchild data sheet
doesn't even publish spects on juction capacitance either..

Re: zener diode regulators.


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If you can find a datasheet somewhere with a curve or two, you can get a
feel for zener diode performance.  Don't take the curves too seriously,
because they vary between manufacturers and rated power.  I have a few
curves in a text book which get me on track, but I have seen good stuff in
datasheets.

Zeners are sloppy - at times I have had to move to the next 10% series
voltage up or down because the supplied parts were different to the
development parts !


Here is a selection of manufacturers for a single diode type:

http://www.datasheetcatalog.com/datasheets_pdf/1/N/4/7/1N4749A.shtml

A quick look showed the Philips 1N4749A datasheet had some I/V data.  Also
the "DC Components" company 1N4749A datasheet.  So the data is out there.

Fairchild datasheets are often sketchy.  Philips datasheets are often OK.

Roger






Re: zener diode regulators.



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Ah. That was my trouble. I was looking only at Fairchild data sheets. Silly
me.



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Yes, Roger's  link below sent me in the right direction and I did indeed
find some particularly educational curves in the DC Components datasheet.
Looking at the current vs volts curves for lower voltage ones I see what
Phil meant when he said they were soggy as wet toast. On t'other hand, the
15V one I'm currently  (pun unintentional) working with seems to have a much
more definite "knee" and become relatively stable voltage wise at a smaller
current than the lower value ones.

I suppose if I was really keen I'd breadboard a few examples of the same
diode with a variable power supply, load etc and draw my own curves from
voltage and current measurements. Be interesting to see how much variation
there was between specimens or even between manufacturers. I could even turn
the hot air blower on the test bed and see what ambient temp does to them.

My whole problem is that I've been spoiled by the ready availability of
cheapish three terminal regs that are easily beefed up with a pass
transistor or even trimmed upwards with a diode or two in the ground lead.
Now I'm building something where cost is most definitely an object so I'm
forced to fiddle with a low cost Zener shunt arrangement.


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Anyway, I thank all of you for your valuable input.

PH



Re: zener diode regulators.


On Tue, 11 Oct 2005 10:26:06 GMT, "Peter Howard"

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The tempco of zeners changes from negative to positive as the zener
voltage passes through about 5.6V. If temperature stability is an
issue, then one can sometimes get better results by using two zeners
in series. However, in your case the effects of tempco may be swamped
by dV/dI.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: zener diode regulators.


On Mon, 10 Oct 2005 10:34:41 GMT, "Peter Howard"

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I think you should be looking for something that quantifies the
dynamic resistance, ie dV/dI. That'll give you an indication of the
change in zener voltage to be expected from a variation in the load.
Some datasheets indicate the zener's dynamic impedance, Zzt, at a test
current of Izt. In the example below, Izt is 25% of the rated current.
The datasheets also quote a much higher figure of Zzk @ Izk for the
knee of the I-V characteristic curve. These numbers can vary from 5
ohm in the linear region to 1Kohm at the knee.

I found this app note useful:
 http://sound.westhost.com/appnotes/an008.htm

The device referred to in the article (BZX2C16V, 16V, 2W) has an Izk
of 1mA and a max current of 114mA.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

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