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ArtG 15th January 2007 12:33 AM

Minimum Zener Current with Pass Device
 
Could anyone with experience with Zener circuits shed some light on a area in which I'm finding conflicting information? I'm building a very simple zener referenced, pass series regulator for a lightly loaded tube circuit. I thought that I'd use some 5 Watt zeners that I have lying around. Searching the web, I find recommendations for about 20 to 25% of the Zener rating, as the minumum current needed for good regulation. Going to the manfacturer's data sheets, I find that min. current isn't specified. Using 7-8 5W diodes, to keep the impedance low, I end up with a circuit that uses far more current (Zeners and series resistor combined) than it is supplying to the circuit!

I built a similiar circuit a long time ago for my preamp, where the designer specified 1.5 W Zeners. I couldn't find that size, so I built it with 5 W devices. Now, I just calced out the current in each of the 6 Zeners in this circuit and find it's flowing 1.5 ma per diode, or just under 1/10th of a watt (62V Zeners). This, of course, appears to be way, way under the minumum current that's needed to get the diode into the avalanche region, at least from my research! And, yet I've monitored the output voltage, and find the regulator works just fine!

Can anyone give me some additional input on this subject?

pmillett 15th January 2007 03:46 AM

I guess maybe it depends on what you mean by "good" regulation...

Zeners don't have a minimum current spec; they have a test current (Izt) at which point the voltage and tolerance are specified. Izt varies between diodes and voltages - it is higher for higher powers and lower voltages.

For example, the 100V 5W 1N5378 (http://www.onsemi.com/pub/Collateral/1N5333B-D.PDF) has an Izt of 12mA, while the 1W 1N4764A has an Izt of 2.5mA (http://www.vishay.com/docs/85816/85816.pdf)

In general you don't want to go below Izt if you want guaranteed performance. So you probably want to use smaller and/or higher voltage zeners...

All that said, my experience has been that zeners will regulate at much less current than Izt. Semi companies have to spec worst-case, over temperature and process. I've run 100uA through zeners with Izt specs of 10mA and had them be right on. So you can either go by the data, or experiment.

Pete

refference 15th January 2007 05:12 AM

Hi ArtG ,

I always use zener diodes in the power supplies .

As a “rule of thumb” I do the following :

I calculate the nominal current allowable , dividing
the nominal power dissipation by the nominal zener
voltage . Then I calculate all the regulation circuit
( ballast resistor and zener or zeners in series for
a given load ) to allow a current swing between
1/20th of nominal current allowable ( maximum load
condition ) and 1/10th ( sometimes 1/5th , for low power
and low voltage zeners ) of nominal current allowable
( minimum load condition ) , using this method ,
I can get a good regulation and a VERY GOOD thermal
stability ( thermal stability is the weakness point of
zener diodes ) .

Examples : A 100 V x 5 W zener diode , will have a
current swing between 2.5 mA and 5.0 mA
A 15 V x 1 W zener diode , will have a current swing
between 3.4 mA and 6.7 mA .
A 6.2 V x 0.5 W zener diode , between 4.1 mA and
16.2 mA ( 1/5th ) , and so on .

Above I described what I do , starting at this point make
your own experiments .

Regards ,

Carlos

zigzagflux 15th January 2007 12:51 PM

I concur with ref:

Choose the zener operating point based on its power dissipation. Control the amount of heat, keeps the zener much more stable.

I usually go for somewhere around 20% of its rating. You are guaranteed to be well beyond the knee, but only mildly warm.

Brian Beck 15th January 2007 02:56 PM

All this depends on how you intend to use a zener. At low zener currents, the slope of the zener’s V-I curve knee is lower than at higher currents, so the increment resistance of the zener will be higher. It will act less and less like an ideal voltage source. This will increase noise too. Incremental changes in zener current will result in greater zener voltage changes with a lower standing current than with a higher current. If the zener is fed by a constant current source or by a large resistor to a fairly high supply voltage, then the voltage across the zener could remain sufficiently constant under these low current conditions for many uses. If there is no audio current drawn from the zener, such as when a zener merely “drives” a tube’s grid or a MOSFET in a power supply regulator, then all is well. But if the zener’s current will vary with audio, such as when the zener is used as a simple shunt regulator or to directly bias a tube’s cathode, then the larger resulting voltage swing across the zener (which isalso non-linear and will add distortion) could be a problem. Here a higher bias current will lower the voltage swings due to audio current through the zener. This might also be a problem in a simple series-pass regulator with a zener reference if the series pass device is a BJT, because base current will vary with emitter signal current demands and the zener will see those current changes (divided by hfe+1 at the base), with resultant zener voltage variations occurring. Here a high-hfe transistor would help, but if this is a high-voltage supply, high-voltage BJTs usually have low hfe values.

I too would keep zener dissipation below rated values for reliability, just not too far below (maybe 25% to 50%) depending on several factors, as above.

ArtG 16th January 2007 12:25 AM

Thank-you all for the replies! First, I just noticed that I mistakenly divided the series resistor value into the zener string voltage (360), squared (rather than by the 50 volt drop across the resistor) to come up with an absurdly high power dissipation, which started my whole problem!
I probably also didn't make it very clear that this will be used only as a reference to the base of a BJT pass device (driving voltage amp plate loads), so zener current fluctuation should be minimal. I'm looking at using a "BUX85", which has an hFE of 30-50, which is fairly high as 450V rated BJTs go, and I don't think going to a Darlington is necessary, in my application. As a result of your kind advice, I'll be looking at using fewer, higher voltage zeners, possibly rated at 1, rather than 5 watts. I'm wondering though, if 1 watt devices at, say 35% of Izt will be much more stable here, than 5 watt zeners at 7% Izt. Some of your replies strongly hint at that, but I can't seem to find anything that specifically states this, although other sources also advise keeping the current up. However, I'd like to use higher power zeners, as insurance against "jolts", and also for their lower impedance. Decisions, decisions.................. trade-offs, trade-offs................

ArtG 16th January 2007 12:44 AM

Correction: where I stated above -- Izt (test current), should have been- Izm (max rated current).


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