Series Zeners

[positronicle]

--Edited by Positronicle--

[dyarker]

Two 5V 1W zeners in series is equivelant to a 10V 2W zener with a less distinct zener knee. (knee = transistion from cutoff to current avalache)<p>A 45V 1W zener in series with a 5V 1W zener is equivelant to a 50V 1.1W zener, but not in practice. Yes, to be a higher voltage zener at a fixed power, the current must be smaller. The voltage should be divided as equally as possible. If the max current of the high voltage zener is less than the knee current of the low voltage zener, the result is ... nonlinear varying voltage mess.

[Robert Reed]

Bear in mind that multiple seriesed zeners will also have a higher source impedance than a single zener of the same value would. Also zeners above 20 volts tend to have poor temp. coeffients., making it desirable to series up smaller ones. Agaiun, as you mentioned --more tradeoffs.

[Dean Huster]

And all this translates to poorer regulation. It's a lousy trade-off that is useful if you need to have something working while you wait for the correct parts to come in. You're best off to use a 10V (or thereabouts) zener.<p>Dean

[Carl Pugh]

The best zeners are usually around 6 or 6.2 VDC.
Comparing 1N5233 (6.0 V) and 1N5242 (12 V)<p>1N5233
Zzt=7.0 ohm/20mA,
Zzt=1600 ohm/0.25mA,
Temp Coef=+0.038<p>1N5242
Zzt=30 ohm/20mA,
Zzt=600 ohm/0.25mA,
Temp Coef=+0.077<p>So depending on the current operated at, two 6.0 volt zeners in series may be quite a bit better than one 12 volt zener.
Or at 0.25 mA, two 6.0 volt zeners in series have higher resistance than one 12 volt zener.
The temperature coeficient for two 6.0 volt zeners in series is better than the temperature coeficient for one 12 volt zener.<p>MOTOROLA INC. Book "RECTIFIERS AND ZENER DIODES DATA" DL125 REV1<p>Carl Pugh
Free Lance Consultant

[Chris Smith]

Mixing any two components harvests you only the “average” between the two. Any discrepancies will average out, and all values should be placed at the minimum exception rather than the best case scenario. <p>If you were to work out your parameter based solely on theory, Derate every value by 10% and then apply it and see if you came out better than expected. <p>Never apply such a theory on the first flight test with out some kind of back up.<p>[ July 14, 2005: Message edited by: Chris Smith ]</p>

[Dimbulb]

The zener configuration using additional stages
has thermal drifts its numerous configurations. The original plan to adjust 50 volts. 5.000 X 10
by dividing first at a low current. Give us an example of your output to regulate ?

[Dean Huster]

Gotta be careful comparing apples and oranges, Carl. Well, maybe in this case, oranges and tangerines! At the higher current, the lower voltage device may be better but at the lower current, the higher voltage device seems better. However, the power dissipation is what messes things up here. The 12 volt part cannot handle the current of the 6 volt part because both have the same maximum power dissipation rating. So, it stands to reason that the impedance of the higher voltage part will be higher. Considering the lower operating current for a given power dissipation, I think that both zeners are comparable.<p>Dean

[Ron H]

Dean, a couple of days ago I almost made the same comments as Carl, but after looking at some datasheets, came to the same conclusion you did.
There may be some minor gains in impedance with certain combinations, but I'm betting they are minor.
It does seem that tempco could be improved on by using series combinations.