Should I use high voltage value zeners or multiple lower voltage zeners in series? - diyAudio
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Old 30th May 2006, 07:10 AM   #1
jarthel is offline jarthel  Australia
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Default Should I use high voltage value zeners or multiple lower voltage zeners in series?

as title.

any pros/cons?

Thanks for the help.
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Old 30th May 2006, 07:17 AM   #2
quasi is offline quasi  Australia
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This depends on the voltage sought, the zener current and the application. Generally for higher voltages I prefer using multiple zeners because I can better spread the heat via several junctions and packages rather than one.

With respect to the application, note that every zener juntion will generate some noise.

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Old 30th May 2006, 07:19 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
depends on the power dissipation.

example.
500mW 36V Zener has a maximum current of 13.8mA and for reliable reference voltage a minimum current of 1.4mA (>10% of max).

A string of three 1.3W 12V Zeners will have current limits of 11mA to 108mA.

Using a string allows selection within the Zener tolerances to get cloer matching and absolute accuracy. But do not mix voltages within a string unless you can ensure that EACH Zener stays within it's limits.
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Old 30th May 2006, 07:21 AM   #4
jarthel is offline jarthel  Australia
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thanks for the help
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Old 30th May 2006, 07:23 AM   #5
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It also depends on the zener dynamic resistance. An ideal zener has zero ohms dynamic resistance, meaning that the zener voltage does not change with zener current. In reality, there is always some resistance. 7V zeners are around 10 ohms at a few mA, for example. Higher voltage zeners have (much) higher dynamic resistances, so multiple low voltage ones may be better zeners than a single high voltage one. You should consult the data sheet for this.

Jan Didden
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Old 30th May 2006, 07:31 AM   #6
jarthel is offline jarthel  Australia
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Quote:
Originally posted by janneman
It also depends on the zener dynamic resistance. An ideal zener has zero ohms dynamic resistance, meaning that the zener voltage does not change with zener current. In reality, there is always some resistance. 7V zeners are around 10 ohms at a few mA, for example. Higher voltage zeners have (much) higher dynamic resistances, so multiple low voltage ones may be better zeners than a single high voltage one. You should consult the data sheet for this.

Jan Didden
was looking at the datasheet for an onsemi one and can't find anything that looks like "resistance".

I guess the best choice is to buy multiples.

thanks for all the help.
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Old 30th May 2006, 07:49 AM   #7
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Quote:
Originally posted by jarthel


was looking at the datasheet for an onsemi one and can't find anything that looks like "resistance".

I guess the best choice is to buy multiples.

thanks for all the help.
Well it pays to check it, you want good zener performance, no?

See the attached, the table on page 3 lists in one column "Zzt @ Izt which is the dynamic resistance at current Izt. That should be as low as possible for the total combination you need.

Then look at Fig 3 which gives in more detail the variations in zener resistance as a function of zener curent.

Jan Didden
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Old 30th May 2006, 04:56 PM   #8
cpemma is offline cpemma  United Kingdom
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Back when I was a lad it was standard text-book to select series zeners such that positive & negative temperature coefficients cancelled out.

Quote:
...up to about 5.6 volts, the zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.

In a 5.6-volt diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6-volt diode is the part of choice in temperature critical applications.
http://www.answers.com/topic/zener-diode
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