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Old 15th June 2009, 08:25 AM   #1
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Default Zener Diode Question

Why do so many circuits use multiple zener diodes in series when Mouser sells about every value imaginable?

Heat disipation maybe?
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Old 15th June 2009, 08:33 AM   #2
rtsang is offline rtsang  Hong Kong
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Default Power dissipation?

Dropping 100V with 10mA means 1W.
Easer to get 3 33V 0.5W zener to do the job.
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Old 15th June 2009, 08:42 AM   #3
mjf is offline mjf  Austria
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hello.
yes the heat dissipation could be a reason,e. g. two 12v/1w zdiodes in series give 2w............and a 24v/1w gives 1w.
another reason is the dynamic (=inner) resistance of the diodes.....values around 8v or so have the lowest dynamic res and this increases with higher z - voltages...................have a look at the datasheets.
greetings...............
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Old 15th June 2009, 08:54 AM   #4
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Default and noise

never done the actual tests, but I understand that (for example) one 91v zener is noisier than a three device string of the same voltage
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Old 15th June 2009, 09:41 AM   #5
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Zener are definitively noisy!

I don´t know the your application, but try to use other parts...
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Old 15th June 2009, 12:56 PM   #6
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hey-Hey!!!,
The Ohms/Volt is also different. It is possible to get a lower impedance string if you build it from the value at the family minimum. Look at the data sheet for Fairchild's BZX55Z8V2( which happens to be the lowest of that group ).
cheers,
Douglas
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Old 15th June 2009, 01:14 PM   #7
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Zener diodes in the 4.7-5.6V range have a temperature coefficient near zero... a string of these will have very little voltage change as they warm up. If you use higher values, you can add a few forward biased ordinary diodes to the string to cancel the positive change with temperature. For example, an 8.2V Zener has about 4 mV / degC change with temperature. A forward biased diode is about -2 mV / degC. So two diodes plus the Zener will be about 9.6V and stable with temperature.
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Old 15th June 2009, 02:57 PM   #8
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Default Re: and noise

Quote:
Originally posted by aardvarkash10
never done the actual tests, but I understand that (for example) one 91v zener is noisier than a three device string of the same voltage

Yup;
Highish voltage zeners are avalanche breakdown types with increasing Zr with voltage and produce horrific fine grass noise and have to be driven fairly hard to reduce noise. With their lousy performance I'm not the only one who doesn't like using them.

Best solution is as others mentioned, string LV (true) zeners; or I use several TL431 in series; for cremé de la cremé performance.
This reggy ic has never let me down.

richy
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Old 15th June 2009, 09:46 PM   #9
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Quote:
Originally posted by Jean-Paul_2
Zener are definitively noisy!

I don´t know the your application, but try to use other parts...

Thanks for all the informative responses. Now that I have a better understanding, I'm wondering if my original idea still has merit.

Here's the deal. I have an old PP EL84 power amp that I like to fool around with. It has fixed bias and I want to ground the cathodes with a 1 ohm resistor and add an adjustable negative bias. The power transformer has unused 12.6 volt taps that I intend to bridge rectify and CRC filter to achieve a -13 volt bias. I want to adjust this bias between -13 and -10 volts. So, I thought I'd connect one end of a 400 ohm 10 turn pot and go to ground through a 10 volt zener, or perhaps a pair of 5 volt zeners as suggested. Actually there will be four of these pots in parallel. Of course the wiper goes to the grid as bias.

It still seems reasonable to me but I'm concerned about noise and a few data sheet specs I don't completely understand. The four parallel pots will act as 100 R and the voltage drop is 3 volt worst case so the zeners will need to accomodate 30ma of current. There is a current spec but I'm not sure if it is maximum allowable or an inherent current (like DCR or impedence).

This is getting over long but I think you folks can get the idea. I need to select an appropriate diode or find a better way if the noise issue is going to be a problem.

Thanks

CD
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Old 16th June 2009, 04:47 AM   #10
rtsang is offline rtsang  Hong Kong
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Quote:
Originally posted by Captn Dave



Thanks for all the informative responses. Now that I have a better understanding, I'm wondering if my original idea still has merit.

Here's the deal. I have an old PP EL84 power amp that I like to fool around with. It has fixed bias and I want to ground the cathodes with a 1 ohm resistor and add an adjustable negative bias. The power transformer has unused 12.6 volt taps that I intend to bridge rectify and CRC filter to achieve a -13 volt bias. I want to adjust this bias between -13 and -10 volts. So, I thought I'd connect one end of a 400 ohm 10 turn pot and go to ground through a 10 volt zener, or perhaps a pair of 5 volt zeners as suggested. Actually there will be four of these pots in parallel. Of course the wiper goes to the grid as bias.

It still seems reasonable to me but I'm concerned about noise and a few data sheet specs I don't completely understand. The four parallel pots will act as 100 R and the voltage drop is 3 volt worst case so the zeners will need to accomodate 30ma of current. There is a current spec but I'm not sure if it is maximum allowable or an inherent current (like DCR or impedence).

This is getting over long but I think you folks can get the idea. I need to select an appropriate diode or find a better way if the noise issue is going to be a problem.

Thanks

CD

Using zener may inject quite a bit of noise to the grid.
Maybe just get DC from an -15V regulator then create an voltage divider with the pot may be a better idea.
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