# zener diodes

• posted

hi, i need a 7.5 volt zener diode, , of course which i havent got, but i do have a 5.1 volt and a 2.4 volt zener,

can i just connect em up in series or do i parralell then up? thanks. mark k

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• posted

series will work fine.

make sure that they can handle the power dissipation of course.

• posted

"mark krawczuk"

** Go and buy a 7.5 volt one.

That series combination will not be equivalent to a single zener.

............. Phil

• posted

Why is that Phil?

Cheers Terry

• posted

Yep. Join them Anode to Cathode, i.e.

-------+---- Cathode of 7V5 zener | | ---/ / \ --- | | ---/ / \ --- | |

-------+---- Anode of 7V5 zener

The 2V4 zener will dissipate 2.4/(2.4 + 5.1) = 32% of the total power, the 5V1 zener will dissipate the remaining 5.1/(2.4 + 5.1) = 68% of the total power, which is of course (2.4 + 5.1)*Izener where Izener is the current flowing through the series-connected devices.

Cheers Terry

• posted

"Terry Given" = another sheep shagger

Greeks Bearing Gifts are no competition.

.......... Phil

• posted

"Terry Given" Phil Allison

** Go look up the Zz figures for a 2.4, 5.1 and then a 7.5 volt zener.

The BZX79C series for example.

Then come back and correct your other post.

........... Phil

• posted

The point Phil makes is that the dynamic zener resistance is different. In the case of a BZX79C zener, a 7V5 is 30-80R, 2V4 is 275-600R and 5V1 is 400-480R at 1mA. At 5mA the numbers are 6-15R, 70-100R and 40-60R respectively. So a 7V5 "compound" zener comprising 2V4 + 5V1 will have a

*different* dynamic resistance. Note that Rz is all over the place like a mad womans shit.

I cant be bothered drawing a plot of Rz vs Vz, but you can clearly see from the datasheet it has a minima around 7V5. higher voltage zeners (really avalanche breakdown diodes) have high Rz, as do low voltage zeners. Vishays BZX85 datasheet gives a much better picture of the crazy behaviour of Rz:

Increased Rz means that the *measured* zener voltage will be higher at a given current. for the numbers given, at 1mA it'll be 645mV - 1000mV higher; at 5mA it will be 520mV - 725mV higher . This can usually be compensated for by fiddling with the current source (or, god forbid, pullup resistor). Note that the change from 1mA to 5mA changes the error due to Rz by about 30%, and is why zeners should be driven with a constant current source - if, of course, you want a constant voltage.

The tempco Sz is also very different. low voltage zeners (actual zeners) have -ve tempcos, high voltage zeners (really avalanche breakdown diodes) have (mostly) positive tempcos. And the tempco itself is a (strong, for true zeners) function of zener current too.

So, if you have any stringent requirements of Vz, its pretty important to sort out the effect of Rz and Sz, and to use a current source to bias the zener to reduce the effects of bias voltage variation.

OTOH for a bench hobbyist cobbling together parts, who cares, bung them together and adjust Iz to get the "right" voltage.

Cheers Terry

• posted

"Terry Given" = one pissweak, Kiwi, bullshit artist

** Zz is an important zener parameter - a fact this mad Kiwi now seeks to conceal.

At 5 mA, a single 7.5 volt is around 10 ohms while the 2.4 + 5.1 combo is a massive 130 ohms.

In any circuit where the zener must act as a *voltage regulator* or a voltage clamp, one 7.5 volt zener is going to do that job VERY much better than a 2.4 + 5.1 combo - about 13 times better regulation. If there is supply ripple voltage on the rail feeding bias current to the zener - there will be 13 times less of it appearing across the zener when a single

7.5 volt one is used.

If the zener is subjected to bursts of current of say 100mA - a 7.5 volt one will rise by less than 1 volt * while * the 2.4 +5.1 combo will rise by about 10 volts !!

" ** Go and buy a 7.5 volt one.

That series combination will not be equivalent to a single zener. "

** Lord forgive the prick for he knoweth not of what he speaks.

.............. Phil

• posted

I hate to stick my finger in the story........... it might get burnt

but just were does this 7.5 volt zener diode go? what is the power rating of the diode? if one of the series diode burnt out, would it cause a whole heap of damage to the circuit, or just cause the circuit to malfunction?

why don't you either: As Phil said "go and buy the correct part", and keep the other diodes for another day.

or just connect the two diodes you already have in series and see how lucky you are.

But goodness me if it is only a 400mW zener it will only cost you 50 cents or so, is the too much?

can i just connect em up in series or do i parralell then up? thanks. mark k

• posted

"Eric"

** Asbestos gloves are handy.......
** Somewhere in an electronic circuit .....
** Almost certainly 400mW or less - 2.4 volt zeners are only made in low powers.
** Err - non relevant to the issues.
** OK - but Phil had a DAMN technical good reason for so saying.
** There is no limit to the **cheapness** some folk long for.

( apologies to Frank Zappa. )

** Ever read Oliver Twist ??

............ Phil

• posted

** Almost certainly 400mW or less - 2.4 volt zeners are only made in low powers.

The aim of the posting was to find the power rating of the 7.5 volt zener diode, after all that is the one the circuit calls for.

and I still think that this is important "if one of the series diode burnt out, would it cause a whole heap of damage to the circuit, or just cause the circuit to malfunction?"

no use saving 50 cents if you wreck a HiFi sound system worth \$xxxx, because it is used in the power supply.

"Eric"

** Asbestos gloves are handy.......
** Somewhere in an electronic circuit .....
** Almost certainly 400mW or less - 2.4 volt zeners are only made in low powers.
** Err - non relevant to the issues.
** OK - but Phil had a DAMN technical good reason for so saying.
** There is no limit to the **cheapness** some folk long for.

( apologies to Frank Zappa. )

** Ever read Oliver Twist ??

............ Phil

• posted

hey, well let me think.....hmmmmmmmmm well it could be a time factor invulved, could be that mail order companies have a minimum \$10 or so order, or could be that i`m 250 km from any decent electronics shop.

or have u ever thought that i just want to do it that way.

do u always get the "correct " part ? so u never make do with wot u got??

mark

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• posted

"Eric"

** And I answered that question.

** Well, the OP is a total moron and cannot answer that.
** But you are an even bigger one - plus a PITA, stinking top poster.

........... Phil

• posted

Hay Mark I just had a idea, ask Phil nicely if you could post him a \$1 or 2 and a self addressed envelope, and maybe you could post him a Christmas card at the end of the year.

Eric

or have u ever thought that i just want to do it that way.

do u always get the "correct " part ? so u never make do with wot u got??

mark

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