zener diode question

[R.Lewis]

Is there any practical difference between the various small (sub 1W) zeners
such as the BZX55,79, & 85's (plus others such as BZY's that I cannot fully
bring to mind at this moment)
All I need is 3v6 and 4v7, 5%, 100mW max, through hole, but neither passing
more than 1mA at 2.0 volts Vf.
..

 

[Robert Baer]

Is there any practical difference between the various small (sub 1W) zeners
such as the BZX55,79, & 85's (plus others such as BZY's that I cannot fully
bring to mind at this moment)
All I need is 3v6 and 4v7, 5%, 100mW max, through hole, but neither passing
more than 1mA at 2.0 volts Vf.
.

Firstly, ordinary zeners below 6V are crappy and get worse as the
voltage gets lower, and seems to get almost as bad as using an ordinary
diode in the forward region.
I would suggest that you use an LM285/LM385 adjustable voltage
reference.

 

[John Fields]

On Tue, 29 Jun 2004 23:17:31 +0100, "R.Lewis" <h.lewis-not this

 

[Joerg]

The LM385 is a good suggestion. In case it has to be rock bottom in cost and you can spare an opamp somewhere, the LMV431 (fixed) would be an alternative.

I can only corroborate what the other posters said about zeners and I replaced a lot of them with this reference after complaints about tolerances.

Regards, Joerg

 

[Joerg]

Correction: The LMV431 is also adjustable.

Regards, Joerg

 

[colin]

as people have said i think, low voltage ones have poor zener efect and even
poorer with low current. ie large slope resistance and variuation with
temperature and wide variation of its knee (ie point at wich zener curent
dominates over leakage curent)

but if its just a simple level shifter where u have a wide margin and dont
care too much about voltage then it can be simplest solution, but u need a
minimum curent to flow through it before you see the rated zener voltage.

i wouldnt count on geting close to 5% of its specified voltage with only
2volts or 1ma tho, u might be too close to its 'knee' to predict what
volt/curent you get.

Colin =^.^=