# Low Zener Voltage?

Discussion in 'Electronic Basics' started by eromlignod, Jul 29, 2007.

1. ### eromlignodGuest

Hi Guys:

This is a simple one. I've got a 0 to 5V square wave that I want to
convert to a 0 to 3.3V square wave. I tried putting a series resistor
and a 3.3V zener diode to ground.

The wave stays nice and square, but it comes out as 0 to 2.0 volts. I
have tried varying the value of the resistor, but it doesn't help.
What am I doing wrong? Is there a better way to do what I'm trying to
do?

Thanks for any replies.

Don

2. ### John PopelishGuest

A better answer depends on several things. What is the
frequency of the square wave? How fast must the rise and
fall times be?

What is the source of the 5 volt wave? How much load can it
tolerate before sagging too much?

What load will be places on the 3.3 volt output?

Some possibilities:

2 resistor divider.

Resistor and green LED divider.

3.3 volt linear regulator, e.g.:
http://www.zetex.com/3.0/pdf/ZMR330.pdf

Resistor and 3.3 volt shunt regulator.

3. ### JamieGuest

output source down to much? or maybe you have a capacitance problem?

You should first verify that the zener is operating correctly.
connect it to a supply via a resistor that places the zener in it's
normal operating range of current. Then perform a Voltage test across
it.

4. ### Phil AllisonGuest

"eromlignod"

** You did not READ the spec for your 3.3 volt zener.

Eg:

The 1N746A is a 400mW, *nominal* 3.3 volt zener, with rated test current of
20mA !!

The 1N4728 is a 1 watt, *nominal* 3.3 volt zener with rated tests current
of 76mA !!!!!

At low current of say 1 mA , the conduction voltage of either type may well
be only 2.0 volts.

Low voltage zeners are *darn soggy * and vague voltage references.

Change the zener to one with a higher * nominal* rated zener voltage, till
you get the 3.3 volts you want.

........ Phil

5. ### EeyoreGuest

Zeners are very 'sloppy' at regulating voltages. You're putting too little
current through the zener.

An 'active clamp' is perhaps the best answer (or a dedicated buffer for voltage
translation) but do you need to implement this with very few parts ?

Graham

6. ### EeyoreGuest

Trust you not to have a clue.

Graham

7. ### neon

1,325
0
Oct 21, 2006
no there are ot bad zeners they jus do not exsists for your application. there are some ref diodes around 2.5 but they are not zeners as diodes. what you want requires an active device