From: Roger Johansson
Subject: Re: In search of special temperature sensitive Zener
Date: Mon, 13 Jan 2003 04:35:37 +0100
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>> - The zener diode looks like a 1/8 w dark yellow axial with clear band in
>> - The operating voltage is about 3v.
>> - The voltage decreases at about 40mV/C
>> - Using a diode checker (with unit powered off) showed 0.6 v drop in one
>> direction only, high resistance is other direction.
You interpret the result incorrectly, it is not high resistance you
read when using the diode check function, it is high voltage.
Let me guess, you have a DMM that goes to 1.999 and you got 1.999Volt
in the other direction.
That means that the zener voltage is higher than 1.999Volt.
The 0.6 Volt value in the first direction is just as it should be.
So everything is normal. That zener should show 3Volt or so, but the
diode check function on your DMM can only show values up to 1.999Volt.
To check it correctly you need to send a certain current through it
and measure the voltage over it, which is the same test as your DMM is
doing when it checks diodes, but now you need to measure higher
voltage, so you need to build a little external circuit instead of
using the built in function in your DMM.
Use an external power supply and a pot to change the current from 1mA
upwards, but not higher than what that zener can take, and measure the
voltage over it to get a curve of current to voltage values.
Set the DMM for DC voltage measurement instead of diode check this
time, and use a scale which allows you to measure 3 Volt.
The maximum current is determined by taking the wattage rating of your
zener divided by the voltage. 100mW/3=33mA, for example.
What resistor values you use is depending on what voltage you use.
If you use a 9 Volt battery to get a maximum of 33mA through a 3 Volt
zener there will be 6 Volt over the resistor and 3 Volt over the
You need a fixed resistor of 6/0.033=180Ohm
Use higher resistances to get lower currents.