# Zener diode test current, Izt help

Discussion in 'General Electronics Chat' started by MbTaNk16, May 11, 2008.

1. ### MbTaNk16New Member

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I'm a beginner at electronics and I wanted to buy a 5.1V zener diode with a Pd of 500mW. I've noticed that the test current (Izt) of these diodes are either 50uA, 5mA, or 20mA. I have two questions:

1) What exactly is zener test current?

2) What's the significance of this and how would it affect an electronic circuit?

2. ### RODALCOWell-Known Member

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5V1 Zener

Most zener diodes 400mW rating, work properly at the rated zenervoltage at a 5 to 10 mA current flow through the zener.

Say you have a 12 volts supply, have the series resistor to allowe a 10 mA currentflow through the zener.

For a 5.1 volt zener 6.9 volts has to be dissipated in a resistor.
From U=I*R 6.9=0.01*R therefore R = 690 ohms.

A 680 ohms E12 ¼ Watt resistor will suffice.

3. ### RODALCOWell-Known Member

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You can use the zener voltage, as a fixed reference voltage, which will remain reasonably stable with power supply voltages ranging from about 8 to 18 Volts in the 12 volts example.

Bear in mind not to exceed the zener power rating and try to keep the zener current within 5 - 15 mA range.

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5. ### eblc1388Active Member

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The voltage across a zener diode is not constant but depends on the current flowing in the diode.

This effect is especially noticable in those low voltage zeners of a few volts. For example, a 3.6V zener in my component box have a breakdown voltage of only 2.95V at less than 1mA current, 3.3V at ~3mA and 3.6V at about 5mA or more.

Therefore to specify a zener diode, the manufacturer will typically use a certain constant current to test them.

For an user, if you want the zener to give you a fairly stable and accurate voltage, you have to supply the zener with a stable current.

6. ### MbTaNk16New Member

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Understood

Thanks guys that cleared everything up! Thanks for the fast responses too!

7. ### audioguruWell-Known MemberMost Helpful Member

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A zener diode that has a test voltage at a low current has a much higher voltage at a higher current.
A zener diode with a high test current might not regulate the voltage at a low current.

EDITED

Last edited: May 12, 2008
8. ### icemonsterNew Member

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Hi all,
Do u know about zener ?? Zener has internal resistance and other action,so could u tell me all bout this,please?? I'm welling to know, who could answer to me? And i hope so,thanks!!

9. ### MikeMlWell-Known MemberMost Helpful Member

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Last edited: Aug 17, 2010
10. ### icemonsterNew Member

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Hello Mike', Thanks for helpin'!!! *