Zener diode accuracy with low bias current.

[eem2am]

Hello,

If a 12V zener only has 130uA flowing in it, will it maintain an accurate 12V at its cathode.?
(the datasheet malheuresement does not tell this)

If not, how much current does a zener need in order that it can give its rated zener voltage?

Here is a schematic of it:-
https://i52.tinypic.com/35k67tx.jpg

Here is the BZX84C12 Zener datasheet
**broken link removed**

 

[LaszloF]

Hi,

Very interesting topic, here is my response, probably will maintain its zener voltage but the zener diode can be modeled as a voltage source in series with a resistor this is the differential resistance of the zener, this increases sharply when the zener current decreases below a critical value, this degrading the shunt stabilizer performance. For low power zeners i usually bias them with a minimum if 1mA in case of lowest input voltage and highest output current, a rule of thumb however states that the bias current should be 5-10% of max Iz, however this would give you 50% of the max dissipated power, if you bias with 10% of the max. dissipated power 2mA will be fine.
If you want the get into the details here's an interesting white paper: **broken link removed**

Summary: 130uA is to small, use 2mA.

 

[Audioguru]

The zener diode is near 12V with a current of 5mA. It is near 8V with a current of only 100uA.
So with your current of 130uA then it might be around 8.5V.

A 1N4699 is near 12V with a current of only 50uA. It will still be close to 12V with your current of 130uA.

 

[biff44]

zener diodes do not output a fixed voltage, as a voltage regulator IC would. They output a voltage, in your case around 12 volts, but it can be considerably higher or lower than exactly 12 volts. The actual voltage depends on: the dc current flowing thru the diode, the temperature, and manufacturing tolerances from diode to diode.

You could probably find a zener diode that puts out 12 volts at 100 uA, but it might not be labeled "12 volt zener". You would have to read the data sheet and find a Vz versus current graph.

 

[Audioguru]

Zener diodes with a voltage of 4.7V to 6.2V are temperature-stable.
Zener diodes with a voltage of about 7V have good voltage regulation.
Zener diodes with a voltage below 4V have a negative temperature coefficient and zener diodes with a voltage above 7V have a positive temperature coefficient.