Zener diode operating current

[rjm]

How much current should go through the Zener diode to make sure it functions properly as the voltage reference element in a regulator circuit?

Let's say I have a 12 V 500 mW type, trying to regulate a nearly constant <1 mA load. The datasheet specifies for 5 mA, but I can't see that I really need to go that high given the low load currents involved. No plots of, eg. impedance vs. current are provided.

[Chris Daly]

Assuming a 15v supply, then a 12v Zener with desired load of 1 ma will require 270 ohms of resistance.

See: Zener Diode Calculator

Cheers / Chris

[RJM1]

Manufactures state the voltage regulation of zener diodes at a specific current level. If the current through the zener is less than recommended current the voltage across the zener will be less than its rating. Also, as the current through the zener decreases its noise and dynamic impedance will increase.
Chris, your zener calculator is calculating for 10ma through the zener. This would be fine if the zener current was rated at 10ma, but in this case it is 5ma. You would need a 15V supply with a 500 ohm resistor in series with the zener to put 5ma through the zener and 1ma into the load.

[rjm]

Hi guys,

As noted the calculator above sets the Zener current at 10 mA. The Zener I will use nominally lists 5 mA in the datasheet, but this is not the "rated" value per se, the diodes are rated for 500 mW so for the 12 V model it will pass up to 40 mA if necessary.

I also appreciate that impedance and other properties of the Zener improve with current draw, but there must be a "reasonable" best practice, diminishing returns and all, which I suppose is about 5 mA. What I'm asking is whether, for very light loads, I can go down to 1 mA or so without losing much.

[Mooly]

Why not measure it for real at various currents. Then you have an absolute answer

[djQUAN]

Try this:

AN008 - How to Use Zener Diodes

[AndrewT]

plot the Zener voltage vs the Zener current.
You will get a very curved plot.

At low current the Zener voltage changes rapidly with very small changes in current.

At high current the Zener voltage changes very little for changes in current.

If you require the Zener to operate as a shunt regulator then usually somewhere 5% and 10% of Pmax is required to get on to the start of the slow slope indicating a low dynamic impedance.

I adopt Iz >10% Imax to save looking at the datasheets for every individual Zener and Manufacturer.

For 12V 500mW Zener I would use >4mA through the Zener. Load current is in addition to that.

You can use a CCS as the current setting device and fine tuning the CCS to get the reference voltage you require. This only works when the load draws constant current.

[rjm]

Originally Posted by AndrewT

If you require the Zener to operate as a shunt regulator then usually somewhere 5% and 10% of Pmax is required to get on to the start of the slow slope indicating a low dynamic impedance.

That was the answer I was looking for. Thanks very much.