How to work with Zener diodes?

[NiGHTS]

I'm sure this is an easy problem to solve but so far I'm perplexed with how to use a Zener diode for the purpose of generating a reference voltage for a differential comparator when a certain voltage is not known or no other reliable voltage available.

Attached is an image showing the basic schematic I have been experimenting with. My only voltage source to work with is anywhere from 12V to 29V and I am aiming for a reference voltage of about 2.4V. My first experiment used a resistor value of 10K but it resulted in an output voltage well below 2.4V -- as low as 1.2V. Other experiments involved using a 1K resistor and yet even then I had an output of 2.2V at 13V. I found that the only way to achieve 2.4V was to use the maximum input voltage with a 1K resistor, but then the resistor overheated.

Given how low the current I need from this zener circuit is, I don't want to pass too much unnecessary current through the current limiting resistor. My required output voltage can range anywhere from 2.0V to 2.8V but must be willing to work with an input voltage of 12V to 29V. Is this possible? If so how can I achieve this?

This is the Zener diode I have been using.

 

[Arouse1973]

Hello
Your diode for requires 20 mA to produce 2.4 Volts, so with a voltage of 29 Volts this requires a resistor (29-2.4) / 20 mA = 1330 Ohms. The power dissipated in the resistor will be (20 mA^2) * 1330 = 0.532 W. At 12Volts this will be (12-2.4) / 20mA = 480 Ohms , and the wattage is (20 mA^2) * 480 = 0.192 W. But you are not going to want to keep changing the value of the resistor for varying voltages. So you would use the lower voltage of 12 Volts and this means at 29 Volts, you have (29-2.4) / 480 = 55.41 mA and a wattage in the resistor of 1.47 W. Umm do you think you might be wasting a bit of energy there, just to get 2.4 Volts. Oh and that's not allowing for any overhead for the zener, you may need another 5 mA of current just to make sure the zener is regulating if the 12 Volt supply drops a bit. If you can accept 2.5 Volts then why not have a look at precision references instead.
Adam

 

[Colin Mitchell]

"Your diode requires 20 mA to produce 2.4 Volts, "
That is not true AT ALL.
The minimum current to keep a zener in breakdown depends on the wattage of the zener. A 400mW zener requires much less current than a 2 watt zener.
The minimum current through a zener needs to be about 5mA for a 400mW zener so that it creates the zener voltage specified on the device.
A 2 watt, 5 watt or 10 watt will require 20mA or more to maintain its zener voltage.

 

[Arouse1973]

"Your diode requires 20 mA to produce 2.4 Volts, "
That is not true AT ALL.
The minimum current to keep a zener in breakdown depends on the wattage of the zener. A 400mW zener requires much less current than a 2 watt zener.
The minimum current through a zener needs to be about 5mA for a 400mW zener so that it creates the zener voltage specified on the device.
A 2 watt, 5 watt or 10 watt will require 20mA or more to maintain its zener voltage.

At low zener voltages, field emission have an effect and these Zeners are much more sensitive to changes in current versus VZ. 2.4 Volt Zeners operate in the soft knee area, where their dynamic impedance is much higher. They need a higher current to regulate correctly. Looking at the datasheet for the above device the manufactures have tested the diode at 20 mA which is where they guarantee the voltage to be approx. 2.4 volts.

It's the VZ that is important and not the max wattage of the device. Although I agree the higher wattage device with the same VZ as a lower wattage device will require more current to raise to the required test temperature to say 30 degC. For example a 75V 3W zener will regulate 75 Volts at 5 mA. Generally only at a certain junction temperature will they specify the zener voltage. Although having a quick look at the datasheet it does appear the graph shows them operating at a much lower current. But I don't always trust the graphs. The graphs don't explain why the OP needs such a large current to get the zener to work correctly does it?

You can't make the general statement that all 3W Zeners need more current that 400 mW Zeners.

Thanks
Adam

 

[Fish4Fun]

A lot depends on what you actually want to use the circuit for...ignoring the actual question for a moment, and assuming what is required is a 2.4V reference and that Vin is really variable from 12V to 29V...it would likely be easiest to simply use a voltage regulator to get a nice stable 5V or 3.3V rail, and then use the stable low Voltage rail to generate your 2.4V reference....might even consider something easy like a 2.4 Vref IC ..;-) You could also use a pair of Zeners...the first limiting the voltage to some stable value below 12V, then using the second to provide your 2.4V ref. Or, perhaps you might just use the zener to limit the voltage to something below 12V and then use a simple voltage divider.....It is much easier to get a stable reference voltage from a stable input.....At least in my world it seems to be.

Good Luck!

Fish