I bought myself a bench power supply (Matrix MPS-3206) and I decided to sort out my Zener diodes 3.3-24V.
I set the power supply to 28.0V @ 10mA and proceeded to check them out in ascending order.

Up to 12V I get a stable value but at 15V and above the values drift up by about 0.5-0.7V over about 20 seconds then stabilize. If I increase the current limiting to 20mA the values are higher and if I reduce to 5mA they are lower. For an 18V nominal Zener stable voltages are :-

Supply
24V @ 10mA = 18.90V
24v@ 5mA = 18.41V
24V @ 1mA = 18.05V (almost no drifting)

Are my testing parameters wrong ?

 

The zener characteristic does have some slope to it, and it is also affected by the junction temperature. This is hardly a surprising development.

 

just compute how much power each diode is consume. this affects joint temperature. you keep current constant but increase voltage. so Pd and temperature are increasing. this is why in many circuits to get more stable result current through Zener is limited and there is an opamp or transistors after the zener as a follower.

 

it is not a good idea to test zeners directly to a PSU, even if current limit is relatively low. Almost all PSU's have a big capacitor at output terminals. ( In past I did used Hyelec HY3005, a 470uF x 100V at the output...). So, if your zener is , say, 8 volt, a huge current impulse will stress it, until the capacitor is discharged from 28V to 8V and PSU's goes to the current limiting mode.
Simply add a resistor of 50-100 Ohm in series to limit the transient current

 

Look at the temperature coefficient of the Zeners you are testing, and calculate the power being dissipated to determine the junction temperature from the devices thermal resistance to air.
You will then see the cause of you observation.
The diodes are checked at the factory with a short pulse to eliminate that problem (also likely noted in the data sheet).

If you want a more stable voltage, then you can use a programmable shunt reference, such as a TL431, which acts like a Zener, but its voltage is programmable, and only slightly affected by temperature or current change.

 

Datasheets for zener diodes show that they are not perfect voltage regulators:

 

What series resistor are you using, supposedly you are using a series resistor?

 

What series resistor are you using, supposedly you are using a series resistor?

He apparently is using the power supply current limit in place of a series resistor.

 

He apparently is using the power supply current limit in place of a series resistor.

Ok. Zeners do not have sharp knee voltages. You can expect the voltage to be higher at higher current.

 

He apparently is using the power supply current limit in place of a series resistor.

In addition, we do not know precisely what the supply is doing when it is in the vicinity of the current limit. It might linearly reduce the output voltage, or it might implement a bang-bang control where the supply alternates between 0V and 24V to keep the average current at whatever limit is being set. I can see these two alternatives giving different and possibly misleading results.

IMHO setting the zener current with a fixed resistor is likely to produce more satisfactory results.

You should check the datasheet for the zeners to make sure you are not exceeding an absolute maximum reverse voltage limit.

 

The bench power supply values are pre-programmed before energizing the circuit. In all cases the power supply indicated "CC" while measuring.

Bearing in mind your comments I "lashed up" a battery based tester limited by a 1K resistor. The values I got were stable on all the Zener I re-tested.

I guess I used an inappropriate testing protocol, I wish I had known about the TL431 before but my field was aqueous polymers (before I retired) so I have learned a little more for my hobby.

Thank you all for your help

 

To summarize, this is the proper way to test a zener diode.



Adjust Vin and R to give the desired zener test current.

 

The bench power supply values are pre-programmed before energizing the circuit. In all cases the power supply indicated "CC" while measuring.

Bearing in mind your comments I "lashed up" a battery based tester limited by a 1K resistor. The values I got were stable on all the Zener I re-tested.

I guess I used an inappropriate testing protocol, I wish I had known about the TL431 before but my field was aqueous polymers (before I retired) so I have learned a little more for my hobby.

Thank you all for your help

It is often the case that different people see different things in the same story. Even if some suppositions are wrong or even off the mark, articulating them raise questions which can usually be quickly answered. Glad things worked out for you.

 

Are my testing parameters wrong ?

You're not testing properly.

Fairchild specifies lead temperature of 30C with junction in thermal equilibrium. Iz isn't 10mA for any diodes in the range of 2.4-56V.

 

Crusty, every zener diode has a part number and a certain test current. Post #14 uses 1N52xx zener diodes where most have a test current of 20mA.
I always use zener diodes that have a test current of 5mA.