Electrical Schematic of the Zener Diode Tester  

The Idz current is adjusted with resistor R1. Infineon provides a very comprehensive Application Note on the operation of Depletion MOSFETs as constant current sources. Since it's an old article that is difficult to find, I have included it .

          Application Note title: Depletion MOSFETs.

          Author: Pradeep Kumar Tamma. 

          Document reference: AN_201410_PL11_003. 

          Revision 1.0, 2015-02-03. 

Here is the relationship between    

                                                             R1 , 

                                                              Id        :  the desired current , 

                                                             Vgs(th) :  the gate threshold voltage of the MOSFET and

                                                             Idss      :  the on-state current at VGS = 0 V .

According to the DN2540N5-G datasheet, VGS(th) ranges from 1.5 V to 3.5 V. Тhe minimum Idss value is 0.15 A, and the maximum value is not specified.

The table below shows the calculated values of R1 for specific values of VGS(th), with Idss and Id held constant.

I have included ODS and XLSX files which contain the implemented formula. You can view the results and perform calculations with different values there.

Practice, however, has shown that instead of calculating the resistor, the easiest way is to place a potentiometer in place of R1 and use it to adjust the current. That's what I did. Afterwards, I replaced the potentiometer with a fixed resistor of an appropriate value. The calculations gave me a guideline for the potentiometer's value.

Below are some photos :

I made a jig for easily...