How To Build A Cheap, Simple And Yet A Powerful Zener Diode Tester

 For those who already own the Testing Electronic Components Ebook you would have known the way on how to properly test a zener diode. By using an analogue meter to place across a zener diode with appropriate range setting you will know if the zener diode is good or bad. However there are few problems arise when test on a zener diode. Problems like missing coding (voltage rating) of a zener diode. How do we know exactly what is the voltage of the zener diode? How do we test on an intermittent problem in zener diode? Another question, how do we test on a zener diode that can breakdown when under full load? The answer for all these questions can be found with the help of a Zener diode tester. Usually you can’t get any zener diode tester from the market and the only choice you have is to build one for yourself. The zener diode tester is cheap, versatile and very easy to build. All of the components used can be easily obtained from any local electronic shops. Here are the parts that you need: 

1)    LM317T IC2)    220 ohm ¼ watt resistor3)    1 Kilo Ohm 1/8 or ¼ watt resistor4)    LED with the holder5)    On/Off switch6)    4 pieces of 1N4007 diode7)    1000uf 50 VDC electrolytic capacitor8)    A 30 VDC analogue panel meter9)    A pair of Test probe10) A 24 VAC Output linear transformer with the rating of 1 Ampere11) 2 pieces of Grommets12) A plastic Box (Casing)13) Some wires14) Power cable15) Three pin plug

 This zener diode tester can be use to test zener diode voltage rating from 2.4 Volt to 30 Volts. The LM317T IC is used to supply a definite amount of

current to the zener diode under test so that it would not damage the component and the DC output from the power supply is about 32 Volts. 

  

Here are the photos of the project 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

Here are the test results of different zener diode voltage rating: 

  

 

 

 

   

The advantages of the Zener diode tester are: 1) Part number missing or no part number printed on its body- By using this meter, we could quickly identify the zener diode voltage as shown in the photos below. If the zener diode is shorted/burnt we can’t find out the value unless we have the equipment’s schematic diagram. 2) Avoid Confusion In Reading Zener Diode Part Number- Some zener diode part number printed on its body could cause confusion to the electronic repairers. I have email that asked me whether the zener diode should read 8.2 Volt or 2.8 Volt. With the help of the tester, you can easily know the exact voltage 2) Find out intermittent problem- All test have to be done off board or one leg out from the circuit board. Carefully look at the scale pointer/needle and it there is any erratic movement then we know that the zener diode have intermittent problem. 3) Zener diode breakdown when under full load- Our normal multimeter don’t have the capability to test zener diode that breakdown when under full operating voltage. The maximum output voltage from our analogue meter is only 12 Volt but this zener diode tester has output voltage of about 32 Volt! If you have confirmed that a particular circuit have problem and could not locate the bad component, I guess is the time for you to use this tester and check on the zener diode voltage. A zener diode that breakdown when under

load will have two zero volt reading while for a good one the pointer will points to the voltage of the zener diode like 18 volt, 22 volts and etc. 4) Short circuit test- A shorted zener diode will registered zero volts readings tested either way with the test probes. 5) Open circuit- If the pointer remains at the 30 Volts range, this could indicate the zener diode has an open circuit or the zener diode under test have voltage rating that is higher than 30 Volts. If the part number printed on the body of zener diode is 18 Volts and the pointer still showing 30 Volts then we can presume that the zener diode under test have an open circuit. Note: Zener diode has tolerance and it can vary from 5 to 20%. That’s why sometimes when you test on a good 15 Volt zener diode, the result that you get could be in the 14.5 to 14.9 Volt range. Here is the powerful part. Besides for the Zener Diode tester to test on zener diode, this tester actually can be use to test on other components as well. Below are the list and explanations of components that can be tested with the Zener diode tester. 1) Battery Voltage-You can use it to test on any batteries whether it is a dry cell or UPS battery. Place the red probe across the positive side of the battery and the black probe to the negative and read from the scale. It can test battery voltage from few volts up to 30 Volts. 

 2) Continuity Test- Yes, you can use this tester to check on a broken cable, fuse, on/off switch, solenoid winding, relay coil and etc. If the pointer moved to zero volts you know that the component is good. 

3) Voltage regulator- Suspect that a 5 volt voltage regulator IC have problem? Don’t need to worry about buying an expensive power supply to test out the voltage regulator. Just place the red probe to pin 1 and black probe to pin 2 of the voltage regulator and check the output voltage (pin 3) with your digital meter. It should read about 5 volts. You can test the voltage regulator up to 24 Volt with this method. 

 4) Testing LED- Some LEDs that need 4 volts to run (like the Blue ESR meter LED) can’t be tested with an analogue meter because the meter can’t produce such a voltage under X 1 Ohm (max only 3 volts). Using the zener diode tester solved the problem. Not only that, you can also use it to test on the normal LEDs that need lesser volt to light up.

 5) Testing Diode- You will be amazed that this tester also can be use to test diode, Schottky Diode and Bridge Rectifier. The good component should show only one reading and if you get two zero volts reading, then the diode is considered shorted. 

 6) Testing Small Non Polarity Capacitor- Checking only the capacitance value and the ESR of a small non polarity capacitor like the .1 uf (typically the 50 volt rating) does not really guarantee they are good. You need to perform the insulation test as well. Some of the small capacitors could breakdown when under load and this Zener Diode tester can do the job for

you. The meter’s scale should not show any zero volt reading when test on a small non polarity capacitor. If you get reading then the capacitor is considered breaking down when under load or it already has short circuit internally. 

 Conclusion- If you are creative enough in fact you can use this tester to test on other components (from your own R& D), install a digital panel meter (for accuracy) instead of the analogue panel, use a smaller and a better casing, modify the circuit to use batteries to power up the circuit instead of from the Main AC and many more. There is no reason as why you don’t want to build one for technical use and lastly the overall cost of building this Zener diode tester was only between USD20.00 to USD25.00. See you again in the next issue of the ERG electronic repair newsletter. Take care and have a brilliant day!