Zener diode tester module


Diodes whose reverse-breakdown characteristics are controlled precisely during manufacture are called as Zener diodes and are intended specifically for operation in the reverse breakdown mode primarily as inexpensive voltage references. An uncomplicated circuit application of a ZD is the common shunt voltage regulator. If you have a Zener diode (ZD) of unknown characteristics or if you want to use reverse-biased transistor junctions as ZD, the zener voltage and performance can be tested with this simple circuit. This battery operated compact circuit can be made in a small plastic case with two crocodile clip leads as test probes. The built-in Digital DC Voltmeter (DVM) module obviates the requirement of an extra voltmeter/multimeter. The DVM reads the voltage drop across the device under test. This circuit is suitable for testing of ZD up to 30V rating, if test current is tailored accordingly.

Zener Diode Test Procedure

  • Put S1 in ON position
  • Identify the test leads and connect the ZD to its proper terminals
  • Push S2 and read the DVM. If the readout is > 30V probably the ZD under test (ZUT) is open/have a higher value, and if it is < 1V the ZD is short. Otherwise you can observe a good value (for example 5VDC for a 5V zener)

Lab Note

  • This circuit can also be used to check forward voltage drop across diodes and LEDs at a pre-setted value of current. Here the maximum test current is near 12 mA with the 2.7K ballast resistor (R3)
  • Test current through the device under test can be set to a different value by changing the value of the ballast resistor (R3). Recommended value of test current range is 5 to 15mA ( Vin-Vz/Iz )
  • Concerning safety of the device under test, it is recommended to keep the tryout time as short as possible
  • Keep it in mind that this is an experimental circuit , not the full-fledged design of a diy test instrument!

Schematic of the Zener Tester Circuit

Zener diode tester circuit

Parts List

  • U1: NE555N IC
  • Q1: P55NF0 Power Mosfet
  • D1, D3: 1N5819 Schottky Diode ( 1A/40V)
  • D2: 1N4752 Zener Diode ( 33V/1W)
  • L1: 10µH Inductor (200mA)
  • LED1: 5mm Red
  • R1, R4: 1K ¼ w
  • R2: 10K ¼ w
  • R3: 2K7 ¼ w (see text)
  • C1: 1nF
  • C2: 10nF
  • C3: 100µF/63V
  • C4: 470µF/25V
  • DVM: 3-Wire Digital Voltmeter Module (DVM) / available at eBay!
  • S1: SPST Toggle Switch (N/O)
  • S2: SPST Push Switch (N/O)
  • BATT: 6V (1.5V x 4)
Zener tester prototype

zener tester prototype

The zener tester circuit is a simple dc-dc boost converter designed around the most common IC – the 555 chip. The 555 IC (U1) is wired as an astable oscillator at a frequency (around 68KHz) determined by RC components (R1, R2 and C1). U1′s output directly drives a power mosfet (Q1) that switches the current through power inductor (L1). Diode D1 and capacitor C3 forms the output rectifier and filter circuit. If possible, try to replace D1 with a UF4001 fast diode for better efficiency. Zener diode D2 sets the output voltage to near 33V, and resistor R3 is the ballast resistor for limiting test current through the device under test.


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