DIY LED and Zener diode tester

The device presented here is a zener diode for testing the voltage value of an unknown zener diode. A zener diode is an electronic component that maintains a constant voltage across its contacts, and the source voltage Vs must be greater than the zener diode's own voltage Vz , and the current is limited by a resistance Rs , so that its current value was always less than its maximum power.

Diagram of the simplest method for checking zener diode voltage

Radio amateurs and all those who are good friends with electronics know that the task of finding a zener diode with the required characteristics (operating voltage) is boring and painstaking. It happens that you need to go through a lot of different instances until you find the desired Vz value. Checking the status of the zener diode is usually done using a regular multimeter diode scale, this test gives us an accurate idea of the condition of the component, but does not allow us to determine the Vz value. In general, a zener diode tester is a really convenient device when we want to quickly find out the value of the voltage Vz.

Zener diode tester

It only takes a few hours to make this device. It is designed to check the serviceability, determine the pinout and stabilization voltage of zener diodes. But it can also be used to test other semiconductor devices, for example, to determine the breakdown voltage of the emitter junction of a transistor, which are sometimes used as zener diodes. During the test, the task was not to determine the dependence of the stabilization voltage on the flowing current.

The device diagram is shown in Fig. 1. It consists of a step-up voltage converter assembled on a DD1 chip and a VT1 transistor, as well as a specialized module F08508-G. On the Internet, this module (Fig. 2) is positioned as a car battery tester and is a three-digit voltage meter with a digital LED indicator. It allows you to measure DC voltage up to 99.9 V.

A pulse generator is assembled on logic elements DD1.1 - DD1.3, element DD1.4 is a buffer one. The frequency is set by the parameters of elements C2 and R1, and for those indicated in the diagram it is approximately 9 kHz. Pulses from its output through resistor R2 arrive at the base of transistor VT1, which operates in switch mode. When it is open, current flows through the inductor and energy accumulates in its magnetic field.

When the transistor closes, a self-inductive emf appears on the collector and a voltage pulse with an amplitude of about 60 V is formed, which is then rectified by diode VD1, and capacitor C3 is charged to this voltage. Through current-limiting resistor R3, this voltage is supplied to the zener diode under test and to the module input. Using switch SA2, the polarity of the voltage on the zener diode is changed, but not at the input of the module. By taking readings from the module indicator, you can determine the stabilization voltage and pinout of the zener diode. It should be taken into account that if the zener diode is ordinary, it includes one pn junction (VD1 in Fig. 3). Therefore, with a voltage of reverse polarity (plus - to the cathode, minus - to the anode), the breakdown voltage will be indicated; for a zener diode this is the stabilization voltage. When the polarity is changed, there will be a forward voltage at the pn junction; if it is silicon, then this is about 0.6 V. If the zener diode is symmetrical (VD2 Fig. 2), when the polarity is changed, the stabilization voltage changes slightly. But there are also so-called temperature-compensated zener diodes, which include an additional diode (VD3 in Fig. 3). In this case, with one connection polarity, the stabilization voltage will be supplied to the input of module A1, and with the other, the output voltage of the converter will be supplied.

The pulse generator can also be assembled on other microcircuits; fragments of the device circuit in the case of using K561LN2 and K561LA7 (K561LE5) microcircuits are shown in Fig. 4 and fig. 5 respectively.

The device elements are mounted on a breadboard (Fig. 6) using wired wiring. The resistor used is MLT, C2-23, oxide capacitors are imported, capacitor C2 is K10-17. Transistor - any of the KT815 and KT817 series. The power switch and switch are small-sized of any type. The choke is a standard choke from a CFL, which is wound on an W-shaped ferrite magnetic core (Fig. 7).

The usual inductance of such chokes is several millihenries. To connect the devices under study, you can use alligator clips (XS1, XS2). Instead of the module, you can use a digital multimeter in DC voltage measurement mode.

The adjustment comes down to changing the frequency of the generator to obtain an output voltage (without load) of about 60 V. This can be done by selecting capacitor C2 (increasing or decreasing the capacity) or resistor R1 (only in the direction of increasing resistance). The device is powered by a 6F22 battery (Krona), the maximum current consumption is 38 mA.

Author: O. KOLCHURIN, Nizhnyaya Tura, Sverdlovsk region. Source: Radio No. 6/2016

Diagram of a device for testing zener diodes

As you can see, the scheme is simple. The voltage from the transformer with two 24V secondary windings is rectified and filtered to obtain a constant voltage of about 80V, then goes to a voltage stabilizer formed by the elements (R1, R2, D1, D2 and Q1), which reduces the voltage to 52V to avoid exceeding the maximum operating voltage limit of the LM317AHV .

Pay attention to the letter index of the microcircuit. For LM317AHV, the input voltage, unlike LM317T , can reach a maximum of 57V.

The LM317AHV contains a DC generator, where a switch (S2) is added along with a resistor (R4) to select two test modes (5 mA and 15 mA) as the current source for the zener diode under test.

This tester is easy to assemble from standard components. A ready-made switching power supply from some DVD or satellite system tuner, and a voltmeter either in the form of an industrial module on a microcontroller, or take a D-830 multimeter.

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DIY LED and Zener diode tester

In a repair shop, it is often necessary to check the serviceability of various single LEDs and arrays of LEDs and LED matrices. To quickly check such LED assemblies, there are devices for checking the entire matrix or line of LEDs at once, which speeds up repairs; at the output, on its probes, it produces a voltage of more than 200V, at a very low current, which allows you to check even a single LED with a low current at such a high voltage voltage without damaging it.

Unfortunately, such testers are not cheap and they are usually connected to an outlet using a power cord. But you can assemble an LED tester yourself and it’s actually not difficult, and its big advantage compared to a commercial device is that it is completely autonomous and has a built-in battery. In addition to checking LEDs, the device can also test zener diodes; the tester indicator indicates the operating stabilization voltage, and the low current at the output of the device will not damage it during testing. When connecting an LED or a line of LEDs, the indicator will display the rated operating voltage of the LED or the total voltage of the entire line.

Parts needed to create an LED tester:

Transistor IRF840 or similar powerful ones, for example IRF740;
Pulse diode FR107 or UF4004;
Resistor 1 kOhm;
Resistor 100 kOhm (any one up to 150 kOhm will do);
Resistor 330 kOhm;
The capacitor will come from an energy-saving lamp, which usually has a voltage of 400V, the capacity can be from 4.7 to 10 µF;
A ferrite rod 8x32 mm was taken from the power supply choke from the computer;
Li-Ion battery 3.7 V;
Winding wire in varnish insulation with a diameter of 0.8 mm;
Winding wire in varnish insulation with a diameter of 0.5 mm.
Mini-voltmeter, you can order one on Aliexpress;
Battery protection and charging module TP4056, buy one on Aliexpress;
Phone charging case (or any other suitable size).