Zener diode tester

When I select a zener diode from my parts cabinet, I always want to check its failure voltage before I utilize it in a job– just to make sure there had not been a mix-up in my cabinet drawer. This treatment requires to be duplicated when putting unused diodes back into my cupboard, or when sorting amongst a bunch of diodes I picked up at an excess electrical outlet.
It’s time eating to set up a power supply as well as voltmeter, pick a collection resistor to limit current, and also measure the voltage for every diode. Alternatives like checking out the component number and looking it up, or establishing a curve tracer aren’t any faster. Numerous mistakes, burned out diodes, and also broken jobs have actually encouraged me there have to be a far better means. While try out self-oscillating changing power products for one more application, it dawned on me that this modern technology was the response to my problem.
The tester defined in this post is a basic two-transistor circuit operating from a 9V battery which examines zener diodes with breakdown voltages up to 52 volts. Absolutely nothing greater than a multimeter is essential to build, test, and utilize the circuit. Its transformer-coupled design changes the result voltage automatically to the voltage of the zener diode under examination, while adjusting diode existing to maintain a reasonably continuous diode power throughout the measurement range.
There’s no need to select a current-limiting resistor. Just connect a voltmeter throughout the diode as well as measure its failure voltage at a regular and also secure power level. The circuit will certainly likewise safely test LEDs– including white LEDs that multimeters can not test and other reduced voltage diodes in their forward direction, along with MOVs (metal-oxide varistors) and also various other safety devices with greater voltage breakdowns.
Connecting an outside power supply to the tester permits tools that damage down well above 50V to be tested conveniently. With all my projects and also the various uncharacterized devices existing around my shop, this zener tester has actually verified to be one of the most frequently utilized equipment on my bench!


Zener Diode History
If you’re not accustomed to zener diodes, after that an intro is appropriate. Zeners are so valuable they ought to be discovered in every experimenter’s element collection. The zener diode is a specialized silicon diode that appears like a normal silicon diode in the forward instructions. Nonetheless, in the reverse voltage instructions, it exhibits a low leak current like a typical diode till a voltage– the supposed “zener voltage”– is reached. Then, it sharply shows a controlled constant malfunction voltage regardless of raising existing.
All diodes start to heavily conduct at some voltage in the reverse instructions, yet are operated well below this failure voltage to stay clear of damages. Zeners, on the other hand, are created to be operated in their malfunction area, which breakdown is meticulously engineered for certain voltages. For instance, the 1N4728 to 1N4764 household of zener diodes consists of break down voltages from 3.3 volts to 100 volts in 37 steps– a much more comprehensive array than common fixed-voltage three-terminal IC regulators. These diodes can additionally be put in collection to attain almost any type of voltage desired, as well as various diode families can be found in numerous power level rankings from 200 mW to over 10W.
Zeners are frequently utilized as voltage regulators, reference-voltage criteria for op-amp layouts, and as security devices to secure components from over-voltage problems. Applications like relay or solenoid vehicle drivers and switching over power materials– like the one explained in this write-up– commonly use zeners for defense.

They also locate usage (rather than a simple collection resistor) in dropping one voltage to a reduced voltage, and also in clipping AC voltage waveforms. They are also utilized as noise generators in RF resistance bridges. Most importantly– specifically for experimenters on a budget plan– regular low power zener diodes expense simply pennies each.

The Zener Diode Tester

The diode to be evaluated is connected between the yellow as well as red terminals, and also the voltage reads throughout these terminals when the push button is dispirited to examination. Linking an existing meter between the yellow and black terminals enables present measurement while testing if preferred, or changing the existing meter with an external power supply permits extending the tester’s range to well over 52 volts.
A blinking red LED shows when voltage exists throughout the examination terminals. A yellow LED shows that the output goes to the limit of 55V. When no diode exists or the zener diode has a break down voltage over 55V, this LED is brightened. The LED likewise suggests correct performance of the internal circuit for first testing as well as works as a security reminder that a possibly harmful voltage is present throughout the outcome terminals. When the examination button is launched, the outcome is promptly discharged to zero volts for security.
The inner circuitry of the tester is easily changed as well as simulated to change the voltage array or for use in other applications. These adjustments and simulation are talked about later on in this article.

Circuit Description as well as Procedure

The schematic of the tester is received Number 1. The key to procedure and building convenience is using a transformer, T1, which is easily available from numerous suppliers and designed for usage in tiny changing power products. T1 has 6 the same and independent polarized windings of which 4 are utilized in the tester: T1-L1 is the “main” driving the “responses” winding, T1-L4, and keeping energy in the transformer’s magnetic field; after that T1-L2 as well as T1-L3 are attached in series to create the “additional,” discharging the saved magnetic energy into the diode under test (DUT).
A few of you will certainly acknowledge this standard circuit as a straightforward “obstructing oscillator,” commonly utilized with vacuum tubes for radar applications in WWII, as well as later on embraced as a transistor configuration for the earliest generation of solid-state changing power products. In the power supply world, it’s currently labelled a “flyback” circuit, harking back to CRT-based/TV horizontal-output circuits utilizing this geography, or usually described as a “increase” circuit. It’s simple, deals with many different transistors, and doesn’t require specialized integrated circuits.
S1 is an SPDT brief pushbutton button. Until S1 is pressed, the 9V battery is detached from the circuit, and outcome filter capacitor, C4, is released through R8. Pressing S1 to the ON or TEST placement removes R8 from the outcome, as well as uses 9V to the key, T1-L1, and also to the start-up resistor, R2. It likewise turns on LED1– a blinking red LED that indicates to the user that the circuit is invigorated and also there are possibly unsafe voltages across the outcome terminals.
Start-up existing circulations via R2 (as well as R4), switching on drive transistor, Q1. As Q1 activates, it draws pin 1 of T1 towards ground which, in turn, triggers the voltage across the responses winding of T1/T1-L 4 to climb from ground towards +9 V, because the turns ratio of the two windings is 1:1. The climbing voltage across T1-L4 is shared to the base of Q1 through D1, r3, and also c3. This current adds to the current via R2, further activating Q1 and rapidly driving it into saturation.
In the saturated state, the voltage across Q1 is a couple of tenths of a volt and virtually the complete 9V battery voltage is throughout T1-L1. Currently, the current via T1-L1 as well as R6 begins to increase, keeping magnetic energy in the core. No current flows with D3 considering that it’s reverse-biased during this part of the oscillation cycle.
When the voltage decline across R6 rises above 0.7 V, the “choke” transistor, Q2, starts to turn on and also shunt the base current of Q1 to ground, requiring Q1 to come out of saturation and also the voltage at Q1’s collection agency to rise. This action reduces the voltage throughout T1-L1, which similarly goes down the voltage throughout the comments winding, T1-L4, more decreasing base drive to Q1 and also shutting Q1 off rapidly through this regenerative action.
When Q1 appears of saturation and begins to shut off, the voltage at its enthusiast climbs quickly as a result of inductive action, and the voltage across T1-L1 turns around, driving Q1’s collector voltage above 9V. At the same time, the additional voltage reverses as well as D3 begins to carry out.
When the stored power in the core is fully released through the additional, the voltages on all windings collapse, activating Q1 again through C1 (the voltage at pin 11 goes from a negative voltage toward ground). The cycle repeats till C2 is billed to a voltage degree where the DUT begins to carry out, at which point the oscillation stabilizes as well as proceeds to feed power right into the DUT.

Voltage levels (referenced to ground) are shown for a generalised zener diode voltage, Vz, under examination. The voltages received parentheses are for a 12V zener diode as the DUT, and matching actual circuit waveforms.

no DUT exists when the circuit is running, then the voltage throughout C4 will remain to rise, as will the peak voltage at the enthusiast of Q1. The voltage at pin 2 of T1 and the result voltage would keep rising with each cycle, as would certainly the peak voltage (half the result voltage plus 9V) at the collection agency of Q1. This is a feature of a flyback circuit arrangement, which allows zener diodes to be checked well over the battery voltage of 9V.
Some defense is essential to maintain the optimal voltage at Q1’s enthusiast from exceeding its maximum collection agency voltage ranking of 40V. The collection mix of zener diode D2 and also yellow LED2 provide this defense by restricting the peak voltage as well as soaking up the energy of T1’s electromagnetic field if no DUT exists, or if the DUT break down voltage is higher than the tester’s optimal output voltage. LED2 lights when there’s current via D2 in this condition.

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