ELECTRONICS FOR YOU� ❚❚❚❚❚ �JUNE 2001

C I R C U I T I D E A S

Zener diodes available in the mar-ket are specified according to theirbreakdown voltage as well as tol-

erance. The tolerance may vary from 5per cent to 20 per cent. The circuit of aversatile zener diode tester presented hereenables you to verify the specified break-down voltage and tolerance values. In ad-dition, you can check the dynamic imped-ance of a zener diode.

The dynamic imped-ance characteristics of azener diode determineas to how well the ze-ner diode regulates itsown breakdown voltage.Thus this circuit can beused to compare the dy-namic impedance char-acteristics of zener di-odes from a lot and seg-regate/categorise themaccordingly.

For full-fledged ze-ner diode testing youwill have to refer to the manufacturer’sdatasheet to check zener diode parameterssuch as zener voltage, power, and cur-rent (maximum/nominal) ratings. In ad-dition, temperature coefficient and dy-namic impedance have also to be checkedif zener diode is to be used for criticalfunctions such as voltage reference for

operation. In quick-test mode, you canperform a rough check of zener diode’sbreakdown voltage up to 47 volts. In qual-ity-test mode, you can check dynamic im-pedance characteristic for zener diodesfrom 3.3V to 120V.

Commonly available step-down trans-formers X1 and X2 (230V AC primary to9V AC, 750 mA sec. each) are connectedback-to-back as shown in the figure. Abridge rectifier followed by filter capaci-tor C1 converts the output from X2 trans-former to DC. Neon lamp L1 indicatesthe presence of higher DC voltage (220Vapproximately) across capacitor C1, whichis used to test various zener diode valuesfrom 3.3V to 120V.

An advantage of using this high-volt-age circuit is that the current gets re-stricted to a low value. It delivers only 3mA (approx.) when testing zener diodeswith higher breakdown values (e.g. 120Vzener diode), but while testing zener di-odes of low breakdown values, such as3.3V, it delivers a current slightly above20 mA. Such power-supply characteris-tics suit our requirement, as stated ear-lier. Since a small current is used for test-ing of zener diodes, there is no danger ofzener diodes getting damaged during test-ing using the dynamic impedance method.

Before using the circuit, check DC volt-age across test terminals A and B withoutconnecting any zener diode and then fliptoggle switch S2 to quick-test position. DCvoltage available across terminals A andB will be around 200V DC. Now put toggleswitch to quality-test position. DC voltagecan now be adjusted from 6V DC to 200VDC (approx.) with the help of potentiom-eter VR1. After these preliminary checks,the circuit is ready for operation.

To test zener diode by quick-testmethod, connect zener diode across termi-

digital voltmeters, control systems, andprecision power-supply circuits. However,for a common hobbyist it is not necessaryto check zener diodes critically, and onlychecking its dynamic impedance charac-teristic is sufficient.

Dynamic impedance implies the de-gree of change in a zener diode’s voltagewith the change in current. Expressed inohms, it equals the small change in zener

voltage divided by the correspondingchange in zener current (centered aroundthe test current figure prescribed indatasheets by manufacturers). Fromdatasheets it is observed that test cur-rent value is high for low-voltage zenerdiodes and low for higher-voltage zenerdiodes. However, the dynamic impedancevalue will be low for low-voltage zenerdiodes and vice versa for higher-voltagezener diodes.

To test 3.3V to 120V zener diodes bythe practical dynamic impedance method,you need to have a variable voltage (0 toabove 120V) and current (1 mA to 150mA) supply source. Designing this type ofpower supply is quite complicated and isprone to damage if excess current isdrawn accidentally.

The zener diode tester circuit pre-sented here has been designed consider-ing the above factors. It is capable of test-ing zener diodes of breakdown voltage rat-ings of upto 120V and wattage ratings of250 mW, 400 mW, 500 mW, and 1W.

The circuit can be deployed in quick-test mode as also in quality-test mode of

SUNIL KUMAR

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K. UDHAYA KUMARAN

TABLE IIMinimum and Maximum Test Current

ValuesZener diode values IT(min) IT(max)3.3V to 12V 10mA 15mA13V to 27V 5mA 10mA30V to 43V 2mA 5mA47V to 75V 1.5mA 3mA82V to 120V 1mA 2mANote: Zener diode power rating is 1 watt.

TABLE IMinimum and Maximum Test Current

ValuesZener diode values IT(min) IT(max)3.3V to 4.3V 10mA 15mA4.7V to 18V 5mA 10mA20V to 39V 2mA 4mANote: Zener diode power ratings are 250 mW,400 mW, and 500 mW.

ELECTRONICS FOR YOU� ❚❚❚❚❚ �JUNE 2001

C I R C U I T I D E A S

nals A and B and flip switch S1 to ‘on’position. Note down DC voltage in digitalmultimeter M2, which is the rough break-down voltage. In quick-test method youcan test zener diode values up to 47 voltssafely. For higher-value zener diodes youwill have to increase the value of resistorR3 suitably. If zener diode presents a short,digital multimeter M2 will read ‘0’ volts.

To perform quality test on the samezener diode, turn switch S1 ‘off’ and re-move zener diode from across terminals Aand B. Now turn switch S1 ‘on’ and adjustpotentiometer VR1 to obtain DC voltage(on digital multimeter) across terminals A

and B equal to the one found during quicktest method. Now keep potentiometer VR2in mid position and connect zener diodeacross terminals A and B.

(Note. Before testing zener diode, re-fer Table I and Table II for the minimumtest current (ITmin) and maximum testcurrent (ITmax) required for various ze-ner diode values, depending upon theirwattage rating.)

Test current is adjusted using poten-tiometer VR2 and measured using meterM1 (A 0-25mA analogue milliamperemeter or a 0-20mA digital multimeter canbe used.)

Now adjust potentiometer VR2 andnote down changes in zener voltage dur-ing ITmin and ITmax conditions. If the re-quired current is not available, increaseDC voltage by adjusting potentiometerVR1 suitably. While changing test currentfrom ITmin to ITmax, the voltage variationacross zener diode should be less than 1volt for lower-value zener diodes and afew volts for higher-value zener diodes. Avoltage variation of more than this valueindicates that zener diode is not properlyregulating. When comparing zener diodesof same values, the zeners showing lessvoltage deviation would regulate better.