breakdown voltage as well as tolerance. The tolerance may vary from 5
per cent to 20 per cent.
The Apparatus need to Prepare this::
- LED
- Transformer
- Bridge rectifier
- Multi -meter
- Neon lamp
- Zener diode
- Transistor
- miliAmmeter
- ON OFF switch
- Resistor & capacitor
enables you to verify the specified breakdown voltage and tolerance values.
In addition, you can check the dynamic impedance of a zener diode.
The dynamic impedance characteristics of a zener diode
determine as to how well the zener diode regulates its
own breakdown voltage. Thus this circuit can be
used to compare the dynamic impedance characteristics
of zener diodes from a lot and segregate/ categorise them accordingly.
For full-fledged zener diode testing you will have to refer to the manufacturer’s
datasheet to check zener diode parameters such as zener voltage, power, and current
(maximum/nominal) ratings. In addition, temperature coefficient and dynamic
impedance have also to be checked if zener diode is to be used for critical
functions such as voltage reference for termidigital
voltmeters, control systems, and precision power-supply circuits. However,
for a common hobbyist it is not necessary to check zener diodes critically, and only
checking its dynamic impedance characteristic is sufficient.
Dynamic impedance implies the degree of change in a zener diode’s voltage
with the change in current. Expressed in ohms, it equals the small change in zener
the test current figure prescribed in datasheets by manufacturers). From
datasheets it is observed that test current value is high for low-voltage zener
diodes and low for higher-voltage zener diodes. However, the dynamic impedance
value will be low for low-voltage zener diodes and vice versa for higher-voltage
zener diodes.
To test 3.3V to 120V zener diodes by the practical dynamic impedance method,
you need to have a variable voltage (0 to above 120V) and current (1 mA to 150
mA) supply source. Designing this type of power supply is quite complicated and is
prone to damage if excess current is drawn accidentally.
The zener diode tester circuit presented here has been designed considering
the above factors. It is capable of testing zener diodes of breakdown voltage ratings
of upto 120V and wattage ratings of 250 mW, 400 mW, 500 mW, and 1W.
The circuit can be deployed in quicktest mode as also in quality-test mode ofoperation.
breakdown voltage up to 47 volts. In quality- test mode, you can check dynamic impedance
characteristic for zener diodes from 3.3V to 120V.
Commonly available step-down transformers X1 and X2 (230V AC primary to
9V AC, 750 mA sec. each) are connected back-to-back as shown in the figure. A
bridge rectifier followed by filter capacitor C1 converts the output from X2 transformer
to DC. Neon lamp L1 indicates the presence of higher DC voltage (220V
approximately) across capacitor C1, which is used to test various zener diode values
from 3.3V to 120V.circuit is that the current gets restricted to a low value. It delivers only 3
mA (approx.) when testing zener diodes with higher breakdown values (e.g. 120V
zener diode), but while testing zener diodes of low breakdown values, such as
3.3V, it delivers a current slightly above 20 mA. Such power-supply characteristics
suit our requirement, as stated earlier. Since a small current is used for testing
of zener diodes, there is no danger of zener diodes getting damaged during testing
using the dynamic impedance method. Before using the circuit, check DC voltage
across test terminals A and B without connecting any zener diode and then flip
toggle switch S2 to quick-test position. DC voltage available across terminals A and
B will be around 200V DC. Now put toggle switch to quality-test position. DC voltage
can now be adjusted from 6V DC to 200V DC (approx.) with the help of potentiometer
VR1. After these preliminary checks, the circuit is ready for operation.
To test zener diode by quick-test method, connect zener diode across termidigitalnals
A and B and flip switch S1 to ‘on’ position. Note down DC voltage in digital
multimeter M2, which is the rough breakdown voltage. In quick-test method you
can test zener diode values up to 47 volts safely. For higher-value zener diodes you
will have to increase the value of resistor R3 suitably. If zener diode presents a short,
digital multimeter M2 will read ‘0’ volts. To perform quality test on the same
zener diode, turn switch S1 ‘off’ and remove zener diode from across terminals A
and B. Now turn switch S1 ‘on’ and adjust potentiometer VR1 to obtain DC voltage
(on digital multimeter) across terminals A and B equal to the one found during quick
test method. Now keep potentiometer VR2 in mid position and connect zener diode
across terminals A and B. (Note. Before testing zener diode, refer
Table I and Table II for the minimum test current (ITmin) and maximum test
current (ITmax) required for various zener diode values, depending upon their
wattage rating.)
Test current is adjusted using potentiometer VR2 and measured using meter
M1 (A 0-25mA analogue milliampere meter or a 0-20mA digital multimeter can
be used.)
Now adjust potentiometer VR2 and note down changes in zener voltage during
ITmin and ITmax conditions. If the required current is not available, increase
DC voltage by adjusting potentiometer VR1 suitably. While changing test current
from ITmin to ITmax, the voltage variation across zener diode should be less than 1
volt for lower-value zener diodes and a few volts for higher-value zener diodes. A
voltage variation of more than this value indicates that zener diode is not properly
regulating. When comparing zener diodes of same values, the zeners showing less
voltage deviation would regulate better
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