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Shree Ram Electronics
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ZENER DIODE TESTER

S&T - INST 003

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The Zener Diode Tester can be used to check zener diodes of 3.3V to 18V. The breakdown voltage of the unknown zener diode is indicated on the pre-calibrated dial of the potentiometer VR1. The tester can also identify the polarity of zener diodes. 

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Project Description

The power supply section of the Zener Diode Tester provides approx. 20V DC stabilised voltage to the sensor section. The sensor circuit comprises two resistors (R2 and R30, a potentiometer (VR1), a red LED and an NPN transistor BC547 (T2).

When the linear potentiometer VR1 is adjusted such that the voltage at its wiper arm (red crocodile clip) exceeds the breakdown voltage of the zener diode, the zener diode
conducts and applies the bias voltage at the base of transistor T2, which causes red LED to light up. When the voltage at the wiper arm (red clip) is less than the breakdown voltage, the zener diode does not conduct and red LED does not glow. For calibration of the zener diode tester, initially the pointer knob of VR1 is set towards the zero-resistance position. Short red clip of the potentiometer and black clip of the transistor and switch on the tester. The pointer knob of potentiometer VR1 is slowly rotated in clockwise direction until LED just starts to glow. This setting
of the knob is marked on the paper dial as 0V. Now a zener diode of a known value (for example, 3.3V) is connected between both the clips (red clip to the cathode and black clip to the anode of the zener diode). The knob of potentiometer VR1 is further rotated in the clockwise direction until the red LED just starts to glow. This setting of the knob is marked on the paper dial as 3.3V. Likewise, the dial of potentiometer VR1 is calibrated for other values of zener diodes by connecting known zener diodes to the tester. 

For testing an unknown zener diode, it is connected across the clips in correct polarity
and the knob of potentiometer VR1 is rotated until red LED1 just starts to glow. The
voltage shown by the pointer knob on the dial at this setting is the breakdown voltage
value of the zener diode under test. If the zener diode is connected in reverse polarity (red clip to the anode and black clip to the cathode), the LED glows brightly at all settings of the knob above the zero reading, indicating that the zener diode is wrongly
connected. The anode and cathode terminals of rectifier diodes can also be identified
in this way. 


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ZENER DIODE TESTER

ZENER DIODE TESTER

The Zener Diode Tester can be used to check zener diodes of 3.3V to 18V. The breakdown voltage of the unknown zener diode is indicated on the pre-calibrated dial of the potentiometer VR1. The tester can also identify the polarity of zener diodes. 

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