The Purpose of the circuit diagram illustrated in Fig.1 is to help in measuring the value of an unidentified Zener diode. It is centered around a common LM317 voltage regulator (IC1), which is connected in its constant-current mode. The current is set to under 5mA, being calculated by the formula I = 1-25V/R, and this flows through the Zener diode being tested. A voltmeter can be  connected in parallel with the Zener and the Zener voltage read directly. By adjusting control potentiometer VR1, it is possible to vary the voltage on transistor TR1 emitter (e), which provides an adjustable voltage to the current limiter.

VR1 is then adjusted until the LED D10 illuminates to indicate that current is flowing, when the test voltage may be read. The circuit operates from a mains power supply. To measure diodes up to, say, 33V DC, it is normally necessary to use a transformer of 30V AC output, but by using a voltage trebler circuit, based around diodes D1 and D2 and capacitors C1 and C2, it is easy to use a  much lower secondary voltage (e.g. 10V AC) and raise this to a useful 42V DC (10V AC x 1-414 x 3) which is current limited to  about 35mA. Due care must be taken when working with these higher voltages, and extreme care should be exercised to ensure that all diodes and electrolytic capacitors are correctly polarized.

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