Zener test circuit serves as dc source
John Jardine, JJ Designs, West Yorkshire, UK - November 25, 2004
This Design Idea describes a versatile test circuit for zener diodes after yet another misread zener diode had infiltrated the ranks of 1N4148 diodes assembled on a pc board. As a bonus, the circuit can serve as a moderate-voltage, power-limited adjustable dc source. Although conventional multimeters' resistance ranges typically apply enough voltage to forward-bias most diodes, few can drive a zener diode into reverse conduction. Figure 1a shows a simple variable-frequency dc/dc step-up converter whose output voltage depends on the device under test's breakdown voltage.
Upon power application, Pin 3 of IC1 (one section of a 74HC132 quad dual-input Schmitt-trigger NAND gate) goes to logic one and switches on Q1, an n-channel logic-level power MOSFET. Current flows through Q1 and R6 and stores energy in inductor L1's magnetic field. Zener diode D1 limits the voltage at IC1's Pin 1 to 4.7V. Simultaneously, diode D2 and resistor R3 charge C2 and establish a logic one at IC1's Pin 2. When the voltage at point E1 reaches approximately 2.7V, IC1's input-voltage threshold, IC1's output goes to logic zero, switching off Q1.
Energy stored in L1's magnetic field discharges through fast-recovery diode D3 and charges C3. Capacitor C1 helps remove diode D1's stored charge and helps restart the charging cycle.
After several cycles, the voltage at E2 reaches the device under test's reverse-breakdown voltage and feeds current via R1 to IC1's Pin 1. As a result, the voltage at E2 stabilizes at the sum of the device under test's reverse-breakdown voltage and a constant offset voltage of 5.4V comprising the voltage across D1—4.7V—plus the forward voltage across D3—0.7V. Thus, for a 100V zener as the device under test, the voltage at E2 measures approximately 105.4V.
At start-up and under fault conditions, resistor R4, diode D2, and resistor R3 produce an asymmetrical oscillation at approximately 2 kHz, which reduces the average current through L1 and Q1 to a safe level.
To use the circuit as a variable medium-voltage power supply, replace the device under test with the network in Figure 1b. Adjusting the potentiometer varies the voltage at point E2 from 22 to 120V. Maximum current available from the circuit depends on the dc resistance, L1's magnetic-saturation characteristics, and Q1's on-resistance. For a nominal 5V power supply and 430 mA of input current, the circuit delivers 10 mA at 100V for a 100V output, yielding an efficiency of approximately 50%. Feeding L1 from a separate 12V power supply improves efficiency.
If you design your own inductor for L1, aim for a nominal inductance of 330 µH at 2A and a dc winding resistance of less than 0.5Ω. For optimum operation, use a fast-recovery diode for D3 and a logic-level n-channel MOSFET with a breakdown voltage of 200V or greater and an on-resistance of less than 0.3Ω for Q1. Note that zener-diode manufacturers specify breakdown voltages at specific test currents. Also, when you subject them to high reverse voltages, signal diodes exhibit zener behavior.
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