Simple zener diode tester
Ever found yourself with a handful of zener diodes, wondering if they’re still good or what their actual breakdown voltage is? Zener diodes are incredibly useful components for voltage regulation, but sometimes, their markings can be ambiguous, or you just need to quickly verify their functionality. That’s where a simple zener diode tester comes in, and today, we’re looking at a fantastic, straightforward design by US7IJW.
This circuit is a gem for its simplicity and effectiveness, making it an excellent project for beginners and a time-saver for experienced hobbyists alike.
Why You Need a Zener Diode Tester
Zener diodes are designed to maintain a stable voltage across their terminals once a certain reverse breakdown voltage is reached. This makes them perfect for creating reference voltages, clamping circuits, and simple regulators. However, unlike regular diodes, their exact breakdown voltage can sometimes vary, and it’s crucial to know this value for accurate circuit design. A dedicated tester allows you to quickly:
Dissecting the US7IJW Simple Zener Tester Circuit
Let’s break down the components and how this clever little circuit works:
- Power Supply (Transformer, Diodes, and Capacitor):
- Transformer (220/24V): The circuit starts with a step-down transformer, converting your mains voltage (e.g., 220V or 240V) to a safer, lower AC voltage, typically around 24V. The designer notes that a secondary voltage of 15V to 20V also works well, offering flexibility.
- Fuses (F1, F2): Essential for safety! F1 (1A) protects the primary side, and F2 (200mA) protects the secondary, safeguarding the circuit from overcurrents.
- Diode Bridge (4 x 1N4001): These four 1N4001 diodes form a full-wave bridge rectifier. This converts the AC voltage from the transformer’s secondary into pulsating DC.
- Smoothing Capacitor (1000uF): This large electrolytic capacitor filters the pulsating DC from the rectifier, smoothing it out into a relatively stable DC voltage.
- Current Limiting and Voltage Source (15K Resistor and Transistor):
- 15K Resistor: This resistor works in conjunction with the transistor to provide a suitable current path for the zener diode under test.
- Transistor (2N2218 or EQV): This NPN transistor (like a 2N2218) acts as a constant current source. When a zener diode (ZD) is connected to the “TEST POINT,” the transistor ensures a relatively constant current flows through it, allowing the zener to break down at its characteristic voltage.
- Capacitor (47uF 63v): This capacitor further smooths the voltage supplied to the transistor and the zener, ensuring a stable test environment.
- Measurement (M1 – VDC):
- DC Voltmeter (M1): This is where the magic happens! You connect a DC voltmeter (or a multimeter set to DC voltage mode) across the zener diode under test. When the zener breaks down, the voltmeter will display its breakdown voltage. The note states that “M1 can be any DC voltmeter or a multimeter,” highlighting the flexibility.
Building this tester is quite simple. You can use a perfboard, stripboard, or even design a small PCB for a more permanent solution.
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