LCD-LED DisplayZener Diode

Fuse Failure Indicator Schematic Circuit Diagram

This circuit indicates when a device is functioning or when its fuse has blown. It is a development of the Mains/Fuse Failure Indicator published in Elektor Electronics in July/August 1995. It is smaller and cheaper than the previous design, even though it works on any main supply voltage. A single bi-color LED (D2) with separate anode connections indicates operating (green) or fuse failure (red). Resistor R1 limits the current through the LED to around 2 mA: the LED is thus reasonably bright. If higher brightness is desired, the resistor value can be reduced.

Fuse Failure Indicator Schematic Circuit Diagram

The Zener diode prevents the red and green LEDs from lighting simultaneously in normal operation. With the fuse intact, the LEDs are effectively in parallel, but the greater voltage drop in the red LED’s arm of the circuit means that only the green LED lights are. General-purpose diodes D3 and D4 prevent damage to the LEDs in the negative half-cycle of the AC supply. If the circuit is used on a DC supply, the diodes can be removed.

If the circuit is used to monitor the fuse on mains-operated equipment, it is vital to note that the components are not isolated from the mains and the voltages present on them can be lethal: do not touch.

A Zener diode is a special type of diode designed to reliably allow current to flow “backward” when a certain set reverse voltage, known as the Zener voltage, is reached. Zener diodes are manufactured with a great variety of Zener voltages and some are even variable.
Zener diodes are used for voltage regulation, as reference elements, as surge suppressors, and in switching applications and clipper circuits. The load voltage equals the breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.
Therefore a Zener diode is sometimes called a Voltage-regulator diode. For example, the output of half-wave, full-wave, or bridge rectifiers consists of ripples superimposed on a DC voltage. By connecting a simple Zener diode across the output of the rectifier, we can obtain a more stable DC output voltage.

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