When We Apply 5V AC to an LED: Why it Does Not Glow

Light Emitting Diodes (LEDs) are optimized to operate with direct current (DC) rather than alternating current (AC). This article explores the reasons why applying 5V AC to an LED will not cause it to glow as expected.

Forward Bias and AC Voltage

LEDs conduct current in only one direction, which is referred to as forward bias. Alternating current (AC) changes direction periodically, thus making it impossible for the LED to remain forward biased for a full cycle. For this reason, when 5V AC is applied, the LED will not glow as it is unable to maintain a consistent current flow in one direction.

Voltage Rating and LED Operation

The standard operating voltage for most LEDs is around 2V to 3.5V, depending on the color. When 5V AC is applied, the LED will experience a voltage during the negative half-cycle that exceeds its maximum reverse voltage rating. This excessive voltage can cause permanent damage to the LED, preventing it from functioning correctly.

Flickering and Implications

Even if the LED managed to light up during the positive half-cycle, it would do so at a flickering rate equal to the AC supply frequency, typically 50Hz or 60Hz. This flickering is generally undesirable and can affect the LED's efficiency and longevity.

Safe Practices for Using LEDs in AC Circuits

To safely use an LED in an AC circuit, a rectifier is typically employed to convert AC to DC. Additionally, a current-limiting resistor is necessary to prevent excessive current from flowing through the LED, ensuring safe operation and extended lifespan.

Example: LED Voltage and Current Drop

To understand how to use an LED in a 5V AC circuit, let's consider a specific example. For instance, if using a blue LED that has a forward voltage of 3.7V and a current rating of 20mA, the calculation goes as follows:

LED voltage: 3.7V Supply voltage: 5V Voltage drop across the resistor: 5V - 3.7V 1.3V

To find the required resistance:

R 1.3V / 0.02A 65 ohms

A standard 65 ohm resistor can be used in series with the LED to ensure safe operation.

While the LED may work in a 5V AC circuit with some flicker, it is important to note that the LED is only lit during the positive half-cycle of the AC supply. This can result in reduced light output and inconsistent performance.

Conclusion: Proper rectification and current limiting are crucial for ensuring the safe and effective use of LEDs in AC circuits.

Note: SMD LEDs specifically designed for AC-driven applications like LED lamps can operate at higher voltages, such as 12V, without requiring additional current limiting.