AC LED Driving Circuit - How To Drive Low Voltage LEDs?

12 min read Sep 25, 2024

AC LED Driving Circuit - How To Drive Low Voltage LEDs?

Driving AC LEDs directly from the mains voltage is not a practical approach due to the high voltage and alternating current. LEDs are designed for direct current (DC) operation and are typically rated for low voltages. To power LEDs from an AC source, it's crucial to convert the alternating current into a stable direct current. This conversion process involves rectifying the AC signal, filtering out any remaining AC components, and regulating the DC voltage to a level suitable for the LEDs. This article will delve into the details of designing and understanding AC LED driving circuits, focusing on how to drive low-voltage LEDs effectively.

AC LED Driving Circuit: Understanding the Basics

Before diving into specific circuit designs, let's first clarify the fundamental principles behind AC LED driving circuits. The main components of such a circuit include:

AC to DC Converter: This is the core component that transforms the alternating current from the mains into a direct current.
Rectifier: This component converts the alternating current into a pulsating DC signal. Diodes are commonly used as rectifiers.
Capacitor: This component smooths out the pulsating DC, filtering out the AC ripple and providing a more stable DC output.
Voltage Regulator: This component ensures the DC voltage output is at the desired level suitable for the LEDs.
LED Driver: This component regulates the current flowing through the LEDs, protecting them from excessive current and ensuring optimal brightness.

Rectification and Filtering

The AC to DC conversion process begins with rectification. A full-wave rectifier, which uses four diodes, is commonly used in AC LED driving circuits. This configuration allows for the conversion of both positive and negative halves of the AC waveform into a pulsating DC signal. This pulsating DC still contains significant AC ripple, which must be filtered out. A capacitor is connected in parallel to the output of the rectifier to smooth out the ripple. The capacitor charges during the peaks of the pulsating DC waveform and discharges during the troughs, effectively reducing the AC ripple component.

Voltage Regulation

The output of the rectifier and filter provides a relatively stable DC voltage, but it may not be at the desired level for the LEDs. A voltage regulator is necessary to bring the DC voltage down to the appropriate level. The voltage regulator can be a linear regulator, a switching regulator, or a combination of both. Linear regulators work by dissipating excess voltage as heat, while switching regulators use a switching element to efficiently convert the input voltage to the desired output voltage.

Driving Low Voltage LEDs: Practical Considerations

Now, let's consider some practical aspects of driving low voltage LEDs from AC power:

Choosing the Right LED Driver

Selecting the correct LED driver is crucial for optimal performance and longevity of the LEDs. Key factors to consider include:

Current Rating: LED drivers are rated in terms of the current they can deliver. Choose a driver that can provide the required current for the LEDs you are using.
Voltage Rating: The driver should be rated for the input voltage you are using.
Efficiency: Consider the efficiency of the driver, as this will impact the overall power consumption of the circuit.
Protection Features: Some drivers include protection features such as over-current and over-voltage protection, which can help to prevent damage to the LEDs.

Series and Parallel Connection of LEDs

Low voltage LEDs are commonly connected in series or parallel configurations. In a series configuration, the LEDs are connected one after another, with the current flowing through all of them. In a parallel configuration, the LEDs are connected side-by-side, with the current being divided among them.

Series Connection: The main advantage of series connection is that it simplifies the circuit design by requiring only one current-limiting resistor for all LEDs. However, the voltage drop across the series string must be less than or equal to the output voltage of the driver.
Parallel Connection: Parallel connection provides more flexibility in terms of voltage and current, allowing for a higher overall current. However, it requires a separate current-limiting resistor for each LED.

Current Limiting Resistors

Even with a dedicated LED driver, it is generally recommended to use current-limiting resistors in series with the LEDs. These resistors help to protect the LEDs from excessive current, which could damage them. The resistance value for the current-limiting resistor can be calculated using Ohm's Law:

Resistance (R) = Voltage (V) / Current (I)

Where:

V is the voltage drop across the resistor (the difference between the driver's output voltage and the LED's forward voltage).
I is the desired current flowing through the LED.

Thermal Considerations

LEDs generate heat during operation. It's essential to consider thermal management to prevent overheating and damage to the LEDs. Factors to consider include:

Heatsink: Use a heatsink to dissipate the heat generated by the LEDs and driver.
Airflow: Ensure adequate airflow around the LEDs to facilitate heat dissipation.
Thermal Interface Material: Use a thermal interface material (TIM) to improve thermal contact between the LEDs and the heatsink.

Practical AC LED Driving Circuit Examples

Here are two practical circuit examples demonstrating how to drive low voltage LEDs from AC power:

Simple AC LED Driving Circuit with a Linear Regulator

This circuit utilizes a full-wave rectifier, a capacitor, and a linear voltage regulator to power the LEDs.

Components:

Transformer (12V AC)
Full-wave Bridge Rectifier
Capacitor (1000µF, 25V)
Linear Voltage Regulator (7805)
Current Limiting Resistor (100 ohms)
LEDs (10mm, 2V, 20mA)

Circuit Diagram:

Circuit Operation:

The transformer steps down the mains voltage to 12V AC.
The full-wave rectifier converts the AC voltage to pulsating DC.
The capacitor filters out the AC ripple, providing a relatively stable DC output.
The 7805 linear voltage regulator regulates the DC voltage to 5V.
The current-limiting resistor limits the current flowing through the LEDs.

AC LED Driving Circuit with a Switching Regulator

This circuit utilizes a switching regulator for improved efficiency and reduced heat dissipation.