Working of Bipolar LED Driver Circuit

A LED driver or bipolar LED driver is an electrical circuit which a regulated amount of current and voltage to a LED or LED lamp. A LED lamp is a light that contains an arrangement of LEDs configured in an electrical circuit that is designed to operate efficiently. Bipolar LED driver circuits are power supplies optimized for LEDs and are generally known as “LED drivers”.

The LED drivers receive power from the main alternating current (AC) source (primary voltage). The driver rectifies this primary voltage to generate a constant DC voltage on the secondary side to drive the LED lamp. The LED drivers can have bulky iron core transformers to step down the main high voltage to a lower voltage for the LED lamp (12V for example).

Most households use a power inverter to step down the voltage for the LED lamp because of their lower cost and small form factor.

The Basic Structure of Bipolar LED

Light-emitting diodes (LEDs) are two-terminal semiconductor devices. A LED’s PN-junction releases photons when a current flows through it in a process called thermoluminescence. The color of a LED is set by the type of material used – which sets the characteristics of the energy band gap specific to the semiconductor.

Structure of an LED and circuit symbol
Structure of a LED and circuit symbol

A LED is also made from a P-N junction, but silicon is unsuitable because the energy barrier is too low. The first LEDs were made from gallium arsenide (GaAs) and produced infrared light at about 905 nm.

The reason for producing this color is the energy difference between the conduction band and the lowest energy level (valence band) in GaAs. When a voltage is applied across the LED, electrons are given enough energy to jump into the conduction band and current flows. When an electron loses energy and falls back into the valence band, a photon (light) is often emitted.

Photon Light Emission in Semiconductor
Photon Light Emission in Semiconductor

Bipolar LED Driver Circuit using Microcontroller

This is a simple circuit given below and the design involves the interfacing of a Microcontroller, the oscillator and resets circuits for the Microcontroller and selection of the LED resistor.

Bipolar LED Driver Circuit using a Microcontroller
Bipolar LED Driver Circuit using a Microcontroller

The LED used here has a forward voltage drop of 2.2V and hence can be biased using a 5V supply. The circuit uses a Microcontroller to drive the bipolar LED. The control over the LED driver circuit is done by the Microcontroller program, based on the input pushbuttons. The Microcontroller is accordingly programmed to send appropriate signals to the two output pins. These output pins are connected to the terminals of the bipolar LED.

The microcontroller interfacing is accomplished by connecting two push button switches to port P1 and connecting the two terminals of bi-color LED to port P2. The oscillator design is done by selecting two 10pF ceramic capacitors to provide stability. The clock signal is generated using an 11MHz Crystal Oscillator.

The reset circuit is designed by selecting an electrolyte capacitor of 10uF and a resistor of 10K to achieve a reset pulse width of 100ms. The voltage drop across the resistor is kept around 1.2V.

Working of Bipolar LED Driver Circuit

Once the circuit is powered ON, the microcontroller always scans the input pins at port P1. If the first button is pressed, the microcontroller receives a low logic signal at the corresponding input pin and accordingly the compiler assigns a high logic signal to pin P0.0 and low logic signal to pin P0.1. This roots the red light of the LED to glow.

Now when the second button is pressed, the compiler will accordingly assign a low logic signal that will be assigned to both the output pins and the LED will be switched off.

LED Driver Circuit for Brightness Control of LED by 555 Timer

LED Driver Circuit for Brightness Control of LED by 555 Timer is usually achieved by rapidly switching the power supply to the LED, controlling the ON/OFF ratio of the power supply using a process called pulse width modulation (PWM). LED drivers also have a control loop built into them to maintain a constant current.

LED Driver Circuit for Brightness Control of LED by 555 Timer
LED Driver Circuit for Brightness Control of LED by 555 Timer

This circuit shown above is designed based on a 555 timer IC. Power ON the circuit (5V), because the voltage at the trigger pin of 555 IC is less than 1/3 Vcc.

The input voltage will reach the capacitor via the 10kΩ potentiometer and diode D2 so that the capacitor starts charging with a time constant RdR1C (where Rd is the forward resistance of Diode D2).

When the capacitor voltage exceeds 2/3 Vcc, the 555 timer gets reset. Then the output will be zero volts. At this instant, the capacitor discharges via the diode D1 and potentiometer R1 to the output pin since it is at ground potential. When the capacitor voltage goes below 1/3 Vcc, the output of the 555 IC again rises to 5V. This process continues.

Here the charging and discharging path is entirely different since it is isolated by diodes D1 and D2 (refer above images). If the potentiometer midpoint is at 50% (middle), we will able to get a 50% duty cycle (square waves of equal pulse width).

The pulse width can be varied by changing the charging and discharging time, this is possible by adjusting the potentiometer. Thus we get the PWM signal as per our intensity level needed.

This signal is applied to the LED via a 4.7kΩ resistor. The brightness of the LED is proportional to the average value of the square wave. For high pulse width, it is possible to get the huge brightness of the LED. Also, if it is a low pulse with, brightness decrease.

Applications of Bipolar LED Drivers

Some applications for LED drivers are:

  • Industrial/outdoor lighting
  • Auto Intensity Control of Street Lights
  • Commercial lighting
  • Residential lighting
  • Cell phone camera flash
  • Automotive interior or tail lights
  • Portable flashlight / torch
  • Signage
  • Elevator lighting
  • LCD backlighting

Thus, this is all about Bipolar LED driver circuit design, its construction using a microcontroller, 555 timer IC, and applications. We hope that you have got a better understanding of this information.

Furthermore, any queries regarding this concept or electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, What is the function of the potentiometer in an LED dimmer circuit?

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