Electronic Ballast: Working Principle & Circuit Diagram

Contents

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Key learnings:

Electronic Ballast Definition: An electronic ballast controls the starting voltage and operating currents of lighting devices through high-frequency power conversion.
Working Principle: The electronic ballast converts AC voltage to a high-frequency output to start and regulate the gas discharge in lamps, ensuring efficient lighting.
Circuit Diagram Importance: Understanding the circuit diagram of an electronic ballast is crucial as it includes components like EMI filters and rectifiers that ensure efficient operation.
Advantages of Using Electronic Ballast: Electronic ballasts enhance lamp longevity and light quality by operating at high frequencies and reducing flickering and noise.
Ballast Types and Selection: Different types of ballasts, including electronic and HID versions, are suited for various lighting applications, influencing efficiency and performance.

What is Electronic Ballast?

An electronic ballast is a high-frequency power supply that sets the starting voltage and the running current of a discharge lamp. It is one kind of electrical ballast, not another name for the older magnetic ballast.

It works by electrical gas discharge. Mains frequency is converted to a much higher frequency so a Fluorescent Lamps can strike. The ballast then holds the lamp voltage and the lamp current in the running range.

Use of Electronics Ballast

An electronic ballast has several practical advantages over an electromagnetic ballast.

Operating at a low supply voltage, the electronic ballast generates a high frequency that produces an initial high output voltage essential for initiating the discharge process in lamps.
It creates very low noise during operation.
It greatly reduces visible flicker. High-frequency switching can still make RF noise, which is why the circuit includes an EMI filter.
As it works with a very high frequency, it helps to start the lamp operation instantaneously.
It does not require any starter that is used in electromagnetic ballast.
It never creates flickering.
No start-up vibration occurs.
Its weight is very minimal.
Ballast loss is very less. Hence energy saving is possible.
It increases the life of the Lamp.
Due to operation at a higher frequency, the discharge process in a fluorescent lamp is at a higher rate. Hence the quality of light is increased.

Working Principle of an Electronic Ballast

The ballast takes a 50-60 Hz supply and rectifies it to DC. A capacitor filters that DC. A high-frequency inverter then makes a square wave, typically between 20 kHz and 80 kHz.

Thus, the output current is at a very high frequency. A small inductance is included to manage the rapid changes in current at high frequencies, ensuring optimal performance.

A fluorescent tube often needs more than 400 V to strike. At switch-on the open-circuit voltage can be around 1000 V, so the discharge starts at once.

After the arc is running the lamp voltage falls, often to about 125 V and below 230 V. The ballast then limits the current through the lamp.

The ballast control circuit sets that voltage and current. In run it is a current limiter, not a user dimmer unless the design is made for dimming.

Basic Circuitry of an Electronic Ballast

In present days, electronic ballast design is so robust and somewhat complicated to work very smoothly with high leveled controlling ability. The basic components used in the Electronic Ballast are listed below.

EMI filter: Blocks any electromagnetic Interference
Rectifier: Converts AC power to DC power
PFC: It does Power Factor Correction
Half-Bridge Resonant Output: Converts DC to square waved voltage with high frequency (20 kHz to 80 kHz).
Control Circuit: Controls voltage and current across and through the lamp respectively.

What is HID Ballast?

An HID ballast (High-Intensity Discharge) sets the voltage and the arc current of an HID lamp from start through run. The common HID ballast circuits are shown below.

Types of HID Ballast

HID ballasts can be classified into four different categories/types:

Reactor ballast
Lag ballast
Regulator ballast
Auto regulator ballast

A short description of each type is given below.

Reactor Ballast

This reactor ballast is essentially a wire coil on an iron core placed in series with the lamp.
A capacitor is introduced to correct the power factor and this capacitor must be inserted across the line.
The voltage change in the lamp due to the reactor is 18%, for wattage, it is 5% change and 5% line voltage change.
It regulates the lamp voltage very well but regulates the line voltage very poorly.
The reactor ballast provides a low current crest factor of about 1.5.
The amount of the starting voltage it can provide to the lamp has limitations up to the line voltage.

The Regulator ballast is shown below.

Lag Ballast

A combination of an autotransformer and a reactor forms lag ballast.
This lag ballast has the same regulation characteristics as the reactor ballast.
But lag ballast overcomes the starting voltage limitation, i.e. more than line voltage.
It is bulk in size with a greater loss.
A lag ballast cost higher.

A circuit diagram of a lag ballast is shown below.

Regulator Ballast

Regulator ballast has isolated primary and secondary windings.
It achieves current limiting through a series capacitor.
This capacitor leads the current to lead the secondary voltage.
With a regulator ballast, an excellent regulation is obtained.
By using this regulator ballast, the change in line voltage is ± 13 % and about ± 3 % is the lamp wattage change.
Its power factor is about 0.95.
It minimizes the grounding and fusing problems.
The higher current crest factor is only its disadvantage as this crest factor lies between 1.65 to 2.0.

A circuit diagram of a regulator ballast is shown below.

Auto Regulator Ballast

The auto regulator ballast has the features of both the lag ballasts and the regulator ballasts.
This auto regulator ballast is most popular and it is intended as trade off.
Its cost is less and it does not provide isolation between primary and secondary.
It has bad regulation property.
It causes a variation in the line voltage with ± 10 % and wattage variation is ± 5 %.

A circuit diagram of an auto regulator ballast is shown below.

What is the Current Crest Factor for a HID Ballast?

The current crest factor is the ratio of the peak to RMS current of the HID ballast, i.e.

Which Ballast is Used in Sodium Lamp?

A different kind of ballast is used in sodium lamps. The crest factor should not exceed 1.8 for proper lamp operation. As in the sodium lamp, ionization of xenon gas needs very high voltage, so starting voltage with a higher value must be obtained from this kind of special ballast.

The lamp wattage is controlled closely to control the amalgam vaporization. The feature of this ballast is given below.

It is a bulky electromagnetic ballast.
It is combined with igniters.
It has much better lumen maintenance capability.
Recently electronic ballasts have been introduced to perform the same tasks more efficiently.