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What is a Ballast: Types, Function and Replacement Guide

Author: Apogeeweb Date: 29 May 2020  2339


I Introduction

The ballast has become an important additional device in the gas discharge light source circuit. Because most gas discharge lamps are made using arc discharge characteristics and have negative characteristics (also known as negative resistance characteristics) where the voltage decreases with increasing current, it is impossible to establish a stable operating point. In order to stabilize the discharge and limit the working current of the lamp, ballast must be installed in the gas discharge light source circuit.


This article will introduce what is a ballast, how does the ballast work, its function, type, fault symptoms, and how to replace a broken ballast.

What is a ballast?


I Introduction

II What is a Ballast?

III Types of Ballasts

 3.1 Inductive Ballast(Magnetic Ballast)

 3.2 Electrical Ballast

 3.3 Resistance Ballast

 3.4 Magnetic Leakage Ballast

 3.5 Capacitive Ballast

 3.6 LC Ballast

IV The Function of the Ballast

V How to Replace a Ballast?

 5.1 How to Tell If the Lamp is Broken or the Ballast is Broken

 5.2 Symptoms of Ballast Failure

 5.3 Replacement Procedure

VI Several Common Terms for Ballasts

 6.1 Ballast Loss

 6.2 Ballast Factor

 6.3 Ballast Efficacy Factor

 6.4 Crest Factor

 6.5 Power Factor

VII One Quiz about the Ballast

II What is a Ballast?

The ballast is a device that acts as a current limiter and generates an instantaneous high voltage on the fluorescent lamp. It is made by wrapping the enameled wire around an iron core made of silicon steel. Such a coil with an iron core, when instantaneously turned on / off and powered on, will generate a high voltage by self-induction, which is added to the electrodes (filament) at both ends of the fluorescent tube.


This action is carried out alternately. When the starter (bubble jump) is closed, the filament of the lamp tube conducts heat through the current limit of the ballast; when the starter is open, the ballast will generate a high voltage on the filament at both ends of the lamp tube. The filament emits electrons to hit the fluorescent powder of the tube wall, and the starter repeatedly turns on and off a few times, thereby turning on the lamp. When the lamp emits light normally, the internal resistance becomes smaller, and the starter will always maintain an open-circuit state so that the current will work stably through the lamp and the ballast to make the lamp emit light normally.


Figure1. Ballast

III Types of Ballasts

3.1 Inductive Ballast(Magnetic Ballast)

3.1.1 Definition

Inductive ballast, also known as magnetic ballast, is an iron core inductance coil. The nature of the inductance is that when the current in the coil changes, the magnetic flux will change in the coil, which will generate an induced electromotive force. Thus hindering the current changes.


3.1.2 How Does the Inductive Ballast Work?

When the switch is a closed circuit of 220V, 50Hz AC power, the electric current flows through the ballast to the starter, lamp filament, the filament heating (at the beginning of the starter is broken, due to a greater than 180V AC voltage, the starter has jumped the gas inside the bubble glow discharge, jump bimetallic strip is heated inside the bubble expansion deformation, two electrodes together, forming pathways to the filament heating).

The Circuit of Inductive Ballast

Figure2. The Circuit of Inductive Ballast


When the two electrodes of the starter are close together because there is no arc discharge, the bimetallic sheet cools, and the two electrodes are disconnected. Due to the inductance of the inductive ballast, when the two electrodes are disconnected, the current in the circuit suddenly disappears. Therefore, the ballast generates a high pulse voltage, which is superimposed with the power supply voltage and is added to both ends of the lamp tube to ionize the inert gas in the lamp tube and cause arc discharge. (High pulse voltage-time is about 1ms 600V ~ 1500V, the exact voltage value depends on the type of lamp).


During the normal lighting process, the self-inductance of the ballast serves to stabilize the current in the circuit.


3.2 Electrical Ballast

3.2.1 Definition

An electrical ballast is a type of ballast, which refers to an electronic device that uses electronic technology to drive an electric light source to produce the required lighting. The electronic components convert the energy of the power grid to meet the voltage and current required by the matching lamps with AC or DC power.

Electrical Ballast

Figure3. Electrical Ballast


3.2.2 How Does the Electrical Ballast Work?

The power frequency power supply becomes a DC power supply after passing through a radio frequency interference (RFI) filter, full-wave rectification, and passive (or active) power factor corrector (PPFC or APFC). Through the DC / AC converter, output high-frequency AC power of 20K-100KHZ, which is added to the LC series resonant circuit connected to the lamp to heat the filament, and at the same time, a resonant high voltage is generated on the capacitor and added to both ends of the lamp tube. However, the "discharge" of the lamp tube is turned into the "on" state and then enters the light-emitting state. At this time, the high-frequency inductance plays a role in limiting the increase of the current and ensuring that the lamp tube obtains the lamp voltage and lamp current required for normal operation.

How an Electrical Ballast Work

Figure4. How an Electrical Ballast Work


3.3 Resistance Ballast

It regulates the lamp current by the voltage on the resistor proportional to the current. In the gas discharge light source circuit equipped with DC power supply, the application of resistance ballast is relatively simple in design and processing, but the power consumption is large and the efficiency is low.


Resistance gas ballasts are also used in gas discharge light source circuits equipped with AC power sources. For example, in self-ballasted high-pressure mercury lamp circuits, tungsten wires are used as resistance ballasts. But generally speaking, the application of a resistor ballast in an AC circuit will affect the lamp current waveform, and make the luminous efficiency of the lamp decrease and the stability deteriorate, but it can improve the circuit power factor.


3.4 Magnetic Leakage Ballast

The magnetic leakage ballast is a type of LC leading peak type circuit. The fundamental wave is mutated by auto-coupling boost and local magnetic saturation, and then it resonates with the working capacitor to obtain a higher open-circuit voltage and longer-lasting lamp operating current. It is a kind of high power factor lighting circuit, the line power factor reaches 0.90 ~ 0.97, which has its unique advantages for point HID light sources.

Magnetic Leakage Ballast

Figure5. Magnetic Leakage Ballast


3.5 Capacitive Ballast

A capacitor is used as a ballast in a gas discharge light source circuit equipped with an AC power source. It cannot limit the instantaneous current of the lamp, only the total amount of charge that passes through the circuit in each half-cycle. In the low-frequency AC circuit, the lamp current waveform will be seriously distorted, forming a high pulse peak current (effective value is not large), which has a very harmful effect on the lamp electrode, resulting in a significant reduction in lamp life. Therefore, capacitors are rarely used as ballasts in low-frequency AC circuits. In higher frequency (20 ~ 100kHz) AC circuits, capacitors can be used as ballasts to obtain satisfactory results. The power consumption is small, the current waveform distortion is small, the volume is small, the weight is light, and there is no noise.

Circuit for Ballast-Ignitor-Capacitor-Lamp

Figure6. Circuit for Ballast-Ignitor-Capacitor-Lamp


3.6 LC Ballast

There are the following two forms.


(1) Ballast composed of inductor and capacitor connected in series. The designed capacitive reactance is usually about twice the inductive reactance, and the total impedance is capacitive. It belongs to the advanced ballast, that is, the phase of the lamp current leads to the phase of the power supply voltage. Compared with resistive ballast and inductive ballast, the power consumption is small, especially with good steady current characteristics, and the short-circuit characteristics when the lamp is started are also good. Combined with the use of hysteretic inductance ballast, it can greatly improve the power factor of the circuit. However, the repetitive ionization voltage provided every half cycle when the power supply voltage is commutated is low, and the repetitive ignition ability is poor.

Circuit of the LC Ballast for T5 28W Lamps

Figure7. Circuit of the LC Ballast for T5 28W Lamps


(2) Ballast composed of magnetic leakage transformer and capacitor. Generally, it belongs to the advanced ballast. If the parameters are selected properly, the circuit power factor can be better improved. Its main performance is the same as the previous form of LC ballast. If a special leakage magnetic transformer structure is selected in the design to form an LC leading peak ballast, in addition to the above advantages, it can also greatly improve repeated ignition ability. This is an ideal ballast. The disadvantage is that the design and processing are more complicated.


To better understand this part, you can see more details about LC circuit.


IV The Function of the Ballast

  • Limit the Starting Current of the Lamp to A Suitable Range

Starting current refers to the current through the lamp within 30 seconds after the lamp is powered on or during the lamp preheating process. In general (especially in the state of lowest temperature), the starting current is much larger than the operating current of the lamp, so each lamp has a maximum starting current. If the starting current is too large, the service life of the lamp will be shortened; if the current is too small, the lamp cannot be preheated to the normal starting state or the process from glow discharge to arc discharge cannot be completed. The starting current provided by the ballast should not only start the lamp in a short time but also not affect the normal service life of the lamp.


  • The Open Circuit Voltage Provided Is Sufficient for the Lamp to Start Smoothly

When the open-circuit peak voltage of the ballast is used as the starting voltage of the lamp, it must be sufficient to ionize the gas in the gas discharge lamp, that is, to generate a peak current that causes a glow-to-arc transition discharge between the electrodes, so that the lamp can start to work. High-pressure mercury lamps and metal halide lamps are more difficult to start at low temperatures, and the open-circuit peak voltage provided by the ballast must be sufficiently high.

How Electronic Ballast Functions

Figure8. How Electronic Ballast Functions


  • Prevent the Lamp Power from Changing Greatly

Although the lamp has a certain range of voltage values during the design and delivery of the lamp, the voltage value of the lamp changes during actual use and throughout its life. This requires the matching ballast to adjust it within a certain range so that the lamp power does not change significantly. The ideal ballast should be such that the lamp power of the newly used lamp and the lamp near the end of its life are not too different.


  • Working Current of Automatic Control Lamp

Stable impedance within a certain voltage range is the basic condition that the impedance ballast can control the working current of the lamp. The ballast uses the time change rate of the voltage proportional to the current to adjust the working current of the lamp. When the open-circuit voltage in a certain period causes the lamp operating current to increase, the inductive effect of the ballast will limit the rate of current increase; when the current starts to decrease, the inductive effect will prevent the rate of current decrease.


V How to Replace a Ballast?

5.1 How to Tell If the Lamp is Broken or the Ballast is Broken

(1) The fluorescent lamp requires a ballast to meet the voltage required for the fluorescent lamp to start and work. If the lamp is broken, you can try another lamp to trouble the lamp cover. If it can be lit, the lamp is broken, if it is still not lit, the ballast is broken;


(2) Using a universal meter to measure the disconnection of the lamp filament can also be judged: use a universal meter to measure the two ends of the fluorescent lamp separately. If the resistance is not zero, it means that the fluorescent lamp is broken, and the light can be shorted According to the method, if the inspection of the two components of the lamp tube and the light emitter is good, the ballast is broken.

Fluorescent Light Troubleshooting and Repair: starter, bulb or ballast?

5.2 Symptoms of Ballast Failure

(1) The shell of the lamp or ballast appears black.

(2) Use an electric pen to detect the ballast. There is no electricity at the incoming and outgoing ends.

(3) Use an electric pen to detect that there is electricity at the incoming end, but no electricity at the outgoing end, which means that there is a disconnection inside the ballast.

(4) If the shell of the ballast is charged with an electric pen, it means that the ballast has a leakage problem.

(5) Use an electric pen to detect that there is electricity at the incoming and outgoing ends. The housing is not live, but the light is off. The light is still off when the trigger is replaced.

(6) Use a multimeter with a resistance level of 200 to measure the resistance of the coil. The resistance is infinite.

Flurescent Ballast Tear Down

Figure9. Flurescent Ballast Tear Down

5.3 Replacement Procedure

(1) Open the lampshade: There are three clips around the lampshade that can be rotated. Use a screwdriver to turn the clip away to remove the lamp cover. Place the removed clip in a fixed place so that it can be reinstalled.

(2) Observe the position of each component

(3) Check whether the old lamp is broken. In general, a section with a longer ring lamp will appear gray. In this case, we should know that it is caused by the sublimation of a substance inside the lamp tube under a high-temperature environment.

(4) Remove the lamp: After the lamp cover is opened, remove it, you can see there is a wire slot on the ring lamp. A wire extending from the lamp holder in the center is inserted into this slot. This wire is plug-in type, just pull it out! The ring tube inside is clamped and fixed by three bent metal pieces. This piece of metal is elastic and breaks apart with a screwdriver. You can remove the ring lamp. Pay attention to the wiring inside.

(5) Replace the ballast: the ballast of the ring lamp, the two white wires are 220V power cords, there is no difference between positive and negative. The four small round holes on the box are the sockets for the lamp feet. Remove the chassis, disconnect the power cord, put the ballast down, and replace it with a new one.

(6) Ring lamp installation: The steps are exactly the reverse when they are removed. Fix the new lamp, and fix the lamp with the three bent metal cards. Use a screwdriver to tighten. Then connect the wires. Stick it with tape and fix it. At the same time, clamp the wire slot on the ring lamp.

(7) Install the lampshade: put back the three clips that were just removed. Fix the lampshade. Turn on the power. You can test whether the light is on. If it's on, it's a good replacement lamp.



(1) Be sure to cut off the main power switch before operation;

(2) When removing the lamp cover, small parts such as cards and screws must be placed. In order to reinstall it finally.

Ballast Kits

Figure10. Ballast Kits

VI Several Common Terms for Ballasts

6.1 Ballast Loss

This value represents that the energy consumed by the electronic ballast itself is converted into heat energy instead of light energy. This value can be calculated by subtracting the power consumed by all lamp tubes from the total output power. Generally speaking, the traditional 40W dual-lamp ballast consumes about 22W, while the electronic ballast consumes about 7W.


6.2 Ballast Factor

This value can show the relative effect of the light output of the electronic ballast. The value is the percentage obtained by dividing the measured light output of the electronic ballast by the light output of the standard ballast light. Generally speaking, the higher the value, the better the light output effect. For electronic ballasts, it must not be less than 0.9, but there are also electronic ballasts designed to emphasize high output values and its light output ratio can be up to 1.18 to 1.28.


6.3 Ballast Efficacy Factor

This value can be obtained by dividing the light output ratio (Ballast Factor) by the ballast input power value (Input Power). In the US market, sellers usually use this value to measure and compare the pros and cons of the efficiency of various electronic ballasts. The higher the value, the better the efficiency of the electronic ballast.

Impedance Ballast

Figure11. Impedance Ballast

6.4 Crest Factor

It is also called wave height rate. This value has a direct and critical impact on the life of the lamp tube. Most lamp tube manufacturers recommend that this value is preferably less than 1.7. Excessively high values can easily cause blackening of the lamp tube and shorten the service life of the lamp tube. The definition of the crest factor refers to the peak current divided by the average current when an electronic ballast is used to light a fluorescent tube.


6.5 Power Factor

This value can represent the efficiency value of the electronic ballast to convert the external input voltage and current into available power. The higher the power factor value, the better the company that supplies the power system (referred to as the power company). In order to encourage consumers to use electronic ballasts with high power factors, foreign power companies have adopted a subsidy policy, but consumers generally think that the higher the PF value, the more power they save. This is a wrong concept because the amount of power saved is not related to the PF value.

Inductive Ballast

Figure12. Inductive Ballast

VII One Quiz about the Ballast

Question: Which of the following ballasts are required to have thermal protection?

A. The ballast of a fluorescent luminaire installed indoors, including a replacement ballast for this type of luminaire

B. A simple reactance ballast in a fluorescent luminaire with straight tubular lamps

C. A ballast in a fluorescent exit luminaire

D. A ballast in a fluorescent luminaire used for egress lighting and energized only during a failure of the normal supply

E. All of the above

Answer: A


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