The complete guide to ballasts for fluorescent lights

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A fluorescent tube uses electricity to make mercury gas emit ultraviolet (UV) light. When that UV light (which is invisible to the naked eye) interacts with the coating of phosphor powder inside the tube, it glows and produces the light we see and use in our homes.

But whenever we use electricity we must control it, otherwise, we risk destroying the device and even putting ourselves in danger. To regulate the current flowing throughout fluorescent lights, we use what’s known as a ballast.

What is the ballast in a fluorescent light?

The ballast (sometimes called control gear) is a small device wired to the light’s circuitry which restricts the amount of electrical current travelling through it.

Because your home’s mains power has a higher voltage than the light needs to operate, the control gear gives the light a small boost of voltage to start and then just enough of a supply to stay running safely.

Why are ballasts needed?

The process that occurs inside a fluorescent light involves molecules of mercury gas being heated by electricity and made more conductive. Without a ballast to control this, there would be too much current for the light to take, and it would burn out and possibly even catch fire.

The process that occurs inside a fluorescent light involves molecules of mercury gas being heated by electricity and made more conductive. Without a ballast to control this, there would be too much current for the light to take, and it would burn out and possibly even catch fire.

Do you require a fluorescent tube?

View our full range of fluorescent tubes here or take a look at our curated products shown here.

How does a fluourscent light ballast work?

Fluorescent lights use either an electronic or a magnetic ballast. Nowadays, magnetic ballasts are a rather outdated technology that manufacturers are phasing out, and so they’re usually only found on older types of lights.

Magnetic Ballasts

These rely on the principles of electromagnetism, in that when an electrical current travels through a wire, it naturally generates a magnetic force around itself.

A magnetic ballast (also called a choke) contains a coil of copper wire. The magnetic field produced by the wire traps most of the current so only the right amount gets through to the fluorescent light. That amount can fluctuate depending on the thickness and length of the copper wire. If you sometimes hear your light buzz or see it flicker, it’s this changing flow of current that’s causing it.

Less advanced in their design than electronic models, some magnetic ballasts can’t function without the aid of a starter. This small cylinder-shaped component sits behind the light fixture and is filled with gas which, when heated, enables the light to start. This is called the pre-heat method.

Pre-heat method

  1. The light switch is turned on. Inside both ends of the light are metal electrodes with filaments attached. The current enters the filaments but at this point is too low to fire up the light, though it is enough to heat the gas (neon or argon) inside the starter.
  2. The heated gas causes components inside the starter to allow the full current into the filaments. This quickly heats the mercury gas inside the light.

  3. As the starter cools, it blocks the current’s path to the filaments and makes it seek another route. If the mercury gas is heated sufficiently, it’ll conduct the current, generate light and then remain lit. If it isn’t hot enough, the electricity will go back through the starter and begin the process again. This is what causes some old fluorescent lights to flicker.
  4. Now there’s more electricity coming in, the control gear starts to do its job of regulating it.

As it can take several seconds for this process to complete, you may see a delay between the moment you flick the switch and when the fluorescent light begins to glow.

Rapid-start Method

If your light fixture has two or more fluorescent tubes, it’ll likely use another different method known as rapid-start. Used in older T12 and some T8 tubes, this method functions without a starter.

  1. Unlike pre-heat—where the filaments receive current via the starter only to heat the mercury gas—with rapid start, the ballast keeps a small amount of current flowing continuously through the filaments.
  2. This causes the mercury gas to become ionised—that is, charged in a way that enables it to conduct electricity.
  3. Because it’s only a gentle current, the light will glow quite dimly at first. But as the ballast continues to push current through the filaments, the gas gets hotter and more charged and the light brightens as a result. If your light comes on immediately but takes a few seconds to get fully bright, it has a rapid-start ballast.

One advantage of the rapid-start method is that by providing a low, continuous current rather than a strong surge, it prolongs the life of the fluorescent light and can be better for your overall light output. However, keep in mind that it does use more energy.

Electronic Ballasts

Using more sophisticated circuitry and components, electronic ballasts can control the current running through fluorescent lights with greater precision. Compared to their magnetic counterparts, they’re smaller, lighter, more efficient and—by supplying power at a much higher frequency—less likely to cause flickering or buzzing sounds. Overall this makes for a more efficient lighting system.

Some older electronic ballasts employ the rapid-start method described above, while newer and more advanced models use what are known as instant-start and programmed-start.

Instant-start Method

These control gears were developed so lights could be turned on and operated at their brightest at the first flick of the switch. Rather than pre-heat the electrodes, the electronic ballast uses a high-voltage boost (around 600 volts) to heat and light the filaments and then the mercury gas. Though this makes them energy efficient, it also shortens their life, as the surge of voltage every time they’re switched on damages them over time.

For this reason, they’re commonly used in spaces where the lights are left on for long periods, such as offices, shops and warehouses.

Programmed-start method

Designed for areas in which the lights are constantly switched on and off, these devices pre-heat the electrodes with controlled amounts of current before applying a higher voltage to start the light.

They’re often a feature of lighting that’s activated by motion-detection sensors (for example, toilets in workplaces or public venues) and allows the fluorescent light to last for a long time.

Signs that your magnetic ballasts has broken

When magnetic ballasts break, it is often blamed on the bulb. Look at for these signs indicating that it’s your ballast:

  • Delayed start
  • Buzzing
  • Flickering
  • Low output
  • Inconsistent lighting levels

You can find out whether the issue is with the device, starter or the lamp with our guide - Easy Fixes for Slow to Start, Flickering or Faulty Fluorescent Tubes.

Testing a ballast with a multimeter/volt-ohm meter

In order to ensure that the issue is with the ballast, you will want to test it with a multimeter. A multimeter is designed to measure electric current, voltage and resistance. They are inexpensive and can be found at most electronics shop.

These instructions are for guidance purposes only – ensure you reference the manufacturers wiring diagrams. If you are missing the instruction manual, most major manufacturers will have opies on their website.

You will need

  • Screwdriver
  • Multimeter


  1. Switch off power to the light fixture.
  2. Remove the light casing.
  3. Remove the bulbs.
  4. Remove the ballast from the fixture.
  5. If the ballast looks burnt, it needs replacing.
  6. Set your multimeter to the ohm setting.
  7. Insert the first probe of the multimeter to the wire connecting the red wires.
  8. Touch the second probe to the green and yellow colour wires.

If the multimeter doesn’t move: This means the ballast is dead

If the multimeter needle moves across to the right: The multimeter is still working.

If the ballast is not the problem, you may need to replace your fluorescent tube or other components of your lighting . You can find out how to do this safely with the guide Replacing and Recycling Fluorescent Tubes Safely. If you know what fluorescent tube you need, browse our full range here.

Swapping magnetic ballasts for electronic

The process for swapping out magnetic ballasts for electronic ballasts is pretty simple and straightforward. This is the direction the lighting industry is headed in, so why not swap them sooner rather than later to optimise your space with better, quieter lighting?

You will need:

  • Electronic ballast
  • Wire cutters
  • Wire nuts


  1. Turn off power to the fixture.
  2. Open fixture and remove the bulb and ballast casing.
  3. Using the wire cutters, cut both power (brown) and neutral (blue) wires coming into the fixture.
  4. Cap the wires with wire nuts.
  5. Use the wire cutters to cut the wires connected to the sockets.
  6. Remove the magnetic ballast.
  7. Screw the electronic ballast into the fixture, in the same place where the magnetic one was.
  8. Use wire nuts to connect the socket wires.
  9. Connect the power and neutral wires to the corresponding ballast wires.
  10. Secure the wires with wire nuts.
  11. Place the bulb and ballast casing back.
  12. Switch power back on.

There is a risk of electrocution when changing a ballast so if you’re unsure ask an electrician to do the job for you.

Does my fluorescent tube need a starter as well as a ballast?

Separate starters are only found in older control gear, so if a fixture is less than 15 years old it probably won’t have a starter. In newer lamps the process provided by a starter is built in, making the function of a separate starter redundant. If the lamp fixture does have a starter, it will be obvious. You should find a small grey cylinder plugged into the light fitting.

What's the difference between switch start and high frequency control gear?

High Frequency

High frequency control gear is a modern single ballast that performs the functions of all the different components in the standard switch start circuit. Lights which are operated with a high frequency ballast do not flicker, but instead light up instantly due to the frequency being much quicker.

Switch start

Switch start is the control gear that has been used by the industry for years. They are general considered old technology and less manufacturers are creating them. Switch start requires a wire wound magnet choke of ballast. Various parts can be replaced for switch start, rather than the whole unit, which could been seen as an advantage in all types of ballasts.

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