How to Check Whether a Fluorescent Light Ballast Has Gone Bad (2024)

Fluorescent lamps were first developed commercially in the 1930s, although ideas about how to develop them began as early as the 1880s. This type of lighting can be ideal because the light produced is energy efficient and well diffused. Additionally, the components that make up fluorescent light have long life spans.

But it can be frustrating when one of these components goes out, causing incessant buzzing noises or flickering of light. If you’ve already replaced your fluorescent bulb recently and are still having issues, your problem might be with the ballast.

There are several components to fluorescent lamps. Light is produced within the glass tube by ionizing mercury vapor, which causes electrons in the gas to emit photons at UV frequencies.

These frequencies are converted into standard visible light using a phosphor coating on the inside of the tube.

High voltage is used to get the electrons to pass from one electrode to another, forming an arc. Colder lamps require a higher voltage, which is why some fluorescent lamps turn on slowly before they achieve full brightness—the lamp is warming up the gas.

A ballast is used to help regulate the current created by this process. Without it, the current would rapidly increase and cause the lamp to overheat. The ballast provides a quick supply of high voltage, establishes the arc, reduces the voltage, then regulates the ongoing current to create a steady light output.

Low output

If the lights remain dim for several minutes after you’ve turned them on, you could have an issue with your ballast, especially if you’ve just replaced your bulb.

Flickering

Periodic flickering or strobing can be particularly frustrating. It could also be a sign that your ballast isn’t working properly.

Buzzing

A dying ballast often starts to hum or buzz on its way out.

Inconsistent lighting levels

Fading light and color variation are common with fluorescent lamps. However, dark corners or inconsistent light across a space could be an indicator that your ballast isn’t working properly.

Delayed start

Remember that the ballast provides the voltage to get the inner workings of your lamp to light up. While a warming start might be expected, an increased delay is a sign of a bad ballast.

If you can’t find obvious signs of a bad ballast and you’ve already tried replacing the bulb, there are steps you can take to test whether your ballast is the issue.

Step 1. Power off the fixture

Go to your circuit breaker panel and power down the area that includes the lamp you want to check.

Step 2. Remove fluorescent bulbs from the fixture

Fixtures are made differently. You may have to unwrap the lens cover, unscrew it, or simply slide it out.

Step 3. Disconnect the ballast

In circular fluorescent bulbs, unplug the ballast from the bulb. For U-shaped bulbs, pull the bulb away from the springs and then release them from the socket. For straight bulbs, give the bulb a twist to release it from its socket.

Step 4. Remove the ballast

If the ballast is not already exposed, remove its cover. At this point, signs of burning, swelling, or leaking are a strong indicator that you need to replace your ballast. If these signs aren’t present, continue to the next step.

Step 5. Use a multimeter

With your multimeter set to ohm, or “Xl1” if there are multiple ohms on your meter, insert one of the probes into the wire connector containing the white wires. Touch the other end to the remaining blue, red, and/or yellow wires attached to the ballast. Your multimeter won’t move if your ballast has gone bad.

To purchase a ballast that’s compatible with your existing light fixture, you’ll need your lamp type, lamp quantity, and line voltage. You can typically find this information on the label attached to your ballast. It’s worth noting that T12 fluorescent tubes have been discontinued because of poor energy efficiency, so finding a replacement ballast for it may be a challenge.

The most common types of lamps and ballasts are T12, T8, and T5, with the T standing for tubular and the number indicating the diameter in 1/8 of an inch. Your light fixture and ballast must match; for example, a T8 ballast must use a T8 bulb.

Fluorescent light can be an ideal solution, especially in larger areas and commercial spaces. However, flickering lights, buzzing, and discoloration can make for a poor work setting. Checking the function of your ballast and keeping fresh bulbs in stock is a good way to keep productivity levels high, without unnecessary strain.

Product Compliance and Suitability

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.