How to Check for a Bad Ballast on a Fluorescent Light Fixture

Written by dale yalanovsky
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How to Check for a Bad Ballast on a Fluorescent Light Fixture
A bad ballast is the most common form of fluorescent light failure. (Fluorescent overhead lamp in office image by Alexey Kuznetsov from

Every fluorescent light fixture uses a ballast to start the bulb. You can think of a ballast as a large transformer that sends an initial burst of voltage to light the bulb. Once lit, the ballast supplies a steady stream of electricity to keep the bulb glowing. All fluorescent tube fixtures have ballasts that power the bulbs, regardless of the length or diameter of the bulbs that are being used.

Skill level:

Things you need

  • Volt meter
  • Ammeter

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  1. 1

    Change the fluorescent light bulbs in the fixture if you notice flickering, the bulbs refuse to light, or they have been in use for some time. This is the simplest and easiest test to determine if you have a bad ballast. Older bulbs on the verge of burning out will mimic the exact same symptoms of a bad ballast, so replace them with new bulbs. When new bulbs are installed but they still flicker or don't light up at all, almost certainly the ballast is bad.

  2. 2

    Check the line voltage with a volt meter. Set the meter to read AC voltage and then read the voltage at the wall receptacle. If the line voltage reads less than 115 volts, the ballast is still good, but there is a short in the main line. Even a small 10 per cent deviation of line voltage will cause a ballast malfunction. If the line voltage is good but the fixture does not work properly, the ballast is defective.

  3. 3

    Use an ammeter to check the output ballast amperage at the light socket. Virtually all ballasts have an amperage specification rating printed on the case that you can use as a reference. If the amperage coming from the ballast is lower than indicated, the ballast itself has malfunctioned or shorted out. This means that the ballast will not allow the correct amperage to reach the fluorescent bulb. As a result, they will either flicker or not light up at all.

Tips and warnings

  • In many cases replacing the entire fixture is more cost effective than just replacing the bad ballast.

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