How to Check a Ballast

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Things You'll Need

  • Owner's manual

  • Replacement bulb (if needed)

  • Screwdriver

  • Multimeter

Check the ballast to find out why your lights don't work.

The ballast of a light fixture ignites the vapor inside the bulb. Depending on the type of ballast you use, the light may start immediately or take a few seconds to warm up to full brightness. Newer bulbs and ballasts have quick-start technology that uses a small internal transformer to generate enough current to heat the filaments of the bulb. When your lightbulbs will not turn on after the proper warm-up period, troubleshoot the ballast before replacing the entire light fixture.


Step 1

Check the circuit breaker switch to make sure the light fixture is getting electrical power. Reset the switch if it has been tripped and change any blown fuses. If you are testing a portable fluorescent light fixture, plug in another appliance to see if the wall outlet works properly. If the second appliance has no power, have an electrician inspect the outlet.


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Step 2

Ground the ballast according to the instructions in the owner's manual or installation guide. This usually involves attaching the ground wire to a screw on the metal portion of the ballast. For portable light fixtures, make sure to use a three-pronged wall outlet.

Step 3

Inspect the bulb and replace it if you see burned or blackened spots. If you are installing a tube-style fluorescent bulb, be sure the pins on each end are in contact with the ballast's socket. Only use lightbulbs that are the proper style and voltage rating for your ballast.


Step 4

Turn off the circuit breaker switch or unplug the ballast's electrical cord. Tighten all wiring connections with a screwdriver and check the condition of the wires. Replace the wires if they are cracked or frayed. Use a scrub brush to remove any debris from the connection points. Plug the light fixture back into the wall outlet or switch the circuit breaker back on to restore power.


Step 5

Test the ballast's voltage with a multimeter. Attach the multimeter's leads to the sockets at the ends of the ballast. If the reading is not between five and nine volts, replace the ballast. Adjust the multimeter to measure frequency and replace the ballast if the reading is not between 20 and 50 kilohertz.


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