How Do the Ballast and Bulb Relate?

The ballast bulb relationship of a fluorescent light is complicated but this article will explain the basics. The ballast of a fluorescent light is the electronic part situated between the actual light and the base which screws into the socket. Regular tube style fluorescent lights can be taken apart to replace individual components and those are what this article discusses. 

How Fluorescent Lights Work

When the light switch is turned on, the current passes through the left hand prong of the fixture plug, through the ballast, through one of the filaments, through the switch in the starter, through the other lamp filament and out the right prong of the base.

As the current flows, it heats the two small end elements of the tube, then the starter opens and the current flows through the lamp which ionizes the gas in the fluorescent tube. 

What the Ballast Is

The current through the tube is adjusted by the ballast, which is a magnetic coil. The ballast makes a surge of current when the starter opens, then keeps it flowing at the correct rate. Regular fluorescent lights provide more light by having two lighting tubes. There are individual starters and ballasts for each tube; both the ballasts are usually built into the one case.

Basically the ballast limits the current and provides the starting kick to ionize the gas. Older lights use iron ballasts, new ones use the smaller ballasts that are an integrated high-frequency inverter/switcher. The smaller ballast tends to reduce or eliminate the buzz and flicker associated with older types of fluorescent lights. 

Different Types of Fluorescent Lights

Regular long tube fluorescent lights have separate parts so you are able to replace one part, such as the starter or the tube, without needing to buy a whole new light. Compact home fluorescent lights on the other hand, come as one whole unit and cannot be taken apart. 

Matching the Ballast to the Light Tube

Generally, the ballast and lamp must be fairly closely matched in diameter and wattage. There are a number of different types of ballasts and also different bulbs. This will not bother those with compact lights but may be of interest to those installing regular tube lights. Instant start bulbs/tubes cannot be used with rapid start ballasts.

Ballasts are designed to work at a particular wattage, size and type. When you shop, take the old ballast with you. The sockets come in different types also, which is another reason you should take the old one with you when buying the new.

Replacing a tube with one for a different wattage to that for the ballast can result in a fire hazard, even it they physically fit together. Don’t replace 40 watt tubes with energy saving 34 watt tubes, for example. The new tubes and ballast must be matched  and simply installing the wrong tubes will result in excessive current flow and over heat the ballast. You have to check the wattage carefully as some 40 watt 'shop' lights are actually duel 25 watt fixtures.