Essentially, a magnetic ballast is a transformer coil that uses a very simple principle to generate the high startup voltages needed to create an arc in a cold lamp. The coil may be tapped in several places to provide for various input voltages (120v or 240v) and a high start-up voltage. Capacitors are also included to compensate for the inductance of the coil and improve the power factor.

An arc lamp or arc light is the general term for a class of lamps that produce light by an electric arc. The lamp consists of two electrodes, typically made of tungsten, which are separated by a gas. The type of lamp is often named by the gas contained in the bulb; including Neon, Argon, Xenon, Krypton, Sodium, Metal Halide, and Mercury.

The electric arc in an arc lamp consists of gas which is initially ignited by high voltage and therefore becomes electrically conductive. To start an arc lamp, a very high voltage is pulsed across the lamp to "ignite" or "strike" the arc across the gas. This requires an electrical circuit with an igniter and a ballast.

The ballast is wired in series with the lamp and performs two functions; when the power is first switched on, the igniter/starter (which is wired in parallel across the lamp) sets up a small current through the ballast and starter. This creates a small magnetic field within the ballast windings. A moment later, the starter interrupts the current flow from the ballast, the ballast has a high inductance therefore it tries to maintain the current flow (the ballast opposes any change in current through it) It cannot, as there is no longer a "circuit".

As a result, a high voltage appears across the ballast momentarily - to which the lamp is connected. Therefore the lamp receives this high voltage across it which "strikes" the arc within the tube/lamp. The circuit will repeat this action until the lamp is ionized enough to sustain the arc.

When the lamp sustains the arc, the ballast performs its second function to limit the current to that needed to operate the lamp. The lamp, ballast, and igniter are rated "matched" to each other so you must replace these parts with the same rating as the failed component or the lamp will not work.

 


       L.P. Associates, Inc.               

Ballasts