Ballast Function

Part of Lighting

How ballasts limit and stabilize current in arc lamps, fluorescent tubes, and other negative-resistance discharge devices.

Why This Matters

Any light source that operates by electrical discharge — arc lamps, fluorescent tubes, mercury vapor lamps, sodium lamps — has a critical electrical property: negative resistance. Once ignited, as voltage across the device increases, current increases faster than voltage would suggest, and the effective resistance drops. Left to itself, connected directly to a voltage source, current would rise without limit until the device or the circuit fails.

A ballast prevents this. It provides series impedance that limits current to the design value and keeps the discharge stable. Without a proper ballast, any discharge lamp burns too brightly for a fraction of a second before destroying itself. With a proper ballast, the lamp operates at design current, design brightness, and design life.

Understanding ballast function allows you to build or salvage appropriate ballasts for any discharge lighting system, diagnose failures, and design circuits that operate lamps at correct conditions even when standard components are unavailable.

Why Discharge Devices Have Negative Resistance

In a resistor, voltage and current are proportional: V = I × R. In a gas discharge, the relationship is fundamentally different. Once the gas is ionized and conducting, the voltage required to sustain the arc is nearly constant regardless of current — it depends on the arc length and gas type, not on how much current flows. Increasing current through the same arc gap does not require increasing voltage; if anything, more current heats the gas more, increases ionization, and the arc sustains at slightly lower voltage.

The V-I curve of an arc slopes backward: higher current at lower voltage. This means if the circuit has low external impedance, a small disturbance that increases current produces conditions that allow even more current, which increases further in a positive feedback loop — runaway.

The ballast breaks this runaway by inserting impedance in series. The ballast’s voltage drop increases with current. When current tries to increase, more voltage is dropped across the ballast, leaving less available for the arc, which limits the current. The system finds a stable equilibrium.

Resistive Ballasts: Simple but Lossy

The simplest ballast is a series resistor. For a DC arc at design voltage V_arc and design current I_arc, with supply voltage V_supply:

R_ballast = (V_supply − V_arc) / I_arc

For a 60 V supply, 20 V arc, 5 A design current: R = (60 − 20) / 5 = 8 ohms. Power dissipated in ballast: 8 × 25 = 200 W — equal to the power in the arc itself. Total system efficiency is 50%.

Despite the waste, resistive ballasts are often the right choice for DC arc lamps because they are simple, durable, and built from wire or rebar. A resistive ballast is just a length of resistance wire wound on a ceramic former. For 8 ohms at 5 A: use 1.6 mm diameter nickel-chrome (Nichrome) wire with resistance of about 0.5 ohm/meter — need 16 meters. Wind on a ceramic cylinder, space the turns to prevent shorting. This ballast runs at elevated temperature (200 W in a small volume) — mount in a ventilated location and keep clear of combustibles.

Alternative resistive ballast material: saline water (salt water) in a tank. The resistance is adjustable by varying salt concentration. Used historically as a simple, adjustable ballast for arc welding. The water also provides cooling. Practical for experimental use; too messy and corrosion-prone for permanent installations.

Inductive Ballasts: Efficient for AC

For AC discharge lamps, an inductor (choke) is a far more efficient ballast. An ideal inductor drops voltage proportional to current (V = L × di/dt) but dissipates no power — voltage and current are 90° out of phase, and power (V × I × cos(θ)) is zero. Real inductors have some resistance in the wire, but a well-designed inductor has efficiency of 95–98% — far better than a resistor.

An inductive ballast is simply a coil of insulated wire wound on a laminated iron core. The iron core increases inductance for a given wire length and provides good magnetic coupling. For a fluorescent lamp ballast, a T-core (toroidal) or E-I core transformer steel assembly works well.

Design parameters: the inductance needed depends on the operating frequency and the required current drop. For a 50 Hz system, the inductive reactance XL = 2π × 50 × L = 314L ohms. For the same example (drop 40 V at 5 A): XL = 40/5 = 8 ohms, L = 8/314 = 25 mH. Wind enough turns on the iron core to achieve this inductance.

The inductive ballast causes the current to lag the voltage — the power factor of the lamp circuit is less than unity. For a ballasted fluorescent lamp with purely inductive ballast, power factor is typically 0.4–0.6. This means the apparent power (volt-amps) drawn from the supply is substantially higher than the real power (watts), requiring larger wiring and generators. A power factor correction capacitor in parallel with the lamp restores power factor toward unity, reducing reactive current demand.

Starting and Ignition

Most discharge lamps cannot self-start from zero. The gas must be ionized initially, which requires a voltage much higher than the sustaining arc voltage. This starting or ignition voltage is typically 2–10 times higher than the operating voltage.

For fluorescent lamps: the starter (a small glow discharge switch) briefly shorts the lamp electrodes through the ballast, driving a surge of current through the electrode coils (pre-heating them to electron-emitting temperature). When the starter opens, the collapsing magnetic field in the inductive ballast produces a high-voltage spike that ignites the mercury vapor. The starter then remains open; the lamp ballast limits current during normal operation.

For carbon arc lamps: the electrodes must be touched together (creating a short circuit through the ballast) and then drawn apart to start the arc. The hot spot left by the short-circuit provides the initial ionization. Electromagnetic or manual means accomplish this.

For high-pressure mercury and sodium lamps: an external high-voltage ignitor (producing 1–5 kV pulses) is required, superimposed on the operating voltage. This ignitor fires at start-up until the lamp reaches operating temperature and the arc self-sustains.

Electronic Ballasts

An electronic ballast uses high-frequency switching circuits to power the lamp at 20–50 kHz instead of line frequency (50–60 Hz). Advantages: no flicker (high-frequency flicker is invisible), much lighter and smaller than iron-core inductors, and better efficiency. Disadvantages: requires semiconductor components (transistors, integrated circuits) that cannot be manufactured without advanced electronics facilities.

For a rebuilding civilization, electronic ballasts are obtained, not built, in the early recovery phase. Iron-core inductive ballasts and resistive ballasts are fully buildable from local materials and are the appropriate primary ballast technology. As electronics manufacturing capability develops, electronic ballasts become achievable.

Practical note: salvaged fluorescent fixtures from buildings almost always have ballasts. Both magnetic (iron core) and electronic types are useful. Test before discarding — a magnetic ballast that hums slightly and works correctly is perfectly serviceable even if less efficient than modern types. An electronic ballast can sometimes be repaired if the failure is a blown fuse or a single failed transistor.

Diagnosing Ballast Problems

A lamp that does not light but shows correct supply voltage: check the ballast — it may be open-circuited (winding broken). A ballast winding can be checked with an ohmmeter: DC resistance should be several to tens of ohms for a typical fluorescent ballast. Open circuit (infinite resistance) indicates a broken winding.

A lamp that lights very dimly or flickers excessively: ballast inductance may have dropped due to core saturation (iron core saturated by DC component from unbalanced circuit) or partial short-circuit in the winding. Replace or rewind.

Overheating ballast: usually indicates the lamp is drawing more current than designed. Check the lamp wattage against ballast rating. A burned or aging lamp may have changed its characteristics and be overloading the ballast. Replace the lamp first; if the ballast is still overheating, it may have internal partial shorts increasing its losses.

A ballast that buzzes loudly: iron core laminations vibrating at 100 Hz (double line frequency). May indicate the lamination stack has loosened (retighten bolts) or that the ballast is being operated above its voltage rating, increasing magnetic flux and vibration amplitude.