Question

The purpose of the choke in a tube light is to:

• A. Induce low resistance
• B. Induce high voltage
• C. Induce low voltage
• D. Induce high resistance

Hint:

Think about what happens to the voltage across an inductor the instant the current through it is suddenly interrupted.

Answer 1

The Correct Option is B

Step 1: A tube light needs a large momentary voltage spike to ionise the gas inside the tube and start the electric discharge that makes the tube glow. The normal mains supply voltage is not high enough by itself to do this.

Step 2: The choke is simply a coil of wire wound on an iron core, which behaves as an inductor. An inductor opposes any sudden change in the current flowing through it.

Step 3: When the starter switch in the circuit suddenly breaks the current flowing through the choke, the coil resists this sudden change by producing a large back electromotive force, following \(EMF = L \dfrac{di}{dt}\), where a fast change in current produces a very large induced voltage.

Step 4: This momentary high voltage spike is exactly what is needed to ionise the gas inside the tube and strike the arc that lights up the tube. Once the tube is lit, the choke then settles into acting as a current limiting element in series with the tube for normal running.

Step 5: So the choke's main starting purpose is to induce a high voltage, matching option B.