Unit – 9 Electrical circuits and Electronics | choke coil

Choke Coil – Principle, Working and Uses | TRB Physics Notes

A choke coil is an important component used in AC circuits to control current without much loss of power. Questions related to choke coil are frequently asked in TRB, BRTE, and other competitive examinations.

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1. What is a Choke Coil?

A choke coil is a coil of insulated copper wire wound on a soft iron core, having very small resistance and large inductance.

It is mainly used to reduce the AC current without appreciable loss of power.

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2. Principle of Choke Coil

A choke coil works on the principle of self-induction.

When alternating current flows through the coil, a back emf is produced which opposes the change in current. This back emf limits the current in the circuit.

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3. Construction of Choke Coil

• Made of insulated copper wire
• Wound on a laminated soft iron core
• Large number of turns
• Very small resistance
• High inductance

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4. Working of Choke Coil in AC Circuit

When a choke coil is connected in series with an AC supply:

• Inductive reactance (X_L) limits the current
• Power loss is very small due to negligible resistance
• Current lags voltage

Inductive reactance:

X_L = 2πfL

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5. Power Loss in Choke Coil

Since a choke coil has very small resistance, the power loss is minimal.

Power loss:

P = I²R

As R is very small, P is also very small.

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6. Choke Coil vs Resistor

Feature	Choke Coil	Resistor
Principle	Self-induction	Ohm’s law
Power loss	Very small	Large (heat loss)
Energy storage	Magnetic field	No energy storage
Efficiency	High	Low
Use in AC circuit	Preferred	Not preferred

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7. Advantages of Choke Coil

• Consumes very little power
• High efficiency
• Reduces AC current effectively
• Used in high-voltage AC circuits

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8. Applications of Choke Coil

• Fluorescent lamps
• AC power supplies
• Radio circuits
• To control current in AC circuits

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9. Shortcut Tricks for TRB Exams

• Choke coil → AC only
• Low resistance → Low power loss
• Works on self-induction
• Used instead of resistor in AC circuits
• Soft iron core → High inductance

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10. TRB Level MCQs (10 Questions)

1. A choke coil is used to:
   (A) Increase voltage
   (B) Decrease frequency
   (C) Reduce AC current
   (D) Convert AC to DC
   Answer: (C)
2. A choke coil works on the principle of:
   (A) Mutual induction
   (B) Self-induction
   (C) Electromagnetic induction
   (D) Resistance
   Answer: (B)
3. Power loss in choke coil is low because:
   (A) High resistance
   (B) Low inductance
   (C) Low resistance
   (D) High voltage
   Answer: (C)
4. Choke coil is mainly used in:
   (A) DC circuits
   (B) AC circuits
   (C) Rectifier circuits
   (D) Battery circuits
   Answer: (B)
5. The core of a choke coil is usually made of:
   (A) Steel
   (B) Copper
   (C) Soft iron
   (D) Aluminium
   Answer: (C)
6. Inductive reactance depends on:
   (A) Voltage
   (B) Resistance
   (C) Frequency
   (D) Power
   Answer: (C)
7. Energy in choke coil is stored in:
   (A) Electric field
   (B) Magnetic field
   (C) Thermal field
   (D) Gravitational field
   Answer: (B)
8. A choke coil is preferred over a resistor because:
   (A) It consumes more power
   (B) It stores energy
   (C) It wastes less power
   (D) It increases resistance
   Answer: (C)
9. Power factor of choke coil circuit is:
   (A) 1
   (B) 0
   (C) Lagging
   (D) Leading
   Answer: (C)
10. Choke coil is used in fluorescent lamp to:
    (A) Increase current
    (B) Control current
    (C) Reduce voltage
    (D) Increase frequency
    Answer: (B)

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Exam Tip: In TRB exams, choke coil questions are mostly conceptual. Remember low resistance, self-induction, and low power loss points.