The coil of a current relay used with small single-phase compressors should have a resistance of approximately 0.1 Ω. Which of the following is closest to that value?

• A. 0.01 Ω
• B. 0.1 Ω
• C. 1 Ω
• D. 10 Ω

Answer: (B)

Coil resistance in a current-relay used with small single‑phase compressors is kept very low so the relay can sense the motor current without wasting significant power or dropping much voltage in the circuit. With typical running currents of a few amperes, a resistance around 0.1 Ω gives only a small voltage drop (I × R) and a small amount of heat, which is acceptable for a compact relay coil. For example, at 5 A the drop is about 0.5 V and the heat is around 2.5 W—easy to manage in a small device. If the resistance were much higher, like 1 Ω or 10 Ω, the motor would lose more voltage and heat up more, which isn’t desirable. If it were much lower, like 0.01 Ω, the coil would draw even less power, but the designed magnetic operation of the relay wouldn’t be as reliably maintained. Therefore, approximately 0.1 Ω is the closest and appropriate value.