The coil of a potential relay used with small single-phase compressors should have a resistance of approximately:

• A. 1 Ω
• B. 10,000 Ω
• C. 100 Ω
• D. 0.1 Ω

Answer: (B)

A potential relay coil in small single-phase compressors is a sensing element that must energize the relay while drawing only a tiny amount of current from the line. That means the coil should have high impedance so it doesn’t load the circuit or waste power during starting. A coil around ten kilo-ohms achieves this, delivering only a few milliamps of current at typical line voltages (for example, about 12 mA at 120 V or 23 mA at 230 V), which is enough to operate the relay without affecting the motor’s start winding.

If the coil were much lower in resistance, it would waste more power and introduce unwanted heating, potentially disturb the start sequence or trip protective devices. Very high or very low values would either fail to energize reliably or waste power; the ten-kilo-ohm range is a practical balance for a sensing coil. That’s why the coil resistance is approximately ten thousand ohms.