Which relays use a magnetic field to transfer signals via a magnetized element and isolation?

• A. Reed-controlled AC relays
• B. Opto-isolated relay
• C. Triac-based relays
• D. Solid-state relays

Answer: (A)

Reed relays use a magnetic field to move a magnetized element and provide isolation between the control coil and the contacts. In these relays, a coil around a small glass capsule creates a magnetic field when energized. This field pulls together the two ferromagnetic reeds inside the capsule, closing the contacts. When the coil is de-energized, the reeds separate and the contacts open. The coil and the contact circuit are physically separated, so the signal is transferred magnetically rather than electrically, delivering isolation.

The other options don’t rely on a magnetized element for signal transfer. Opto-isolated relays use light to couple the signal across an insulating barrier, not a magnetic field. Triac-based and solid-state relays switch signals using semiconductor devices, with isolation typically provided by optical or other non-magnetic means rather than a magnetic field acting on a magnetized element.