By Brian Bennett, updated March 24, 2022
The pull‑in voltage is the minimum voltage that must be applied to an electromagnetic relay coil to attract the armature and close the contacts. It is always higher than the drop‑out voltage, the voltage required to release the armature and open the contacts.
At rest, a small air gap separates the relay armature from the coil core, leaving the magnetic circuit open. The pull‑in voltage must drive enough current through the coil to overcome this reluctance and close the circuit. Once the armature moves into contact with the core, the magnetic circuit closes, creating a magnetic resistance (reluctance) that must be overcome for the relay to release. This is why the drop‑out voltage is typically lower than the pull‑in voltage.
Both the pull‑in and drop‑out voltages correspond to specific coil currents. The operating current is simply the voltage across the coil divided by its resistance:
I = V / R
where I is the coil current, V is the applied voltage, and R is the coil resistance.
Because the relay is a current‑driven device, its performance shifts with temperature. The coil resistance increases as temperature rises, altering the voltage required to achieve the same current. Designers must account for this temperature coefficient when specifying operating voltages.
When operated within its specified limits, an electromagnetic relay offers dependable performance for years. Understanding pull‑in and drop‑out characteristics helps ensure that relays remain within safe operating parameters and maintain circuit reliability.
