Coil Resistance & Temperature

The copper wire used in relay coils has a positive temperature coefficient of resistance ($0.4\% / ^\circ C$). As temperature rises, coil resistance increases.

If the voltage source is constant (e.g., 5V), an increase in resistance means less current flows ($I = V/R$). Since magnetic force is proportional to Ampere-Turns, less current means less magnetic force.

Design Consideration

At high ambient temperatures (e.g., >85°C), the coil current may drop below the “Must Operate” threshold. MiRelay specifies our relays with a safety margin (Overdrive) to ensure reliable operation across the full industrial temperature range (-40°C to +85°C).