Relay Coil Power Calculator

A relay is an electrically operated switch. When current flows through the relay coil, it generates a magnetic field that moves a mechanical armature to open or close one or more switch contacts. The coil is essentially a resistive-inductive load, but for DC relay calculations, the DC resistance dominates.

Key formulas:

1. Coil current (Ohm’s Law): I = V ÷ R Where V = coil supply voltage (V), R = coil resistance (Ω)

2. Power consumption: P = V × I = V² ÷ R = I² × R (in Watts)

3. Driver transistor requirements: The transistor (or MOSFET) driving the relay must handle at least the coil current with a safety margin.

   • Minimum transistor current rating = I × 1.5 (50% safety margin)
   • For BJT: base current = I_coil ÷ h_FE (gain), where h_FE ≥ 100 for saturation

Flyback diode: A relay coil is inductive. When current is switched off, the collapsing magnetic field generates a large voltage spike (back-EMF) that can destroy the driving transistor. Always place a flyback (freewheeling) diode in reverse-bias across the coil terminals. A 1N4007 or 1N4148 is typically sufficient for small relays.

Typical relay coil specifications:

Heat dissipation: All relay coil power is dissipated as heat in the coil winding. For always-on relay applications, calculate total heat load. A 12V relay at 100 mA dissipates 1.2W, which adds up in a sealed enclosure packed with multiple relays.

Relay contacts rating (separate from coil): The coil power is separate from the contact current rating. A relay with a 70mA coil may switch 10A on its contacts. Never confuse coil specifications with contact specifications.

Pick-up voltage vs. drop-out voltage: Two different thresholds, and they are much further apart than people expect. Pick-up (or “must operate”) voltage is the level at which the armature actually pulls in, typically 75 to 80% of the rated coil voltage. Drop-out (or “must release”) voltage is the level it falls back out at, and datasheets usually put it at 10 to 25% of rated.

The gap between them is deliberate hysteresis. It stops a relay chattering on a sagging supply. It also means a relay already energized will happily stay energized on a supply that could never have closed it in the first place, which is a real source of confusion when a circuit works after a warm restart but not from cold.

One coil, one voltage: A relay coil is wound for a single rated voltage. Feeding a 12V coil from 24V roughly doubles the current and quadruples the dissipation, and it will cook the winding, usually within minutes rather than hours. The comparison table in the result panel exists to show that relationship, not to suggest the coil can be run at those voltages.