Relay Coil Power Calculator
Calculate relay coil current, power consumption, and required driver transistor specifications from coil voltage and resistance.
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:
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Coil current (Ohm’s Law): I = V ÷ R Where V = coil supply voltage (V), R = coil resistance (Ω)
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Power consumption: P = V × I = V² ÷ R = I² × R (in Watts)
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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:
| Relay Type | Coil Voltage | Coil Resistance | Coil Current |
|---|---|---|---|
| 5V mini relay | 5V DC | 70–90Ω | 55–70 mA |
| 12V auto relay | 12V DC | 80–160Ω | 75–150 mA |
| 24V industrial | 24V DC | 250–600Ω | 40–96 mA |
| 5V SRD relay | 5V DC | 71Ω | ~70 mA |
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 — this can become significant in enclosed enclosures 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. hold voltage: Most relays require a higher pick-up voltage to initially engage than the minimum hold voltage to remain engaged. This is typically 75–80% of rated voltage for pick-up and 10–25% of rated voltage for hold.