A relay uses a small control current through a coil to move contacts that switch a separate circuit. The coil and the contacts are electrically isolated, so a low-power control circuit can switch a much larger load. A contactor is essentially a heavy-duty relay designed for frequent switching of power circuits, with robust arc-handling contacts and usually an overload protection arrangement.
Coil drive
Relay coil current
Problem. A relay coil is rated 12 V and measures 120 Ω. What current does it draw, and what power does it dissipate?
Solution. I = 12 ÷ 120 = 0.100 A = 100 mA. P = V × I = 12 × 0.100 = 1.2 W.
The contacts are marked normally open (NO), normally closed (NC) and common (COM). The contact rating applies to a stated load type; a rating for a resistive load does not automatically transfer to a motor, a lamp with high inrush current or a capacitive load.
Inductive loads and flyback
When the coil current is interrupted, the collapsing magnetic field induces a voltage that can be far higher than the supply voltage. A flyback diode across a DC coil provides a path for that current and protects the driving transistor. The diode must be rated for the coil current and for the reverse voltage it will see, with margin.
- Coil voltage and resistance determine the control current.
- NO contacts close when energised; NC contacts open when energised.
- Contact ratings are load-dependent and must be checked against the real load.
- A flyback diode is required across a DC relay coil driven by a semiconductor.
- A solid-state relay has no moving contacts but still has leakage and surge limits.
- A relay isolates a small control circuit from a switched load.
- Coil current is V/R and coil power is V × I.
- Contact ratings depend on the load type.
- A flyback diode protects the driver from coil inductive kick.
Worked example
A 24 V relay coil with 480 Ω draws 24 ÷ 480 = 50 mA and dissipates 24 × 0.050 = 1.2 W. A flyback diode rated above the coil current and reverse voltage is placed across the coil.