Introduction

The power relay is the part that stands between the control electronics and the load, and it decides whether a machine, an appliance or a vehicle module switches reliably for years. Unlike a signal relay, a power relay must carry a high current and survive the inrush of a motor, a lamp or a capacitive load, and the contact material and the contact design are what make that possible. This application note explains contact rating, inrush margin and coil drive for Panasonic JS and ACT power relays in appliance, industrial and automotive designs.

The Inrush Problem

The inrush current is the design point that most often trips up a relay choice. A cold lamp filament has a low resistance and draws many times its steady current for a few milliseconds; a motor draws a locked-rotor current at start; an uncharged capacitor draws a large charging current. A relay that is rated only for the steady current will weld or erode its contacts under that inrush, and the failure appears as a relay that sticks on or fails to make contact. The datasheet inrush figure must be checked against the real load profile, not just the resistive rating.

Contact Material

The contact material decides the inrush capability. Silver-tin-oxide is a robust material that resists welding and erosion, which is why it is used in the JS series for appliance and industrial loads. A silver alloy with a copper clad layer, as in the ACT automotive series, handles the capacitive inrush of a lamp or a module in a vehicle. The contact gap and the arc-quenching design also matter, because the arc that forms when the contacts open erodes them over the life of the relay.

Coil Drive

The coil voltage is chosen to match the control supply, and the pick-up voltage is up to 75 percent of nominal, so the drive must supply enough voltage even when the supply sags. The coil current is modest but not negligible, so the drive transistor is sized for it, and a flyback diode across the coil absorbs the turn-off spike that would otherwise stress the transistor. For a circuit that must hold state during a power loss, a latching relay keeps its position without holding power.

Sealing and Life

A sealed relay can be washed after soldering, while a flux-resistant relay cannot, so the sealing follows the assembly process. The expected life is set by the load profile: a relay that is correctly rated for the current and the inrush will last the life of the product, while an under-rated relay fails early. The BeiLuo FAE team helps with the contact, inrush and coil selection and validates the choice on the bench.