Should I use a relay or a contactor?
Both are coil-operated switches; the difference is scale and breaking capability. A relay is for auxiliary circuits with low and medium current that open and close over short periods. A contactor has higher continuous current, a larger contact surface and a structure aimed at extinguishing the DC arc. Continuous current, inrush current and especially arc-quenching capability in DC decide the choice.
Answer three questions in order when deciding.
1. How much continuous current? Automotive-type relays are typically for loads in the tens of amperes. On motor, crane or charging lines reaching into the hundreds of amperes, a contactor is preferred.
2. DC or AC? This is the most frequently skipped point. In AC the current passes through zero every half cycle and the arc extinguishes by itself. In DC there is no such zero crossing; the arc hangs between the contacts. That is why a contact's AC rating is not valid for DC. If you are switching a DC load, look for the component's DC rating.
3. What is the switching frequency and load type? Inductive, high-inrush loads such as motors wear contacts out faster. On a frequently switched line, one step up in size extends life.
Rules common to both cases:
- A reverse diode or a suitable suppression component is fitted across the coil; otherwise the high voltage generated at interruption damages the driver output.
- The cables running to the power contacts are calculated separately; choosing a relay does not remove the need for a cross-section calculation.
- The voltage drop measured across the contact under load is expected to be below 0.2 V; above 0.5 V indicates contact wear.
Full coverage of the topic: Installation, redundancy and field continuity