Relay vs Contactor vs Solid-State Relay: Which One Do You Need?

The short answer

• Control relay – a small coil-operated switch for control circuits and PLC interfacing; contacts are commonly rated 5 to 16 A, usually for resistive loads.
• Contactor – a heavy-duty switch with arc control for motors, heaters and other power loads, rated by utilisation category (AC-1, AC-3).
• Solid-state relay (SSR) – a semiconductor switch: silent, fast, no contact wear; best for resistive loads that switch often.
• Rule of thumb – signals and coils: relay. Motors and big loads: contactor. Heaters switched every few seconds: SSR.

Relay vs contactor: the real difference

Both use a coil to pull in contacts; the difference is what the contacts survive. When a loaded contact opens, an arc forms across the gap, worse with high current and inductive loads. A contactor is built for this, with heavy contacts (often two breaks per pole), arc chambers and firm closing force. A control relay is not, so a relay marked “10 A” is not a 10 A motor starter.

Contactors are covered by IEC 60947-4-1, electromechanical elementary relays by IEC 61810-1, and control-circuit devices by IEC 60947-5-1, which defines AC-15 and DC-13.

What AC-1, AC-3, AC-15 and DC-13 mean

• AC-1 – resistive or slightly inductive loads such as heaters; higher current than AC-3 on the same contactor.
• AC-3 – squirrel-cage motors started and switched off while running. Size motor contactors by this (the kW-to-amps chart helps); jogging and plugging (AC-4) need a larger one.
• AC-15 – control of AC electromagnetic loads such as contactor coils and solenoid valves. It applies when a relay switches a coil, and is often well below the relay’s headline current.
• DC-13 – the same for DC electromagnets. DC ratings are far lower because a DC arc has no current zero to help extinguish it: a contact that switches 10 A at 250 V AC may manage that current only at about 30 V DC.

For safety circuits, look for mirror contacts (a normally closed auxiliary contact that cannot close if a main contact has welded) or forcibly guided relay contacts. Ordinary relays are not safety devices; safety functions need certified safety relays or modules.

Coil voltage: 24 V DC, 24 V AC, 110 V or 230 V AC

Match the coil to the control supply in type and value. An AC coil on DC usually burns out, because only its winding resistance limits the current; a DC coil on AC can hum, chatter or fail to pull in.

• 24 V DC – the usual choice with PLCs: extra-low voltage, silent, and a regulated supply keeps the coil steady through moderate mains sags (see how to size a 24 V DC power supply).
• 230 V AC – a common control voltage in Pakistani panels, taken phase to neutral. Never put a 230 V coil across two phases (400 V).

Coils are typically specified to pull in at 80 to 85% of rated voltage or above (roughly 185 to 195 V on a 230 V coil), so prefer regulated 24 V DC where the supply sags.

Why a PLC output usually drives an interface relay

Many PLC transistor outputs are rated for only a few hundred milliamps per point and relay outputs for about 2 A (check the datasheet), while an AC contactor coil draws several times its holding current as it pulls in. A plug-in interface relay fixes this:

• Current – its coil needs little current; its contacts carry the load.
• Separation – the 24 V DC PLC circuit stays apart from 230 V AC.
• Protection – the contactor coil’s inrush and switch-off spike hit the relay contacts, not the PLC output.

Coil suppression: diode, RC snubber or varistor

Cutting the current in a coil makes the collapsing magnetic field produce a spike far above the supply voltage, which can destroy a transistor, triac or SSR output, erode contacts and upset PLCs. Fit suppression across each coil, including interface relay coils:

• DC coil: diode, cathode (banded end) to the positive terminal. It clamps the spike but slows drop-out; add a Zener in series, or use a varistor, if release time matters. Fitted backwards, it short-circuits the supply.
• AC coil: RC snubber or varistor. Never fit a diode across an AC coil: it conducts on half of each cycle and short-circuits the supply. Clip-on suppressor modules are made for specific coil voltages.

Solid-state relays: strengths and limits

An SSR switches with a thyristor or triac (AC loads only) or a transistor or MOSFET (DC loads), driven through an isolating coupling from a low-current input. With no coil or moving contacts it is silent, fast and free of arcing, which suits heaters run by a temperature controller. SSRs are in our relays & accessories range.

Limits to check before you buy

• Heat – a thyristor-based AC SSR typically drops about 1 to 1.6 V when on, and heat is that voltage times the load current: 25 A at 1.2 V is about 30 W. Use a heat sink and the datasheet derating curve.
• Leakage, no air gap – an AC SSR typically passes a few to over ten milliamps when off, enough to keep a small coil or indicator lamp faintly energised (a resistor across the load, sized from the SSR datasheet, is the usual fix; use a power resistor, as it dissipates real heat on 230 V, about 5 W for 10 kΩ). It is not an isolating device: the load may be live when it is off.
• Failure and protection – SSRs usually fail shorted, leaving the load on, so add an independent over-temperature cut-out on heaters. Their outputs also have a low I²t withstand, so datasheets typically call for a fast semiconductor fuse.

Zero-cross or random turn-on?

Zero-cross SSRs switch on near zero voltage, which limits inrush and noise and suits resistive heaters; turn-on can wait up to half a cycle (10 ms at 50 Hz). Random-fire SSRs switch at any point in the cycle, for phase-angle control and some inductive loads, with more noise. For motors use a contactor, unless the SSR maker gives motor-load guidance.

Load types, inrush and derating

• Resistive (heating elements) – steady current, the easiest case.
• Inductive (motors, solenoids, transformers) – stored energy prolongs the arc, and a motor typically draws five to eight times its full-load current when starting. Use AC-3 for motors, AC-15 or DC-13 for coils.
• Lamp and capacitive (incandescent lamps, LED drivers, capacitor banks) – a brief, very large inrush; a cold incandescent filament can draw roughly 10 to 15 times its running current. Contactor catalogues give separate ratings for lamps (AC-5a, AC-5b) and capacitor banks (AC-6b).

Ratings assume a reference ambient temperature, and panels in Pakistan often reach 40 to 50 °C in summer. Read the derating curve, space devices apart and ventilate the enclosure; SSRs are the most temperature-sensitive.

How to choose: a 6-step checklist

1. Load and device – signals and coils: relay. Motors and heavy loads: contactor. Frequent resistive switching: SSR.
2. Rating – use the right category (AC-3 motors, AC-1 heaters, AC-15 or DC-13 coils) or the DC rating for DC loads, with margin for inrush.
3. Voltage – the 230 V or 400 V load within the contact or SSR rating (plug-in relays are commonly rated 250 V AC, which does not cover 400 V between phases, and a 240 V-class SSR does not either), with the AC or DC type matched to the load.
4. Coil or input – type and value matched to the control supply, with suppression fitted.
5. Contacts and accessories – NO, NC or changeover, poles, socket and auxiliary blocks; check the minimum load for tiny signals.
6. Protection and heat – overload and short-circuit protection, a heat sink for SSRs, and derating for 40 to 50 °C panels.

Browse relays & accessories and contactors & motor starters, or the brand pages for Omron, Schneider Electric and Siemens. For the buttons and lamps that drive these coils, see our push button and indicator lamp guide. Send us the coil voltage, the load and the old part’s label on WhatsApp and our team will recommend a match.