How to choose a control relay: coil voltage (AC/DC), contact rating, 8-pin vs 11-pin sockets, and electromagnetic vs solid-state for panel builders.
A control relay lets a low-power signal — a PLC output, a timer, a pushbutton — operate a higher-power load without the control circuit carrying the load current. Pick the wrong coil voltage and it stays dead; undersize the contacts and they weld shut or burn. This guide covers the parameters on a panel builder's bill of materials.
The relay has two independent sides. The coil side is the low-power control input (often 6–48 V DC or 24–230 V AC). The contact side carries the load — a contactor coil, a solenoid, a lamp, an alarm. Because the two sides are isolated, a 24 V DC PLC can safely switch a 230 V AC load. That isolation is the whole reason relays sit in control gear.
The most common mistake is ordering a 24 V DC coil when the panel runs on 24 V AC, or vice versa. DC and AC coils are not interchangeable — a DC coil on AC hums and runs hot; an AC coil on DC may never pull in. Confirm the control-supply voltage and whether it is AC or DC before the part number is cut.
| Coil | Typical use | Note |
|---|---|---|
| 24 V DC | PLC / DCS output, battery-backed panels | Most common in modern automation |
| 24 V AC | Legacy control transformers | Check 50 / 60 Hz |
| 110–120 V AC | North American control circuits | Confirm 50 vs 60 Hz |
| 230 V AC | European / IEC panels | Often fed from a control transformer |
Also watch the pick-up and drop-out bands: a 24 V DC coil operates down to about 19 V and releases near 10 V, so size it for the worst-case low voltage, not the nominal.
The contact rating is really two numbers. The continuous thermal current (e.g. 10 A) is what the contacts survive carrying. The make/break capacity is what they can actually switch. A contact rated 10 A resistive may only handle 2–3 A of an inductive solenoid. For DC the rating drops sharply because DC arcs do not self-extinguish at an AC zero-crossing, so 10 A AC may fall to 1–2 A DC.
General-purpose relays ship as a relay plus a matching socket. The pin count decides how many contacts you get.
Always order the socket with the relay. A relay with no matching base is a paperweight, and pin pitch is not universal between brands.
An electromagnetic relay uses a coil and armature; it wears mechanically and bounces. A solid-state relay (SSR) switches with semiconductors: silent, fast, no contact wear — but it drops a volt or two (needs a heat sink) and leaks a few milliamps when "off". For high-cycle loads (heaters, valves pulsed thousands of times a day) the SSR wins; for simple interposing the electromagnetic relay is cheaper and simpler. Pick by cycle count and the need for galvanic isolation.
When the relay must protect a motor, the part you want is a thermal overload relay, not a general-purpose control relay. It mounts to the contactor and trips on sustained over-current with a bimetal strip. Size it to the motor full-load current, not the contactor frame size.
ELEHPD stocks LY / MY / MK general-purpose relays, solid-state relays and thermal overload relays in the coil voltages and contact ratings panel builders standardize on, with matching DIN-rail sockets.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.