Relay (electromechanical)
Relay
An electrically operated switch: a small coil current magnetically flips heavy-duty contacts, letting a low-power signal control a high-power load with isolation.
Schematic symbol
Relay
How relay (electromechanical) appears in a circuit diagram.
Types & variants
Key specs
Coil voltage
VWhat energises it: 5V, 12V, 24V DC (or AC types).
Coil current/resistance
mA / ΩDrive current the coil needs - usually needs a transistor.
Contact rating
A @ VLoad it can switch, e.g. 10A 250VAC / 10A 30VDC.
Contact form
-SPST-NO, SPDT (COM/NO/NC), DPDT.
Laws that govern it
- Ampère's circuital law∮ B · dl = μ₀ × I(enc)
Coil current creates the field; N × I ampere-turns is what decides whether the armature actually pulls in.
- Hopkinson's law — reluctance formulaMMF = Φ × S, S = l / (μ₀μᵣA)
MMF = Φ × S explains the click: as the gap closes, reluctance drops, flux jumps and the pull-in becomes decisive rather than gradual.
- Lenz's lawthe minus sign in EMF = −N × dΦ/dt
Switch the coil off and the collapsing field produces hundreds of volts — the flyback diode across every relay coil exists for this law alone.
- Ohm's lawV = I × R
Coil current is simply V/Rcoil: a 12 V, 400 Ω coil draws 30 mA, which is what your driver transistor must handle.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Relay vs Solid-State Relay (SSR)
Side by side with Solid-State Relay (SSR) — specs, variants and which one the job calls for.
Markings
Coil voltage and contact rating printed on the case; pin diagram on top or datasheet.
Standard values
5V/12V SPDT 10A 'sugar cube' (SRD-05VDC-SL-C), 12V automotive, SSRs 2-40A.
How to choose
Match coil voltage to your control rail; contact rating above the load (derate for DC/inductive). Choose contact form (NO/NC). Use an SSR for fast, silent, frequent switching.
Pinout & package
Coil + changeover contact (SPDT)
Check first: Sugar-cube pin positions vary by part, so identify the coil with a multimeter: it reads a few tens to a few hundred ohms, while COM–NC reads near zero and COM–NO reads open. A flyback diode across the coil is mandatory when driving from a transistor.
Coil (2 pins) + contacts (COM/NO/NC). ALWAYS add a flyback diode across the coil and drive via a transistor/driver, not directly from an MCU pin.
Example circuits
- MCU + transistor + relay switching a mains lamp
- Latching control with NO/NC contacts
- Automotive accessory switching
- SSR for a heater (PWM)
Common failures
Contacts arc, pit or weld shut on overload/inductive loads; coil opens; chatter from a weak supply; MCU damage if flyback diode is missing.
How to test
Apply rated coil voltage - it should click and COM-NO go to ~0Ω (COM-NC opens). Continuity-check contacts. No click = open coil or low voltage.
Substitutes
Same coil voltage, equal/greater contact rating, matching form. SSR can replace an EMR if load type suits.