Electromechanical Relay

How a relay works

Strip a relay to its essentials and you find five parts: a coil of fine wire wound on an iron core, a hinged iron armature, a return spring, a set of contacts, and a frame (the yoke) that completes the magnetic circuit. Push current through the coil and it becomes an electromagnet. The field threads through the core, jumps the small air gap to the armature, and pulls the armature toward the pole face. The armature is mechanically coupled to a movable contact, so as it swings in it presses that contact against a fixed one, completing the load circuit.

The crucial point — the whole reason relays exist — is that the coil circuit and the contact circuit are electrically separate. Energy crosses from one to the other only as a magnetic field across an air gap. This is galvanic isolation. A 5 V logic signal drawing 30 mA can therefore switch a 230 V, 10 A mains heater, and a fault on the heater side cannot push dangerous voltage back into your microcontroller.

The force that pulls the armature

The attractive force across the air gap between core and armature is set by the magnetic flux density B in that gap and the pole-face area A:

F = (B² · A) / (2 · μ₀)

where:
  F   = pull force across the air gap (N)
  B   = flux density in the gap (T)
  A   = pole-face area (m²)
  μ₀  = 4π × 10⁻⁷  (T·m/A)

and the flux is driven by the magnetomotive force:

  MMF = N · I        (ampere-turns)
  B   ≈ μ₀ · N · I / g    (g = air-gap length, m)

Two facts fall straight out of these equations and explain almost everything a relay does. First, force scales with the square of flux, so it is highly nonlinear in coil current. Second, B is inversely proportional to the air-gap length g. When the armature is fully open, g is large, B is small, and the force is weak; the relay needs a healthy current to start moving. But the instant the armature begins to close, g shrinks, B climbs, and the force grows explosively — so the armature does not creep shut, it snaps shut. That snap is what gives a clean, bounce-limited contact closure and the audible click.

Pull-in, drop-out and hysteresis

Because force depends on the gap, the voltage to close a relay is much higher than the voltage at which it will release. This is the relay's built-in hysteresis, and it is a feature, not a defect — it stops the armature chattering near the threshold.

Parameter	Typical value	What it means
Pull-in (operate) voltage	~75% of rated coil V	Minimum voltage that seats the armature against the spring
Drop-out (release) voltage	~10–20% of rated coil V	Voltage you must fall below for the spring to reopen contacts
Must-operate guarantee	≤ rated coil V	Manufacturer guarantees pull-in at or below nameplate voltage
Coil resistance (12 V relay)	~320–400 Ω	Sets coil current ≈ 30–40 mA at 12 V
Coil power	~200–500 mW	Holding power once seated

A subtle consequence: because seated force is large, you can often hold a relay closed at far below its pull-in voltage. Latching schemes and "economizer" coils exploit this by pulling in at full voltage, then dropping to a lower hold voltage to save power and heat.

Contact arrangements and ratings

Relay contacts are described by their pole and throw count, using the same notation as manual switches. A pole is an independent switch the relay operates; a throw is a position each pole can connect to.

• SPST-NO (Form A): single contact, normally open — closes when energized. The simplest power switch.
• SPST-NC (Form B): single contact, normally closed — opens when energized. Used for fail-safe cutoffs.
• SPDT (Form C): a common terminal that transfers between a normally-closed and a normally-open contact. The most versatile small relay.
• DPDT / 4PDT: two or four Form-C sets ganged on one armature — switch several circuits at once, in step.

The contact rating is not a single number. A relay marked "10 A 250 VAC / 10 A 30 VDC" can switch far less DC at higher voltage, because DC arcs do not self-extinguish at a current zero-crossing the way AC arcs do. Contact resistance is typically 50–100 mΩ when new and rises as the contacts erode, so relays are poor for switching microvolt-level signals unless they use gold-flashed contacts intended for "dry" low-current duty.

Driving the coil: the flyback diode

The coil is an inductor. When you switch it off, the stored magnetic energy (½LI²) has to go somewhere, and the collapsing field generates a reverse voltage spike governed by:

V_spike = -L · di/dt

A 12 V relay coil with L ≈ 100 mH switched off in 1 µs:
  V_spike ≈ 0.1 × (0.04 A / 1 µs)
          ≈ 4000 V   (clamped in practice, but lethal to a bare transistor)

That spike will punch through the base–collector junction of a switching transistor. The fix is a flyback (freewheeling) diode wired across the coil, reverse-biased in normal use. When the transistor turns off, the diode conducts and lets the coil current recirculate and decay harmlessly. The cost is slower release: the recirculating current keeps the armature held for an extra millisecond or two. If you need fast release, add a Zener in series with the diode so the spike is clamped to a higher (but bounded) voltage, dumping the energy faster.

Relay vs. MOSFET vs. solid-state relay

	Electromechanical relay	Power MOSFET	Solid-state relay (SSR)
Galvanic isolation	Yes, inherent (4–5 kV)	None	Yes, via opto-coupler
On-state loss	~50 mΩ contact, near-zero	I²R in R_DS(on)	~1–1.5 V drop (triac/SCR)
Off-state leakage	Essentially zero (true open)	µA	mA-scale
Switching speed	5–15 ms operate	ns–µs	µs to half-cycle (zero-cross)
Cycle life	10⁵–10⁷ (mechanical wear)	~unlimited	~unlimited (no moving parts)
AC and DC	Both, one part	DC (or with bridge)	AC types common; DC types exist
Audible / silent	Audible click	Silent	Silent
Failure mode	Stuck open or welded shut	Usually shorted	Usually shorted

The headline trade-off: a relay gives you a true mechanical open circuit, isolation, and zero conduction loss, at the price of slow switching, audible noise, and a finite number of operations. Semiconductors switch fast and forever but always leak a little and always dissipate heat while conducting.

Worked example: sizing a coil driver

You want an Arduino (5 V GPIO, ~20 mA limit) to switch a 12 V relay whose coil is 400 Ω. Can the pin drive it directly, and what does the transistor need to handle?

Coil current at 12 V:
  I_coil = V / R = 12 / 400 = 30 mA

A 5 V pin limited to 20 mA cannot supply 30 mA, and the coil
needs 12 V anyway — so a logic-level NPN/MOSFET is required.

Base resistor for a BJT (β ≈ 100, want I_C = 30 mA):
  I_B(min) = I_C / β = 30 mA / 100 = 0.3 mA
  Drive hard (×5 overdrive) → I_B = 1.5 mA
  R_B = (5 V − 0.7 V) / 1.5 mA ≈ 2.9 kΩ → use 2.2 kΩ

Transistor must survive the flyback before the diode clamps:
  Pick V_CEO ≥ 40 V (e.g. 2N2222 at 40 V, or a logic MOSFET)
  Add 1N4148 / 1N4007 flyback diode across the coil.

So: one small transistor, one base resistor, one flyback diode, and the relay's own contacts then handle the 12 V — or 230 V — load. The microcontroller never sees more than 5 V.

Failure modes and trade-offs

• Contact welding. Closing into a high inrush (capacitor banks, incandescent or LED lamps, motors) can momentarily melt the contact spots; if they re-solidify while touching, the relay welds shut and will not release. Mitigate with correctly rated contacts, AgSnO₂ alloys, pre-charge resistors, or inrush-limiting NTCs.
• Arc erosion. Every break draws an arc that vaporizes and transfers metal. DC and inductive loads sustain the arc longest, so relays are derated heavily for DC — a "10 A 250 VAC" relay may be rated only 0.4 A at 110 VDC.
• Contact bounce. The armature snaps in and the contacts physically rebound a few times over the first 0.5–2 ms, generating a burst of make/break edges. Harmless for a heater, but a logic circuit reading the contact must debounce it.
• Coil overheating. Holding power becomes heat; coil resistance rises with temperature, lowering current and force, so a hot relay may drop out. Economizer drives or latching relays avoid continuous holding power.
• Slow switching. 5–15 ms operate time rules relays out of PWM and fast switching duty — that is MOSFET/SSR territory.
• Mechanical wear-out. Springs fatigue and bearings wear; mechanical life (no load) might be 10⁷ cycles while electrical life under full load can be 10⁵ or less. The electrical life is the one that matters in service.
• Orientation and shock. Vibration can momentarily open seated contacts or bounce open contacts closed; aerospace and automotive relays are qualified for shock and specified mounting orientations.