Relay Construction

Part of Basic Electrical Circuits

How to build electromagnetic relays that allow small control currents to switch large power circuits safely.

Why This Matters

A relay is one of the most useful electrical devices: it allows a small, weak signal in one circuit to control a large, powerful circuit—without any direct electrical connection between them. A small switch in a control room can safely switch a heavy motor circuit. A thermostat circuit drawing milliamps can switch a heating element drawing tens of amps. A radio receiver output can trigger an alarm siren.

Relays also provide electrical isolation. The control circuit and the power circuit share no conductors, so faults in the power circuit cannot damage control electronics, and people working the control switches are protected from high voltages in the power circuit.

Historically, relays were the active components of the telegraph network, telephone exchanges, and early computers. A single 1950s telephone exchange might contain hundreds of thousands of relays. A rebuilding community with the ability to make reliable relays can build automatic controls, remote switching, and even basic logical control circuits.

Relay construction requires only iron, copper wire, and a spring—materials available with basic metalworking capability.

How a Relay Works

A relay consists of an electromagnet and a set of switching contacts.

When current flows through the coil, the electromagnet attracts a movable iron armature. The armature is mechanically linked to switch contacts, which close (make) or open (break) the circuit being controlled. When coil current stops, a spring returns the armature and opens the contacts.

Key design parameters:

• Pull-in current: The minimum coil current needed to close the contacts
• Hold current: The minimum current to keep contacts closed (typically 50–70% of pull-in current)
• Contact current rating: The maximum current the contacts can switch safely
• Coil voltage: The voltage the coil is designed to operate at

Building a Simple Relay

Materials:

• Soft iron core: a bolt, rod, or machined cylinder. Soft iron (not hardened steel) is essential—it loses its magnetism when the coil current stops. Hardened steel retains magnetism and will not release.
• Copper wire for the coil: 0.1–0.5mm insulated wire, 50–500 turns depending on application
• Armature: a flat strip or plate of soft iron, 1–3mm thick
• Spring: thin steel strip, spring wire, or thin strip from a tin can
• Contact materials: copper, silver, or brass for the contacts themselves
• Wooden, ceramic, or hard rubber base

Construction sequence:

Step 1 — Build the electromagnet core: Take a soft iron bolt (M6 to M10) approximately 30–40mm long. File the exposed face flat and smooth to ensure good magnetic contact with the armature. Mount it vertically on the base with the head down (it can be threaded into the base) or horizontally.

Step 2 — Wind the coil: Insulate the bolt shank with several layers of paper or cloth tape. Wind coil wire in tight, even layers. For a 12V relay drawing approximately 50–100 mA at activation:

• Use 0.2mm diameter copper wire
• Wind approximately 200–300 turns (calculate resistance = V/I; about 100–240Ω needed)
• Insulate each layer with a thin paper strip before winding the next
• Leave at least 10cm of wire at each end for connections

Step 3 — Shape the armature: Cut a strip of soft iron approximately 5 × 30 mm. One end will hinge (or be held by a spring pivot) near the base of the core. The other end carries the moving contact.

Bend a small tab or drill a hole at the pivot end. Mount it so that when the coil is energized, the end near the core face is pulled in and the far end moves. The stroke should be about 1–2mm.

Step 4 — Install the spring: A spring steel strip (from a clock spring, hacksaw blade, or tin can strip) provides the return force. It must be strong enough to ensure reliable contact separation but light enough that the electromagnet can overcome it.

Mount the spring to hold the armature in the “off” position with light force. Too strong a spring reduces sensitivity; too weak a spring causes contact bounce and sluggish return.

Step 5 — Make the contacts: The stationary contact is a fixed copper or brass rivet or screw mounted on the base. The moving contact is a similar rivet attached to or bent from the armature tip. When the armature pulls in, the moving contact presses against the stationary contact.

For higher reliability, silver-coat the contacts by immersing them in a silver nitrate solution and passing a brief current (silver electroplating). Silver contacts resist oxidation and maintain low contact resistance.

Step 6 — Adjust and test: Connect the coil to its rated voltage through a series resistor (for adjustable voltage supply). The contacts should:

• Open with a clear audible click when voltage is removed
• Close with a definite snap when energized (no slow creep)
• Show near-zero resistance when closed (test with ohmmeter)
• Show infinity when open

Adjust the spring tension to find the threshold between reliable operation and false triggering.

Relay Contact Configurations

Normally Open (NO): Contact is open when coil is de-energized; closes when coil is energized. Most common. Use for “turn on when commanded.”

Normally Closed (NC): Contact is closed when coil is de-energized; opens when coil is energized. Use for “turn off when commanded” or “fail-safe: circuit stays on unless actively disabled.”

Changeover (CO) or SPDT: Has both NO and NC contacts plus a common. The common switches between the two contacts as the coil is energized. Use to route a signal to one of two paths, or to switch direction of a motor by reversing current.

Double-pole (DP): Two independent contact sets operated by a single coil. Use to simultaneously switch both conductors of a power circuit—important for safety isolation of AC circuits.

Protecting the Relay Coil

When coil current is interrupted, the magnetic field collapses. The collapsing field induces a voltage in the coil (back-EMF) that can be several times the supply voltage. This voltage spike:

• Damages transistors or other semiconductor switches driving the relay
• Creates electrical interference in nearby circuits
• Increases contact arcing

Suppression: Connect a diode across the coil (cathode toward positive supply). The diode blocks current during normal operation but clamps the back-EMF spike. If no diode is available, a small capacitor (0.01–0.1µF) or a resistor-capacitor “snubber” in parallel with the coil provides some protection.

Relay Applications

Motor contactor: A heavy-duty relay rated for motor starting current. Switching contacts are sized for 3–5× normal running current to handle the inrush at motor start. Usually includes arc-quenching features (gaps, blow-out coils) to safely extinguish the arc when breaking motor current.

Latching relay: Two coils: one “set” and one “reset.” A brief pulse to the set coil closes the contacts; they remain closed until a brief pulse to the reset coil opens them. No continuous power needed to maintain either state—ideal for remote switching with minimal power consumption.

Thermal relay: The coil carries the protected circuit’s current directly. A bimetallic strip bends with heat when current exceeds the rated value, opening a separate alarm or trip circuit. Used for motor overload protection.

Time-delay relay: A dashpot (oil-filled damper) or RC timing circuit delays the armature movement, providing a fixed time between coil energization and contact switching. Used for starting sequences that require delays between steps.