Enclosed body · 19 × 15.5 × 15.3 mmCoil terminal · 5 V DC; COM terminal at the backBase (sealed)NO terminal (normally open)

Blue plastic box, coil voltage (5 VDC) and contact ratings (10 A) on top, five legs underneath
≈ 19 × 15.5 × 15 mm, five legs

Switches and relays

Relay

In simple terms

What is this?

An electrically pulled magnetic switch; a small circuit presses the button of a big circuit.

What is it for?

A small signal switches a separate powerful circuit (lamp, pump) on and off; the two sides stay insulated from each other.

Where do you see it?

A small box that makes a clicking sound in washing machine or boiler boards.

How do you spot it?

A small box that clicks when it switches on and off, and has five legs.

Caution:

The coil current is more than the pin can supply; drive it with a transistor and put a reverse diode across the coil.

Technical definition

An electromechanical switch pulled by an inductor coil’s magnetic field. Control and switched circuits are electrically separate. Coil current exceeds a microcontroller pin’s capacity, so a transistor drives it.

Inductor5 V, ≈70 Ω

Inductor current≈71 mA

Contact10 A

Schematic symbol IEC and ANSI differ

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 5

1Coil terminal 1Inductor has no polarity; flyback diode cathode connects here (positive)A1

2Coil terminal 2To the transistor driver (minus side)A2

3Common contactSwitched line inputCOM

4Normally openConnects to COM when inductor coil energizesNO

5Normally closedConnected to COM while inductor coil is idleNC

Features typical

Coil voltage: 5 V DC
Coil resistance: ≈70 Ω
Inductor current: ≈71 mA
Inductor power: ≈0.36 W
Pull-in voltage: ≤ 3.75 V
Contact: SPDT (1 form C)
Contact rating: 10 A 250 V AC · 10 A 30 V DC
Warning: Mains voltage is outside this app's scope
Example part: SRD-05VDC-SL-C

Common mistakes

Driving directly from a pinInductor coil draws ≈71 mA; a microcontroller pin cannot supply this. Use an NPN transistor or MOSFET driver.

Forgetting the flyback diodeSwitching off an inductor creates a high reverse voltage pulse. Connect a 1N4007 in reverse parallel, cathode to positive.

Experimenting with mainsAlthough the contacts withstand 250 V AC, mains voltage is outside this app’s scope. Work with loads below 12 V.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Switching a lamp or pumpChanging motor directionDevice control with a timerIsolated switching

Try it in the Lab