Introduction

An electromagnetic relay is a physical mechanical switch that is controlled by an electrical circuit. The electrical circuit controls an electromagnet and the electromagnet controls the switch. There are different types of relay but they all work in a broadly similar way.

An electromagnetic relay has an electromagnet that can attract a soft iron armature when activated. The movement of the armature is used to operate a mechanical switch. A reed relay uses two thin metal reeds inside an evacuated glass capsule. The reed switch is inside the coil of an electromagnet. When current flows in the coil of the electromagnet the reeds are magnetised in such away that they attract, contact and complete the circuit.

Electromagnetic Relay basics

An electromagnetic relay is an electromechanical device. It uses an electromagnet - the relay coil - to physically move a mechanical switch. The switch may be either single pole or double pole and there may be many separate switches all controlled by the same relay coil.

The picture shows a high power relay. The copper coil is clearly visible on the left hand side and the switch contacts are visible at the bottom right-hand corner. The bright shiny soft iron armature and return spring can also be seen. The symbol for the relay shows the coil as a box with a diagonal line, the physical connection (the armature) as a dotted line and the two switch contacts.

Relays are available in many shapes and sizes. The picture shows several PCB mounted relays that would be soldered to a circuit board. Other relays may plug into a socket or be connected using screw terminals.

The relay coil has a normal working voltage and an associated coil resistance. The white relay in the picture is marked as 5V which means the relay coil operates at 5V - the relay itself can switch 120VAC or 24VDC but the actually relay operates at 5V making it ideal for use with logic circuits. The relay datasheet states that the relay coil resistance is 70Ω and so the relay coil will take approximately 5/70 = 0.07A or 70mA in operation. The black relay is rated at 12V and the coil resistance is 400Ω giving a working current of 33mA. Both of these relays provide a pair of double pole switches as shown by the bottom view.

Using a relay in a circuit

A relay provides an interface between two otherwise separate circuits. An electronic circuit, possibly the output from a computer, logic circuit or sensor, working at low voltages can be used to control high power devices such as light bulbs and motors. The electronic control circuit does not provide any current for the output device but simply provides enough current to control the relay. The relay acts as a switch to connect an external power supply to the output device.

The example shown uses a 5V logic circuit to control a mains powered bulb. The electronic control circuit uses 5V D.C. and can provide enough current to turn on the relay. The relay switch connects the 60W bulb to the 230V A.C. mains supply. In this way the logic circuit can control a 60W bulb.

Advantages and Disadvantages of using a Relay

The advantages of using a relay include:

• The control circuit and the output device are electrically isolated from each other
• The output device can use its own power supply e.g. mains voltage
• An electronic circuit can control devices that use a lot of current
• Multiple devices can all be controlled simultaneously as most relays have more than one physical switch all mechanically connected together
• Relay switches can be enclosed or sealed in an inert atmosphere and therefore do not corrode over time or suffer the effects of water and dust

The disadvantages of using a relay include:

• Relays are (very) slow compared to transistor switches
• Relays can be big and bulky and can be unacceptably noisy as they switch
• Mechanical switch contacts my wear out over time
• The relay coil may produce a large back e.m.f. damaging sensitive equipment or causing interference

Using a Transistor to control a Relay

In some cases the electronics being used to control the relay cannot provide enough current for the relay coil. For example, the 5V relay shown above requires a coil current of 70mA but a CMOS logic gate can only source about 10mA of current. In this case a transistor can be used to drive the relay and the electronic control circuit simply turns the transistor on and off. Biploar transistors and MOSFETs are both suitable for controlling relays.

In the circuit shown an astable controls a small npn bipolar transistor and the transistor turns on and off the relay. Note that a diode is connected in reverse bias in parallel with the relay. When the relay turns off it can produce a significant back e.m.f and damage the transistor. The reverse bias protection diode limits the back e.m.f. to a safe value and protects the transistor from damage.

Reed Relay

A reed relay uses a coil of wire i.e. an electromagnet, to operate a reed switch. The contacts of the reed switch are not usually able to carry large currents or work at high voltages. However, a reed relay is much faster to respond than a conventional electromagnetic relay.