What does it do?
|The relay subsystem is an electrically-operated switch. It requires a separate electrical supply to provide power to an output device. It is often used for reversing motors.
How does it operate?
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
Like ordinary switches, relay switches are available as single-pole single-throw (SPST), single-pole double-throw (SPDT), and double-pole double-throw (DPDT). The circuit diagram on the left shows a DPDT relay.
The switching is done by a coil of wire (an electromagnet) that creates a magnetic field when a current passes through it.
The switch contacts in the relay change over due to the force from the magnetic field when a current passes through the coil.
The reverse biased diode is included because, when relays are switched off, they can generate a ‘back e.m.f.’ that can damage the driver. When the relay is switched off the diode conducts current and prevents the damage.
The driver subsystem that provides the input signal to the relay must be able to supply enough current for the coil.
A DPDT relay has three pairs of connections known as common (CO), normally open (NO) and normally closed (NC).
Relay circuit for reversing a motor
A DPDT relay is often used to reverse a motor. The circuit diagram on the left shows how the motor is connected to the relay.
When the input signal to the relay is high there is no current in the relay coil (as on the left), the positive side of the battery B1 is connected to the right-hand terminal of the motor, so the current in the motor flows from right to left.
When the input signal to the relay is low there is current in the relay coil (as on the left) and the switch contacts change over. So now the positive side of the battery is connected to the left-hand terminal of the motor, the current in the motor flows from left to right and so the direction of rotation of the motor reverses.
The circuit diagram shows the basic principles. If it is necessary to stop and start the motor this can be done with a separate driver or a SPST relay.
A few relays need relatively low currents and can be driven directly from a PIC, 555 Timer IC or LM324 op-amp. In these cases the relay coil is connected to the input signal and to 0V.
- Reversing a motor
- Providing electrical isolation between a noisy output device (such as a motor) and the processing electronics.
- Controlling a low voltage a.c. output device, e.g. a low voltage halogen bulb (hotlink to bulb data sheet, section that refers to halogen bulb)
Pins of the Rapid 60-0100 DPDT relay
How part of the PCB might look
The diagram shows the pin arrangements and numbering for the Rapid 60-0100 DPDT relay. Note the unusual pin labelling system. The PCB shows the basic circuit. The separate power supply and output device would be connected to the six upper pins.
Build and test the driver unit that will provide the input signal before building the relay.
Use a 16-pin Dual In Line (DIL) socket for the relay. Before inserting the relay, connect the power supply and use a voltmeter to check that:
- the voltage on pin ‘b’ is high (the supply voltage);
- the voltage on pin ‘a’ (the blue PCB track) goes high and low in response to the driver unit that provides the input signal.
Insert the relay the right way round.
Use a multimeter to test the resistance between the switch contacts and make sure that their resistance changes from high to low when the coil is switched on and off.
If there is a fault, check that:
- The voltage on pin ‘b’ is high
- The relay has been correctly inserted
If there is a fault, check the tracks and solder joints.
- A pair of SPDT relays can be used to provide forward, reverse, stop and start for a motor – this has the advantage of providing a better ‘brake’ for a motor but is more expensive.
- A L293D IC can also be used to provide forward, reverse, stop and start for two motors – this has the advantage of providing a better ‘brake’ for a motor, but is more expensive.
- If the relay is being used to provide electrical isolation from noise, an alternative is the opto-isolator.
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