In
the exam you are expected to:
Know
where a relay is used;
Know
the NO and NC notation;
Know
about the need for a reverse biased diode to protect the circuit.
The
relay is a switch operated by an
electromagnet. It allows the small
voltage and small current produced by an electronic circuit to turn on a big
current at a high voltage. Electronic
circuits cannot, as a rule, produce the currents and voltages required to
operate a large mains motor. Also
electronic circuits tend to work on low d.c. voltages.
Therefore the relay acts as an
interface between the two.
This photograph shows the construction of a relay:
The circuit symbol for a relay is:
Notice
that there are two different kinds:
NO
– normally open. The contacts are open until the coil of the relay is
energised, whereupon they are closed to complete the outside circuit.
NC
- normally closed. The contacts are closed until the coil of the relay is
energised, whereupon they are opened to break the outside circuit, switching
it off.
Many
relays have multiple contacts,
half of which are NO and half NC.
Relays are electromagnetic devices which have a certain amount of inductance. When they are turned off, the collapse of the magnetic field can produce a momentary “spike” of high reverse voltage that can wreck a transistor or integrated circuit. Therefore a reverse biased diode is placed in parallel to short out the voltage spike, thereby protecting the circuit.
In
this circuit we can see clearly the reverse-biased diode to protect the
transistor.
Question 1 Explain why a relay needs a reverse biased diode in parallel to protect the rest of the circuit. ANSWER
The
example here shows a light dependent resistor arranged in series with a variable
resistor, to form a potential divider circuit whose output goes to the base of a
transistor. In this circuit, the transistor is used as a switch.
Question 2 Explain in as much detail as you can how this circuit works. Describe what R1 and R3 do. ANSWER
The picture below shows a few different kinds of relay:
|
Summary The
relay allows a small current to turn on a big current; The
relay is an electromagnetic switch; The
contacts can be normally open or normally closed; The
relay needs a reverse biased diode in parallel to protect the electronic
circuit from high reverse voltage spikes. |