Relays are used extensively in modern vehicles. They can be thought as electro/mechanical switches. They are often used to switch on circuits that require a heavy current e.g. the heated rear windscreen (H.R.W.) circuit.
If a relay was not used the contacts inside the HRW switch would need to be quite large to carry the heavy current. By using a relay, smaller switch contacts can be used. The relay then acts as a switch saver.
Relays can be used to avoid voltage drop and reduce the amount of heavy duty cable required. They can also ensure that contacts make and break cleanly. Relays also make a convenient point for testing individual circuits.
From the circuit diagrams with circuit A the HRW switch has to carry the full current. However, with circuit B the HRW switch only needs to carry enough current to activate the relay. When the relay is activated it closes the relay switch. The feed for the HRW then comes through the 16 amp fuse and the contacts of the relay. By taking this route it saves the contacts of the HRW switch.
Circuit B has the advantage that the relay switch can be located almost anywhere in the vehicle. With circuit A, if the switch was mounted too far away from the heated rear window, it could cause pressure loss i.e. voltage drop.
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| Circuit A |
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| Circuit B |
