Switch Circuits
A relay switch circuit is a circuit where a transistor is used to switch a current or voltage between two circuits. In this circuit, the transistor is connected in series between the supply terminals and the relay coil. When the transistor is in the closed circuit state, it will not allow any current to flow through its collector. In this way, when V1 is zero, no current will flow through the transistor and it will activate the relay coil.
A typical relay switch circuit consists of a NPN transistor and a coil connected in series. The coil acts as a short circuit when a positive voltage is applied to the transistor’s base terminal. A voltage spike can damage the switching transistors. A relay switch will not work properly if it is exposed to a voltage spike or reverse voltage.
Different types of relays use different materials for their contacts. Silver nickel contacts reduce the chances of pitting and result in longer life. However, they are slower than semiconductor switches and have a limited lifetime. Solid state switches are more reliable because they do not suffer from transients outside of their ratings. However, they are prone to contact bounce, which can result in arcing.
Relay Switch Circuits
If you want to check your relay’s circuit, you need to determine the resistance of each of the two terminals. The resistance of the coil wire is usually in the range of a few hundred ohms. In contrast, the resistance of the relay’s side terminals is typically 100-ohms to 500-ohms.
There are many types of relays, and they are usually classified as either normally open or normally closed. Normally open contacts close only when the field current is ON. When closed, the armature is pulled away from the coil by an inductive force. Then, the soft-iron armature will attract the armature and close circuit 2. The steel spring will then return it to the open position. A relay normally has five terminals. Two of them are coils, while the other three are common and open. Using a multimeter, you can measure the resistance in these terminals.
Relay switch circuits can be made using different types of transistors. A PNP transistor is similar to an NPN transistor, but it has two different types of channels. When the Base of the transistor is forward biased, current flows through the Emitter, and when the base is negative, current flows through the Base.
Electromechanical relays have been used as electrical switches for decades. They are more reliable than solid-state switches and can withstand transient voltage surges. However, electromechanical relays tend to wear out more quickly when switching inductive loads. Lastly, solid-state relays require less voltage for switching. However, they are still a great choice for some applications.