5.0 Relays as Switch
What are the advantages of using relays over transistors? Well the following are the advantages of using relays: –
a) relays can switch AC and DC while transistors can only switch on DC current
b) relays can switch on high voltages but transistors can’t
c) relays are a better choice for switching large currents (>5A)
d) relays can switch many contacts at once
The disadvantages of relays are as follows:-
a) relays are bulkier than transistors for switching small currents
b) relays can’t switch rapidly except for reed relays while transistors can switch many times per second
c) relays use more power due to the current flowing through its coil
d) relays require more current than many ICs can provide and hence a low power transistor may be needed to switch the current for the relay’s coil
Let’s just put aside whatever I just said and bear in mind that in next chapter, I will combine both relay and transistor together. I would like you to just compare the following relay circuit with that of the transistor’s circuit in Chapter 4. It looks rather similar. The red and white cables are used to power up or activate the relay (the white cable I am refering to is the white that is on the left not the one on the right side of the relay).
The relay has 5 legs as shown here: –
Here is a close up view of the 9V battery: –
The following photo shows that on the left side, the circuit is open. If I were to close this circuit i.e. to clip the crocodile clip to the cable, the relay would be activated.
Let’s watch a video on how the relay can be used as a switch: –