1) Relay provides galvanic isolation between driving circuit and load whereas transistor does not.
2) If something gets short on your load side, your circuit will be protected it ifts relay based otherwise everything will blow up.
3) Transistors have better swithing speed.
4) Transistors does not generate high voltage spikes during switching as relay does across its coil due to falling magnetic field( can be solved using free-wheeling diode).
5) Transistors require less current to operate than relay.
I use the relay only when don't know what kind of voltage will switching (AC or DC).
With optocoupler and transistor i can make also galvanic isolation.
relays are better when having high current, because a 30mA relay/12V can switch 10A, but if you were to use a transistor you will loose lots of power like heat
how are they placed on the relay?
it they are like 4 in the corneres of a rectangle and one wich is closer to 2 of the 4 then that pin in the one that is connected to n.c.(normally closed) swotch
the other 2 near it are to the coil and the last 2 are for n.o. abd n.c.
now what do you mean by the normally open and closed and stuff? i know what the term means, and i heard the guy at the auto store mention it, but in this case, what is it refering to?