super basic question about transistors

Thread Starter


Joined Jun 29, 2011
Hey all,

I'm super new to electronics and had no clue what transistors were before reading through the "Semiconductor" section in e-book. It was very informative, but I still have some questions.

Isn't the function of a transistor the same as a relay? They both switch/amplify electronic signals. There are obviously important differences with respect to maintenance, how long the part will last, and how quickly the switch occurs, but with respect to function, they do the same exact thing, correct?

And if this is correct, then why do solid state relays involve so many components, that is, the LED, the TRIAC, etc.? Why not just use a transistor?

Answers would great, and direction to further reading would also be splendid!


Joined Dec 26, 2010
Transistors are capable of switching, and also analogue amplification. Relays are restricted to switching only, and are not capable of an analogue response.

The components used in solid-state relays are chosen by to fulfill requirements such as input to output isolation, and low-loss AC switching. (Some solid-state relays offer DC switching, but normally not both.)

Transistors lack electrical isolation between the output and input circuits, which relays can provide. The LED element in the solid-state relay couples the input signal optically to another photo-sensitive component, so that the input and output are electrically isolated.

Transistors are basically DC devices, but TRIACS are suitable for controlling AC, with a type of switching action that can result in relatively low power losses.
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