question

[u/Tamim_mahmoud]

i'm pretty new to electronics and all that stuff but my question is why would i ever need to use a relay or a transistor with a switch instead of just directly wiring the switch to whatever i need

[u/ComputeInMemory]

So you can switch something on a high power circuit automatically with low power. 

So you can use a lighter switch to safely control high power. 

[u/coffeislife67 → u/ComputeInMemory]

Not exactly on the same level as transistors and relays but I think maybe the same principle.

I'm a commercial electrician and sometimes we put entire electrical panels on a contactor for say warehouse or factory lighting.

That way I can put a single 120v switch on the wall, that will operate hundreds of lights.

[u/bobbaddeley]

If you have a microcontroller operating at 5V and you are trying to switch something running at 120V, you can't just attach the 120V thing directly to the 5V thing or else you'll blow it up. Often the thing controlling the switch needs to be isolated in some way from the thing being controlled.

[u/IndividualRites]

What if you wanted something automated to turn that switch on and off? Say, a light bulb at a specific time of day?

[u/throwaway74389247382]

1. Most of the time, relays and transistors are controlled with other logic. Not by a manual switch. Definitely not always, but that's the overwhelmingly predominant use case.
2. Many switches, particularly small hobbyist-grade switches, can only handle so much current. If you try to control a 10 amp circuit with some little $0.02 toy switch, it's going to heat up / burn out / melt / catch fire. Transistors and relays can be designed to handle way way more current.
3. Similarly, switches can only handle so much voltage before failing in some unpleasant (and in some cases extremely dangerous) way

In terms of typical beginner circuits though, you're right, there's often little/no need for them. The point of including them in circuits at that level is to teach you how to use them for when you actually do need them.

[u/Tamim_mahmoud → u/throwaway74389247382]

so basically relays keep you from dying

[u/IndividualRites → u/Tamim_mahmoud]

Or just burning up your circuit.

[u/Fart_knocker5000]

Take a look at the guys who's job it is to switch huge contacts on high power equipment. Arc flash can literally vaporise you. Relays and transistors allow you to switch a much larger power supply without having to interact with it directly

[u/Tamim_mahmoud → u/Fart_knocker5000]

so that's how the switches on like really really big and scary lab machines don't explode

[u/Fart_knocker5000 → u/Tamim_mahmoud]

Totally. The equipment i work on uses contactors, which are just a really large relay in effect. A small 24vdc coil at maybe 0.5A magnetically couples the much larger contacts that carry 415vac at 64A. The dangerous contacting is happening in the panel while the operator is only handling 24vdc

[u/RuthlessIndecision]

The power jumps, so you can control a larger power with low power, for example signal controls or electronic controls.

Isolation, the contacts side of the circuit is isolated from the switch or solenoid side, so you don't do damage if something goes wrong,

and other reasons I can't think of.

[u/Hirtomikko]

Assume you want to switch something on and off 50,000 times a second. How else are you going to accomplish this feat?