Why use a transistor rather than a switch?

Thread Starter

claudeboules

Joined Mar 10, 2021
2
I know this question has been raised previously but as a newcomer to electronics there is still a bit missing for me. As I understand it the small base current allows a larger current to flow through the transistor. So if we need the base current to flow to allow the transistor to operate as a switch, what turns the base current on? If that is another switch, what is the point of the transistor as a switch in the first place, why not just use a mechanical switch?
 

Yaakov

Joined Jan 27, 2019
3,161
One common case would be switching a large load with a DIO (Digital Input/Output) pin of a microcontroller or even discrete logic chips. The pin can handle very limited current so the transistor provides a way to control practical loads.

All the other cases are similar. An existing switch can't handle the load to be switched, or, we want to translate the output of a device, for example an LDR (Light Dependent Resistor) into a switching action for a load. In the latter, the device output varies but the goal isn't necessarily proportional response rather it is threshold switching.
 
Last edited:

Ian0

Joined Aug 7, 2020
2,235
If that is another switch, what is the point of the transistor as a switch in the first place, why not just use a mechanical switch?
If that were true, relays would not need to exist!

Another point - transistors can switch more cleanly. Switch contacts bounce, which causes arcing, which damages the contacts and causes interference.
A triac can switch a mains load exactly at the point the mains crosses zero, again avoiding interference.
Switches wear out - transistors don't, but, on the other hand, they can fail more easily than switches if overloaded.
Transistors are less efficient - they get warm while passing current.
Transistors can switch a load on and off a million times a second - switches can't.
A good designer knows how best to switch a load - transistor, triac, relay or switch.
 

Thread Starter

claudeboules

Joined Mar 10, 2021
2
One common case would be switching a large load with a DIO (Digital Input/Output) pin of a microcontroller or even discrete logic chips. The pin can handle very limited current so the transistor provides a way to control practical loads.

All the other cases are similar. An existing switch can't handle the load to be switched, or, we want to translate the output of a device, for example an LDR (Light Dependent Resistor) into a switching action for a load. In the latter, the device output varies but the goal isn't necessarily proportional response rather it is threshold switching.
Hi Yaakov - thank you - this makes it clear and fills in the bit I was missing. Regards Claude
 

BobTPH

Joined Jun 5, 2013
3,316
The transistor is switched by an electronic signal. The switch is switched by a mechanical motion. Completely different uses, though sometimes overlapping.

Bob
 
Speed control of a motor, or brightness control of a lamp, can also be done by pulse width modulation with a logic chip or CPU, for example.

PWM = Pulse Width Modulation

To put it simply, at a given frequency the logic could switch on at the beginning of the cycle, but after x percent of the cycle time, it turns off until the next cycle. You'll get approximately x percent of the speed or the brightness, within certain limits (i.e. below 20 percent the motor may not turn, there may be no visible light from the lamp, or whatever)
 
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