# I don't understand how transistors are used as a switch

When I see circuits with transistors used as switch I see this circuits.The thing I dont understand is why is this transistor is so important ? Wouldn't we have the same circuit with just a simple "Voltage source- switch-resistor-lamp"? What is the differance what isthe effect of transistor in a switching circuit. Please go easy on me, I really just want to understand

• I'll point out that if VCC is 24V and the lamp is 24V, that doesn't leave room for a resistor of practical value. So your "simple circuit" would be "Voltage source-switch-lamp" Commented Apr 7 at 22:29

In this case the transistor allows a small current to switch a larger current, so a switch with a lower rating can be used. This could also be done with a relay, but transistors are much smaller and less expensive than relays, and relays tend to wear out as they have moving parts and contacts, a transistor operated under proper conditions can last for many decades with little to none degradation of performance.

Another reason a transistor is used instead of a switch is that a transistor can be controlled electronically rather than mechanically. This allows a circuit to turn something on or off as opposed to a person having to operate a switch.

With a transistor you can do things like have a microprocessor or a timer circuit control the lamp.

• And because you can control it electronically, you can switch one transistor with another. And you can switch multiple transistors with one electrical signal. And you can also set up transistors to switch off when a signal turns on. The combination of these three properties enables you to build arbitrarily complex logic like the microprocessor mentioned. Commented Apr 7 at 21:25
• But in order to connect this base to the source we still need to turn the switch mechanically in this circuit. But I guess that is the case for this circuit and controllig it with a signal just solves the problem for me. Thank you !
– kaan
Commented Apr 7 at 21:32
• @kaan Yes, that's where switching a larger current comes in, the pushbutton switch can only handle so much current, by adding the transistor you can use the switch for larger loads. Commented Apr 7 at 21:40
• @kaan Rearranged my answer to emphasis using a transistor with a physical switch. Commented Apr 7 at 21:44
• @kaan: Often simple circuits like this are just used to illustrate basic principles, and would rarely be used as-is in Real Life. Commented Apr 7 at 22:21

The 25 W lamp in your 24 V circuit will draw 1 A from the power supply. What happens if you want to switch this on using a tiny touch button capable of 50 mA max?

The transistor in this circuit behaves like a current amplifier. RB limits the current through the base and through the switch to a safe value. In simple terms, the transistor will pass that much current × its gain. So if our transistor has a gain of 30 we would need a base current of about 33 mA to turn the lamp on fully. The transistor is an amplifier!

In practice there are a few more considerations but that may help get you started.

• I never thought of a switch could have a current limit. That makes sense. Well, thank you.
– kaan
Commented Apr 7 at 21:34

In your "simple voltage source-switch-resistor-lamp" circuit, the switch has to carry the entire current of the lamp. Depending on how much current the lamp takes, that could be a very big switch!

In the schematic you've shown, the switch only needs to carry a comparatively small current.

The switch could also be another transistor, opening up the possibility of fully automatic logic operations--the basis of a computer.

In the circuit you showed us, the transistor isn't really doing anything that you couldn't achieve with just a switch, and as such it doesn't demonstrate the real benefits of transistors.

The purpose of a transistor in any circuit is to amplify, meaning that it is able to "switch" large currents under control of a much smaller current. For instance, to switch a lamp, whose current could be several amps, under control of an Arduino, or temperature sensor, or some other weak signal source able to provide only milliamps, then you will need one or more transistors, or other amplifying element.

In your schematic, there are two interesting values shown, base current $$\I_B\$$ and collector current $$\I_C\$$. The lamp is passing current $$\I_C\$$, and is a little over 1A in this circuit. However, base current $$\I_B\$$ is required to be only about 10mA (one hundred times smaller) for that 1A of lamp current to flow.

While you are using a switch in this example, you could replace that switch with a sensor of some kind. Here's an example using a hall effect sensor to detect the presence of a magnetic field:

simulate this circuit – Schematic created using CircuitLab

A typical sensor won't be able to pass more than a few milliamps of current, meaning that you would not be able to use the sensor directly to switch on and off the 1A of lamp current. The transistor here does the "heavy lifting", taking over responsibility for lamp current, while only requiring that the sensor handle just a few milliamps.

You can use multiple transistors to increase current sensitivity (gain), to the point where only microamps (millionths of an amp) of current are necessary to switch several amps. Using the circuit below, you can use your fingers to pass a tiny current (less than 1mA) by holding onto contacts A and B, and that will be sufficient to light the lamp with the full 1A:

simulate this circuit