How do the transistors work?

How do the transistors work?

Today we are going to have a look at the transistor, what a transistor is and how it works in a simple circuit. So to start off with transistors are semiconductor devices. This means that transistors can be both conductive allowing current to pass through them or non-conductive which blocks current from flowing through it.

Transistors can be used as a switch essentially emulating a mechanical switch but in a solid state form. Before I move on, I have a transistor connected within this circuit. It’s a simple circuit consisting of an approximately 3-volt power supply connected to an LED with the collector and emitter of this transistor connected in series.

Currently, we have everything connected and we are playing this circuit live. What we would expect is current to flow from this power supply through the LED back through the transistor and to the power supply and in that case, the LED should be lit but the LED isn’t lit because no current is flowing through the circuit (see Figure 1). So why is this?

Figure 1

Well, that’s because by default transistors are in their nonconductive state and something needs to change here to turn this transistor on enabling its conductive state. To turn the transistor on to its conductive state, we need to apply a voltage to the base of the transistor, in this case, we have the collector and the emitter but we also have a third terminal which is the base.

So moving on to my second circuit I have exactly the same circuit except now I have a separate power supply which supplies positive voltage to the base of this transistor (see Figure 2) because the voltage is now applied to the base of this transistor, this has turned this transistor into its conductive state because this transistor is now in its conductive state, current from our first power supply is now able to flow from the power supply through the LED through the transistor and back to this power supply.

Figure 2

I have a switch connected to the second power supply which supplies voltage to the base of the transistor. If I turn this power supply off by disconnecting it, you can see the circuit below it stops as well because now the transistor is back to its normal nonconductive state (see Figure 3) because the voltage is not being applied to the base of the transistor.

Figure 3

So why are transistors so common and so useful? Well without transistors today we simply wouldn’t have the luxury of computers and other digital devices. Transistors can act like very fast switches but we can control the switch “on” and “off” at very fast speeds we can switch them on and off at the rate of millions of times a second in fact.

Figure 4

If you think of a computer, you most likely know that they work with binary data essentially 0s and 1s. Those 0s and 1s essentially represent the on and off state of a transistor and there are billions of transistors in your computer’s processor. This means that the computer can understand those transistor switches as 0s and 1s and encode them into usable data and instructions because transistors can turn on and off so quickly, this gives us many advantages when we want to control other devices such as LED lights, motors enabling us to turn them on or off, vary their speeds, vary their brightness, frequency etc.

The other advantage to transistors is that we can use small logic level voltages to turn the transistor on and off and control higher powered devices. I am supplying the base of this transistor with only a 3.3-volt logic level voltage but the transistor is able to switch on and off this 12 volt high powered LED and in fact, we can use bigger transistors to switch even bigger loads such as motors and other higher powered devices (see Figure 4).

So I have this transistor supplied with a low voltage switching on and off a higher powered load. I am using a digitally controlled voltage source here which is pulsing voltage to the base of the transistor every 10 microseconds. Obviously, it’s much slower in every circuit mainly for your understanding but also because every circuit cannot show in real time the pulsing of voltage here to the base of this transistor is switching the circuit below on and off. The pulsing is known as pulse width modulation. So this voltage source can be a digital device like a microcontroller or a computer.

I won’t go too deep into pulse width modulation this is for a later post but the amazing fact here is that I am able to digitally control a high powered device with a low powered voltage source to the base of the transistor.

Figure 5

So obviously depending on the type and size of a transistor, they are able to switch high powered and low powered devices. Along with Simply Electronics as we dive deeper into more uses of transistors building simple circuits and where you can learn about many other components. Just check out our other tutorial notes related to Electronics in our plcgoods.com site.