Summary of PNP Transistor – How Does It Work?
The article explains PNP transistors, comparing them to NPN types and highlighting that currents flow in the opposite direction. A PNP turns on when a small emitter-to-base current flows, requiring the base to be about 0.7V lower than the emitter, which opens a channel allowing larger emitter-to-collector current. It gives a simple example: connect the emitter to the positive supply, and use the collector to drive a load (like an LED with a series resistor) that is switched when the transistor is turned on.
Parts used in the PNP Transistor Circuit:
- PNP transistor
- Power source (positive supply)
- LED
- Series resistor for the LED
The PNP transistor is a mystery to many. But it doesn’t have to be. If you want to design circuits with transistors, it’s really worth knowing about this type of transistor.
For example: Want to automatically turn on a light when it gets dark? The PNP transistor will make this easy for you.
In my article how transistors work, I explained how a standard NPN transistor works. If you haven’t already, I’d really urge you to read that article first.
If you understand the NPN transistor, it will make it easier to understand the PNP transistor. They work pretty much in the same way, with one major difference: The currents in the PNP transistor flow in the opposite directions of the currents in the NPN transistor.
Note: This topic is much easier with an understanding of current flow and voltages.
How PNP Transistors Work
The PNP transistor has the same leg names as the NPN:
- Base
- Emitter
- Collector
A PNP transistor will “turn on” when you have a small current running from emitter to base of the transistor. When I say “turn on”, I mean that the transistor will open up a channel between emitter and collector. And this channel can carry a much larger current.
To get current running from emitter to base, you need a voltage difference of about 0.7V. Since the current goes from emitter to base, the base needs to be 0.7V lower than the emitter.
By setting the base voltage of a PNP transistor to 0.7V lower than the emitter, you “turn the transistor on” and allow for current to flow from emitter to collector.
I know this can sound a bit confusing, so read on to see how you can design a circuit with the PNP transistor.
Example: PNP Transistor Circuit
Let’s see how to create a simple PNP transistor circuit. With this circuit you can use to turn on an LED when it gets dark.
Step 1: The Emitter
First of all, to turn on the PNP transistor, you need the voltage on the base to be lower than the emitter. For a simple circuit like this, it’s common to connect the emitter to the plus from your power source. This way, you know what voltage you have on the emitter.
Step 2: What You Want To Control
When the transistor turns on, the current can flow from the emitter to the collector. So, let’s connect what we want to control: An LED. Since an LED should always have a resistor in series with it, let’s add a resistor too.
Read more: PNP Transistor – How Does It Work?
- What causes a PNP transistor to turn on?
A PNP transistor turns on when a small current flows from emitter to base, which occurs when the base is about 0.7V lower than the emitter. - How are the legs of a PNP transistor named?
The legs are named base, emitter, and collector. - Which direction does current flow in a PNP transistor compared to an NPN transistor?
Currents in a PNP transistor flow in the opposite directions of the currents in an NPN transistor. - How can you ensure the emitter voltage is known in a simple PNP circuit?
By connecting the emitter to the positive supply of the power source. - What voltage difference is typically needed between emitter and base to turn on a PNP transistor?
About 0.7V lower on the base than the emitter is typically needed to turn it on. - How is an LED used in the example PNP circuit?
The LED is connected to the collector with a series resistor so it is driven when the transistor turns on.
