PNP transistor for many a mystery. But it should not be. If you want to design circuits with transistors, you definitely need to know about this type of transistor.
Example: If you want to automatically turn on the lights when it gets dark PNP transistor will make it easy for you.
If you understand the work NPN - the transistor, it is easier to understand PNP-transistor. They work in much the same, with one important difference: the currents in the PNP transistor flow in opposite directions, when compared with the flow of current in the transistor NPN.
How the PNP transistors?
PNP transistor has the same conclusions as the NPN:
- Base
- emitter
- Collector
Transistor PNP «inclusive», when you have a small current flowing from the emitter to the base. When I say "turn", I mean that the transistor will open a channel between the emitter and collector. And through this channel can flow is much more current.
That current flows from the emitter to the base, you need a voltage of about 0.7 V. Since the current flows from the emitter to the base, the base should have a voltage of 0.7 V lower than
, than the voltage at the emitter.Setting the voltage at the base to 0,7 PNP-transistor B is lower than the emitter, you "turn the transistor" and allows current to flow from the emitter to the collector.
I know that this may sound a little confusing, so read on to see how you can design a circuit with a transistor PNP.
Example: PNP transistor circuit
Let's see how to create a simple circuit with transistor PNP. With the help of this scheme you can "ignite" LED when it gets dark.
Step 1: Emitter
First of all, to turn the PNP-transistor, it is necessary that the voltage on the base was lower, than the emitter. To do this, connect the emitter to the positive side of your power source. So you know what you have the emitter voltage.
Step 2: What do you want to monitor
When the transistor is turned on, current flows from the emitter to the collector. So let's connect that we want to control: namely LED.
Since LEDs should always be installed in series resistor, and let's add a resistor.
Step 3: Transistor input
To turn on the LEDs must enable the transistor to the channel from the emitter to the collector opened. To enable transistor, it is necessary that the base voltage was at 0.7 V lower than the emitter, which is 9 V - 0.7 V = 8.3 V.
For example, you can now turn on the LED when it gets dark, using the LDR and the standard resistor is configured as a voltage divider.
The voltage at the base will not behave exactly as said voltage divider formula. This is because the transistor also affects the stress.
But in general, when the resistance value of the photoresistor is high (no light), the voltage will be close to 8.3 V, and the transistor is turned on (which includes LED). When the value of the photoresistor low (much light is present), the voltage will be close to 9 and disable transistor (which will turn off the LED).
I used the following components:
- Transistor PNP- BC557.
- Photoresistor - 10k when light, and 1 mOhm when it is dark.
- Resistor on the base of the transistor - 100 ohms.
- Resistor which is connected in series LED - 470 ohms.