Transistors
Background
 |
| A BC547 NPN type transistor |
It can also be used as a switch, which is either fully ON with maximum current, or fully OFF with no current. The amount of current amplification is called gain.
There are two types of transistors a NPN (Negative, Positive, Negative) or a PNP ( Positive, Negative, Positive) They are made from a semi-conductor material and the letters refer to the different layers of the transistor. Most now days are NPN types as they are the easiest type to make from silicone. Each terminal is labelled either B (base) C (collector) or E (emitter).
Technical Explanation
As I said above transistors are made of three different layers of silicone which can either be one of two types a NPN or a PNP. Each of these three layers have a terminal attached, they are the base, collector, and emitter.
Referring to a NPN type transistor, the base will have a positive charge and the other two terminals will have a negative charge (collector and emitter).
When no current is flowing In the transistor the P-type silicone layer is short of electrons and the two N-type layers have extra electrons. The P-type layer has holes in it, where the electrons should be. This normally prevents current flow and acts like a barrier.
This prevents electron flow from emitter to collector and this means that the transistor is in a OFF state.
When the transistor is connected to a power supply and a positive charge is put through the base the electrons begin to move from emitter to collector. The emitter becomes negatively charged because of the positive voltage to the base, and the collector becomes positively charged and this causes electrons to be pulled from the emitter to the base and then to the collector. This cause the transistor to be switched to its ON state.
The small voltage to the base creates a much larger current flow from the emitter to the collector and it becomes a amplifier and a switch at the same time. Because if no current is flowing to the base there will be no flow from emitter to collector.
Also regulating the voltage to the base allows the current flow to be controlled accordingly to what is required of the circuit.
Testing Transistors
Testing is done using a multimeter on diode test mode and putting the probes across each terminal. If the transistor is working correctly each pair of terminals should get a certain reading:
Problems if faulty
A transistor normally does not fail if it is used in a circuit it is designed for and it is not exposed to excessive heat from a soldering iron for example. But if it is used incorrectly and fails this would be because it has exceeded its usage range. If it fails the circuit would be broken and no current will flow and would need replacing to repair it.
Technical Explanation
As I said above transistors are made of three different layers of silicone which can either be one of two types a NPN or a PNP. Each of these three layers have a terminal attached, they are the base, collector, and emitter.
Referring to a NPN type transistor, the base will have a positive charge and the other two terminals will have a negative charge (collector and emitter).
When no current is flowing In the transistor the P-type silicone layer is short of electrons and the two N-type layers have extra electrons. The P-type layer has holes in it, where the electrons should be. This normally prevents current flow and acts like a barrier.
This prevents electron flow from emitter to collector and this means that the transistor is in a OFF state.
When the transistor is connected to a power supply and a positive charge is put through the base the electrons begin to move from emitter to collector. The emitter becomes negatively charged because of the positive voltage to the base, and the collector becomes positively charged and this causes electrons to be pulled from the emitter to the base and then to the collector. This cause the transistor to be switched to its ON state.
 |
| Diagram showing how current flows from base, to allow flow from emitter to collector |
Also regulating the voltage to the base allows the current flow to be controlled accordingly to what is required of the circuit.
Testing Transistors
Testing is done using a multimeter on diode test mode and putting the probes across each terminal. If the transistor is working correctly each pair of terminals should get a certain reading:
- From Base to Emitter it should behave like a diode and only conduct one way.
- From Base to Collector it should also conduct one way
- From Collector to Emitter there should be no conduction either way.
If you don't no what each terminal is you can also find this out using the multimeter as well. From emitter to base there should be a higher voltage drop than from collector to base. For example With the meter touching each terminal you should get the following readings at each pair:
 |
Example of terminals of a Transistor
this can be different depending on
transistor type and manufacturer
|
- Meter touching 1 and 2 (+-) OL
- Meter touching 1 and 2 (-+) OL
- Meter touching 1 and 3 (+-) 0.665V
- Meter touching 1 and 3 (-+) OL
- Meter touching 2 and 3 (+-) 0.621V
- Meter touching 2 and 3 (-+) OL
Problems if faulty
A transistor normally does not fail if it is used in a circuit it is designed for and it is not exposed to excessive heat from a soldering iron for example. But if it is used incorrectly and fails this would be because it has exceeded its usage range. If it fails the circuit would be broken and no current will flow and would need replacing to repair it.
No comments:
Post a Comment