|
Here it is. After several unsuccessful circuits I present the NPNP-Detector.
The complete circuit cannot be implemented due to lack of space but the one exposed here is the one that performs the detection.
This circuit will tell us if the transistor is NPN or PNP and also the position of the base collector and emitter terminals.
OPERATING PRINCIPLE:
Take advantage of the difference between the three semiconductors that make up a transistor. Both doping and surface, the collector is very different from the emitter producing different behaviors if we invest these two in a circuit. For analysis, just polarize pin to pin and see the result.
DESCRIPTION:
- Top left there are three generators that simulate a 3-bit shift register.
- The three diodes that follow are to ensure that the transistor is isolated from the rest of the circuits.
- Below are three switches that allow us to switch between two transistor configurations more quickly.
- Operational measures the tensions (from top to bottom)
Vcb, Vbe, Vce. Following the law of the arrow what points to the one with the greatest potential.
- The rest of the circuit is responsible for showing us in a color code the results of the analysis.
COLOR CODE:
I will refer to the circuit as it is connected.
- The yellow LED will light when the three pins of the transistor have voltage, so the reading will be valid only when this LED is on.
- The three magenta LEDs illuminate to indicate which transistor pin is being polarized.
- The six LEDs below are paired and have the colors corresponding to the two terminals to which they refer. For example, pin 1 (Vc) and pin 2 (Vb) are blue and green colours. If, after the analysis, the blue has lit, that means that the voltage Vcb > 0. That is, the tension of the collector is greater than the tension of the base.
THE LIGHTED LED REPRESENTS THE ARROW POINT
Vbe > 0 equal to Vb <--- Ve
COLOR DECODIFICATION:
It will only be a correct result when the yellow LED and the other 3 LEDs are on.
To distinguish a NPN transistor from a PNP, it must be set if the yellow LED has been lit once or twice in the three pulse sequence. Once NPN, two PNP. You can see red line showing it.
Do not worry if the yellow LED has been turned on twice on the PNP transistor because the rest of the LEDs will only have done it in one of those configurations.
To identify the terminals of the transistor you have to take paper and pencil (tomorrow, you will do it automatically):
- Draw an equilateral triangle.
- Number 1 to 3 vertices
- When you have found the correct color code, draw arrows respecting the law of greatest potential.
There will always be two arrows pointing to the same terminal. That terminal will be the BASE in the case of NPN and the EMITTER in the case of PNP.
- The other arrow will always point to the COLLECTOR.
- The magenta LED also matches the base in the NPN and the emitter in the PNP.
The algebraic explanation helps to understand about the little arrows. As we know Vce = Vbe + Vcb (because I feel like it, then the equation will solve itself). We also know that the Vce in saturation is practically 0. Thus the equation before is left as 0 = Vbe + Vcb. On the other hand we have that Vbe = 0'7v (for silicon transistors and in normal operating conditions). The equation solves itself resulting in
Vcb = -Vbe
Or what is the same, Vbc = 0'7v.
This explains why the base voltage is greater than collector and emitter and that is why in an NPN transistor in saturation, there will ALWAYS be two arrows pointing to the base. Do not ask me for the PNP that you surely know.
I hope you liked it.
|
