Solve NPN Transistor Puzzle: -0.4V?

In summary, the conversation discusses a puzzle involving a NPN transistor and a Zener diode. The question is what the voltage between the collector and ground is when the base is grounded and +12 volts is applied to the emitter through a series resistor. The answer is revealed to be about -0.4 volts, due to the effects of avalanche breakdown and photoemission in the transistor. The conversation also notes that this experiment may damage the transistor, but it is still an interesting and educational experience.
  • #1
donpacino
Gold Member
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I was just sent an email with the following text.
""
Here is a puzzle I just encountered.

Take any NPN transistor and ground the base. Now apply +12 volts to the emitter through a series resistor of 1k. Yes, it is a Zener diode of about 6 volts so about 6 ma of current flows.What is the voltage between the collector and ground? I couldn't guess. Well I could guess it was zero volts, but then, that would not have been a puzzle. So I gave up.Answer: about -0.4 volts ! (Yes negative.) Why?
""I am at a loss for understanding where they are getting this supposed -0.4V. Any takers?

edit: it is important to note my interpretation of the problem is the the collector is left open
 
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  • #3
Cool problem. Amazing effect.
 
  • #4
Note that most NPN transistors will be damaged by reverse biasing the E-B junction, especially with such large voltages. I'm not discouraging you from doing the experiment (these things are cheap, after all), but don't expect the transistor to function properly afterward.
 
  • #6
meBigGuy said:
Cool problem. Amazing effect.
My thoughts exactly. Thanks for the links everyone. Great reads!
 

1. How does an NPN transistor work?

An NPN (Negative-Positive-Negative) transistor is a type of semiconductor device that is commonly used in electronic circuits. It consists of three layers of doped materials, with a layer of P-type material sandwiched between two layers of N-type material. When a small current is applied to the base of the transistor, it allows a larger current to flow from the collector to the emitter. This makes it useful for amplifying or switching electronic signals.

2. What is the significance of -0.4V in the NPN transistor puzzle?

The -0.4V refers to the base-emitter voltage (Vbe) of the NPN transistor. This is the voltage required to turn on the transistor and allow current to flow from the collector to the emitter. In the puzzle, the goal is to find the correct combination of resistors that will result in a Vbe of -0.4V.

3. How do I solve the NPN transistor puzzle?

To solve the NPN transistor puzzle, you will need to use Ohm's Law (V=IR) to calculate the voltage drop across each resistor in the circuit. The sum of these voltage drops should equal the -0.4V required for the Vbe. From there, you can experiment with different resistor values until you find the correct combination that results in a Vbe of -0.4V.

4. What are some applications of NPN transistors?

NPN transistors have a wide range of applications in electronics. They can be used in amplifiers, switching circuits, and digital logic circuits. They are also commonly used in power supplies, voltage regulators, and audio circuits. NPN transistors are essential components in many electronic devices, including computers, televisions, and smartphones.

5. Are there other types of transistors besides NPN?

Yes, there are two other types of transistors: PNP (Positive-Negative-Positive) and JFET (Junction Field-Effect Transistor). PNP transistors work in the opposite way of NPN transistors, with a layer of N-type material sandwiched between two layers of P-type material. JFETs, on the other hand, use only one type of material and rely on an electric field to control the flow of current. NPN transistors are the most commonly used type, but the other types have their own unique applications as well.

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