V."Calculating Vce: What to Know

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Discussion Overview

The discussion revolves around calculating the collector-emitter voltage (Vce) in a transistor circuit, focusing on the need for a current limiting resistor at the collector and how to determine its value. The context includes theoretical and practical aspects of transistor operation, particularly in switching and amplification applications.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory

Main Points Raised

  • Some participants propose that Vce is the difference between the collector voltage and the emitter voltage, with examples provided for when the transistor is on and off.
  • Others mention the necessity of a collector resistor when the transistor is on to limit current, highlighting that Vce cannot drop below the saturation voltage (Vce,sat).
  • Questions arise regarding the calculation of the current limiting resistor, particularly in relation to a 12V supply.
  • One participant humorously notes the risk of damaging the transistor if a current limiting resistor is not used.
  • Another participant suggests that the calculation of the resistor value can vary in complexity depending on the application, providing a basic formula for a simple switching scenario.
  • Concerns are raised about the power dissipation of the transistor and the implications for resistor sizing, particularly in linear amplifier applications.

Areas of Agreement / Disagreement

Participants generally agree on the importance of a current limiting resistor and the basic relationship of Vce, but there is no consensus on the specific calculations or approaches to take, as multiple views and scenarios are presented.

Contextual Notes

Limitations include the lack of specific values for the circuit components and the dependence on the transistor's characteristics, such as maximum collector current and power dissipation capabilities. The discussion also does not resolve the complexities involved in using the transistor as a linear amplifier versus a simple switch.

nothing909
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Homework Statement


Calculate Vce.

The Attempt at a Solution


Without any values or anything, I just want to know if the Vce is the difference between the voltage at the collector and the voltage at the emitter. So if there's like a 12V supply and the transistor is on, then Vce would be 12-12=0 and if its off Vce will = 12
 
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nothing909 said:

Homework Statement


Calculate Vce.

The Attempt at a Solution


Without any values or anything, I just want to know if the Vce is the difference between the voltage at the collector and the voltage at the emitter. So if there's like a 12V supply and the transistor is on, then Vce would be 12-12=0 and if its off Vce will = 12
Mostly. You need a collector resistor when the transistor is on to limit the current, and the lowest you can go is Vce,sat. But you probably already know that... :smile:
 
Why do I need a current limiting resistor at the collector and how do I calculate at current limiting resistor if I have a 12V supply to it?
 
nothing909 said:
Why do I need a current limiting resistor at the collector and how do I calculate at current limiting resistor if I have a 12V supply to it?
Because if you turn on a transistor that is connected across a 12V supply, you will let the smoke out of it...
 
lol, ok, how do I calculate a the value I need for the resistor?
 
nothing909 said:
lol, ok, how do I calculate a the value I need for the resistor?

That can be very easy to calculate or much harder depending on what you are doing. If you are just using the transistor as an On and Off switch and it's switching relatively slowly then it's not so hard...

How much current can the transistor handle (max collector current)? Let's say it can handle 1A. Then R=V/I = 12/1 = 12 Ohms. eg it must be larger than 12 Ohms.

However you might also need to check how much power the transistor can dissipate. Suppose the transistor can only dissipate 0.2W without a heat sink. If Vcesat is 0.3V then the power dissipated would be 0.3V * 1A = 0.3W which is too much. So either add a heat sink or increase R.

If the transistor is being used as a linear amplifier then it's much more complicated. See the turorial Berkman posted.
 

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