Power dissipated across a transistor regulator

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Discussion Overview

The discussion revolves around the power dissipation across a transistor regulator, specifically focusing on calculations related to voltage and current in the circuit. Participants explore the implications of their calculations for both minimum and maximum power dissipation.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents a calculation for the voltage across the collector-emitter junction and the power dissipated by the transistor, arriving at a specific numerical answer.
  • Another participant suggests that there may be an error in the question and proposes calculating the maximum power dissipation instead, noting its importance for heat sink requirements.
  • A third participant agrees with the previous point, indicating that the power dissipation in the transistor is a critical factor in circuit design for higher power applications.
  • A later reply confirms that the initial participant arrives at the correct answer when considering the second question as having a typo and being about maximum power dissipation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the original question's intent, with some suggesting a potential typo while others focus on the calculations presented. Multiple views on the importance of maximum power dissipation remain evident.

Contextual Notes

There are indications of possible errors in the question, particularly regarding the interpretation of minimum versus maximum power dissipation, which could affect the calculations and conclusions drawn.

Who May Find This Useful

Readers interested in transistor circuits, power dissipation calculations, and thermal management in electronic design may find this discussion relevant.

jaus tail
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Homework Statement


upload_2017-12-13_10-41-38.png
[/B]

Homework Equations


For the 1st part
Ohms law and resistance across zener is 10V[/B]
I can't get 2nd

The Attempt at a Solution


For 1st part I took Vi/p = 18V[/B]
Putting KVL for left loop of source-->collector-->base-->zener-->ground
18 = Vcb + 10
So Vcb = 8V
And Vbe = 0.7V
So Vce = 8.7V Correct answer.

For second question,
P dissipated in transistor = Vce times total current = Vce times Ie
Ie = 10 - 0.7 divided by 1 K
=9.3mA
So P dissipated by transistor = 8.7 times 9.3 = 80.91 mW
Book answer is A.
Where am I wrong?
 

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I haven't checked the numbers as I'm on my phone but I suspect an error in the question. Try calculating the maximum power dissipation rather than the minimum. The max power dissipation is usually a much more important value to calculate as it determines the heat sink requirements for the transistor.
 
PS In this circuit you may not need a heatsink but the power dissipation in the transistor is the main reason why other circuits are sometimes preferred for higher power applications.
 
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Likes   Reactions: jaus tail
Yup you're right. I get the correct answer if the consider the second question to have typo and be maximum. Thanks.
 

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