# Homework Help: Transistor As Switch

1. Oct 13, 2008

### Jason7fd

1. The problem statement, all variables and given/known data
http://img300.imageshack.us/img300/2640/17569077cy5.jpg [Broken]
The transistor is a N-P-N type. When the siren is functional, the resistance of thermistor is 120 ohm. R1 is 600 ohm. The question is "Calculate the collector current if the emitter current is 100mA". I know that current of emitter = current of collector + current of base, but how do I find the base current? Another one, "what is the maximum voltage across R1 if the siren is functional?".

2. Relevant equations
none

3. The attempt at a solution
none

Last edited by a moderator: May 3, 2017
2. Oct 13, 2008

### Defennder

You didn't provide the value of R2. Also you didn't specify which end of the BJT is the emitter or collector. Is the BJT functioning in forward-active mode?

3. Oct 13, 2008

### Jason7fd

I hav updated the schematic diagram. Sorry, Im not sure what are BJT and forward-active mode. The value of R2 is not provided in this question.

4. Oct 13, 2008

### Defennder

Uh, how are you expected to do this question if you don't know how a bipolar junction transistor (BJT) works in circuits? Do you know the equation for collector current in terms of VBE? What is value of B, the beta parameter of the BJT?

5. Oct 13, 2008

### Jason7fd

I wish I was taught about this in school but they are not in the syllabus. It is a question from O-level Physics of Malaysia.

6. Oct 14, 2008

### Defennder

Hi, could you post the question exactly as it is ? Because what you have provided so far is insufficient.