Base current calculation for 2N2222 transistor

Thread Starter

sharath_krishna

Joined Apr 8, 2018
3
I am trying to understand the basics. I am trying to run the motor in uni direction for my robot. Help me out to solve the following problem Please find the attachment for the circuit. when Ic = 50mA and Vb = 3.3V find Beta and Rb. I couldn't able to find any relation between Ic and Beta without knowing Ib and Ie.

bjt.png
https://photos.app.goo.gl/dTjpTWs3osy5RkNk2
 
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WBahn

Joined Mar 31, 2012
25,759
I am trying to understand the basics. I am trying to run the motor in uni direction for my robot. Help me out to solve the following problem Please find the attachment for the circuit. when Ic = 50mA and Vb = 3.3V find Beta and Rb. I couldn't able to find any relation between Ic and Beta without knowing Ib and Ie.
https://photos.app.goo.gl/dTjpTWs3osy5RkNk2
You need to make some assumptions (or declare some conditions). First, since you are using this to run a motor, you might assume (or declare) that you are using the transistor as a switch and you simply want to turn it on and off. Is that a reasonable assumption for you? Or are you trying to use the transistor to control the speed of the motor? We don't know -- this is, after all, YOUR robot and your homework.

Assuming you want to use it as a switch, what do you know about the three operating regions you want the transistor to be in?

Now you might look at a data sheet and see what they have to say about the value of beta when the transistor is in saturation.
 

MrAl

Joined Jun 17, 2014
7,591
I am trying to understand the basics. I am trying to run the motor in uni direction for my robot. Help me out to solve the following problem Please find the attachment for the circuit. when Ic = 50mA and Vb = 3.3V find Beta and Rb. I couldn't able to find any relation between Ic and Beta without knowing Ib and Ie.

View attachment 150032
https://photos.app.goo.gl/dTjpTWs3osy5RkNk2
Hi,

Well then try to calculate Ib first, then you know what Ie is.

What we dont know yet though is what you are allowed to assume for the base emitter voltage drop. Is it 0.7 as many problems like this use? That allows you to calculate the base current easy.

One small point, Vb is shown as 3.3v but that is often called "Vin" because Vb is often reserved for the actual base to ground voltage measurement, like that 0.7v as above. As long as you know where that voltage is measured from though you are ok.
 

WBahn

Joined Mar 31, 2012
25,759
What we dont know yet though is what you are allowed to assume for the base emitter voltage drop. Is it 0.7 as many problems like this use? That allows you to calculate the base current easy.
Not quite as easy as that -- Rb is one of the unknowns the TS is being asked to find.
 

MrAl

Joined Jun 17, 2014
7,591
Not quite as easy as that -- Rb is one of the unknowns the TS is being asked to find.
Hi,

Yes that is a little strange. Maybe then solve for the ratio Rb/Beta instead of each one individually.
Something might be missing from this problem.
 

Bordodynov

Joined May 20, 2015
2,603
If you carefully look at the datasheet, you will see that the ratio of collector current and base current in saturation mode is 10. Therefore, I suggest not to be clever and set base current equal to 50mA / 10 = 5mA. Hence the emitter current will be 55mA.
 

MrAl

Joined Jun 17, 2014
7,591
If you carefully look at the datasheet, you will see that the ratio of collector current and base current in saturation mode is 10. Therefore, I suggest not to be clever and set base current equal to 50mA / 10 = 5mA. Hence the emitter current will be 55mA.
Hi,

Sounds reasonable.
 

ericgibbs

Joined Jan 29, 2010
10,032
Hi,
We should limit our posts regarding Homework questions to giving hints/clues to the TS on how to solve their assignments.
It will not help them learn their class subjects, by just posting the answers to a problem.
Moderation.
 

WBahn

Joined Mar 31, 2012
25,759
If you carefully look at the datasheet, you will see that the ratio of collector current and base current in saturation mode is 10. Therefore, I suggest not to be clever and set base current equal to 50mA / 10 = 5mA. Hence the emitter current will be 55mA.
That has already been suggested as a HINT. Namely:

Now you might look at a data sheet and see what they have to say about the value of beta when the transistor is in saturation.
Since the TS hasn't been back, there's no reason to conclude that they were struggling with the suggestion.

The fact that the assignment didn't come out and tell them what beta to use is a strong indication that part of the point of the assignment was for the students to go down that road. Hence why we need to refrain from just giving them such information and give them HINTS and suggestions on how to discover it.
 

MrAl

Joined Jun 17, 2014
7,591
Hi,
We should limit our posts regarding Homework questions to giving hints/clues to the TS on how to solve their assignments.
It will not help them learn their class subjects, by just posting the answers to a problem.
Moderation.
Hi there Eric,

If you ask me it's all still questionable what the heck they want the student to do, so it seems like all guesswork and the student, if they return, will have to decide who/what is right :) :)

My problem with these kinds of questions has always been that we dont have a view of what the student was learning prior to this like we would if we were teaching the entire class for the entire semester. If they dont even know where to start then it's hard to figure out what technique they need to use to solve it.
 

WBahn

Joined Mar 31, 2012
25,759
Hi there Eric,

If you ask me it's all still questionable what the heck they want the student to do, so it seems like all guesswork and the student, if they return, will have to decide who/what is right :) :)

My problem with these kinds of questions has always been that we dont have a view of what the student was learning prior to this like we would if we were teaching the entire class for the entire semester. If they dont even know where to start then it's hard to figure out what technique they need to use to solve it.
That, sadly, is just the way it is and the way it always will be. Sometimes it's because the student doesn't give us the same information they have. Sometimes it's because the problem just plain lacks the information. Sometimes it's because the question is intended to be read in the context of the course being take and what is being covered at the point.

The best we can do is help the student identify that we lack the necessary information and, if needed, help them understand what is missing and, ideally, why it is needed. It often happens that they respond right away with the rest of the information, but certainly lot always. If they can't or won't provide the needed information, then there is very little that we can do.
 
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