V."Calculating Vce: What to Know

[nothing909]

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

 

[berkeman]

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...

 

[nothing909]

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?

 

[berkeman]

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...

 

[nothing909]

lol, ok, how do I calculate a the value I need for the resistor?

 

[berkeman]

lol, ok, how do I calculate a the value I need for the resistor?

Depends on what you are trying to do. Here is basic design info for a Common Emitter amplifier:

http://www.electronics-tutorials.ws/amplifier/amp_2.html

 

[CWatters]

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.