Please help me bias a transistor!

[icetea]

my name is Nick and i'm new here! a friend of mine from here helps me so i can learn electronics! but many things he says i dont understand them so here i am! i want to bias a transistor so i can light a simple led! i know ohms law and how transistors work but i cant do it! please tell me stepbystep what to do!!!!!!!!!!!!!!!!!!!!!

 

[CynicalMan]

This is a good site.
**broken link removed**

 

[aljamri]

This is a good site.
**broken link removed**

very good site to visit. thanks CynicalMan.

 

[icetea]

ok it helped! now who is going to tell me whats the difference between Vebo (emitter-base voltage) and Vbe (base-emitter voltage)?

 

[audioguru]

Vebo is when the base-emitter of a transistor is reverse-biased. Then the transistor is turned off very quickly. When the base-emitter voltage of a transistor is zero then it turns off pretty quickly but not as fast as when the base-emitter has a reverse voltage. Most silicon transistors have a max allowed reverse emitter-base voltage of only 5V to 7V.

Vbe is when the base-emitter of a transistor is forward bised and the transistor is turned on. The forward voltage is about 0.6V for a low current to 1.5V for a very high current. Most little transistors have a Vbe of about 0.7V at a reasonable current.

 

[icetea]

I got these results is my transistor biased correctly?

Transistor 2n3904
Resistors: 5.8K base, 385Ω collector.

hFE=202
Vcc=14V
Ib=0.0026A ?
Vbe=0,79V
Vce=2.18V
across led=2.05V
Ic=30.7mA

0.0026x5800=15.8

15.8+0,79=16.59V

 

[Nigel Goodwin]

I got these results is my transistor biased correctly?

Depends how you wanted it biased, and what you're trying to do with it? - there seems to be a random mention of an LED there?, so what's that all about?. Post a circuit diagram of what you're doing.

 

[icetea]

heres a photo

 

[Nigel Goodwin]

No 'biasing' required then, it's just a switch - just bang enough current in the base to make sure it's turned hard on.

 

[icetea]

what about the above calculations why i'm getting wrong results?
also the 385Ω resistor gets hot!!!!!!!!!!!!!!!! my friend cant explain whats going on with the Ib and we get such results and about the hot resistor he says that is smaller than the power dissipation about 0,1W and needs bigger than 1/4W is that correct?

 

[audioguru]

The transistor is biased with a base current of 2.3mA and has a collector current of about 30.1mA. The transistor is turned on hard so its collector voltage is about 0.1V.

 

[audioguru]

The 385 ohm resistor has about 11.9V across it so it dissipates about 368mW which is far more than a little 1/4W resistor can dissipate.

Where did your arithmatic fail?

 

[aljamri]

I got these results is my transistor biased correctly?

across led=2.05V
Ic=30.7mA

I guess your LED is very bright with more than 30 mA.

15.8+0,79=16.59V

16.59V from a supply of 14V, are you sure?

 

[icetea]

The transistor is biased with a base current of 2.3mA and has a collector current of about 30.1mA. The transistor is turned on hard so its collector voltage is about 0.1V.

if the base current is 2.3mA the calculation is fine and gets close to supply voltage

0.0023x5800=13.34

13.34+0,79=14.13V

but our indication is 0.0026A and not 0.0023A!!! what was that 0.1V at the collector? i dont understand! the voltage at the collector-emitter is 2.18V

 

[audioguru]

The emitter is at 0V. The base is 0.8V when it has 2.3mA. So the 5800 ohm resistor limits the base current to (14V - 0.8V)/5800= 2.3mA.

The emitter is at 0V. The transistor is turned on hard so its collector voltage is 0.1V. Therefore its Vce is 0.1V.

The Vce would be 2.18V if the transistor did not have enough base current to turn on properly.

 

[icetea]

how do we measure this 0.1V? our transistor it is supposed to be switched on "hard" so the collector is not 0.1V right?
when you say that the emitter is 0V probably you mean that the we got common emitter right?

 

[audioguru]

how do we measure this 0.1V?

Look at the curve of the saturation voltage in the datasheet. With a collector current of 30mA and a base current of 3mA the typical saturation voltage (Vce) is only 0.075V. Your base current is a little less so the saturation voltage is about 0.1V.

our transistor it is supposed to be switched on "hard" so the collector is not 0.1V right?

Your transistor has plenty of base current so it is turned on hard and its collector voltage is only about 0.1V.

when you say that the emitter is 0V probably you mean that the we got common emitter right?

Your schematic shows a common-emitter transistor. Its emitter is connected to zero volts (0V).