the right transistor??

is there any method to know the correct transistor for my switching applications
such as feeding a relay or motor dependently on a micro-controller signal at the base of transistor

what is the value Hfe and how can I choose the correct value??
how can i determine the right base resistor??
how can i avoid the overheating of a transistor??

please help me coz i think this is the most important block in electronics design

Well, I'm not sure I'd say it this most important thing to know...

There are a number of things you need to consider in selecting the transistor, in no particular order:
What voltage will it operate at?
What is your current load?
What is the available drive current and voltage?
How fast do you need to switch your load?

As far as hfe, a rule of thumb is to drive the base with one-tenth the collector current to ensure the transistor goes into saturation. The base resistor is determined by the drive voltage and base current.
You can avoid overheating the transistor by not dissipating more than its rated power (I'm sorry if that sounds like circular reasoning). The power in the transistor is equal to load current multiplied by the saturation voltage.

Hi
Before going to anywhere else, you said a word a "switching regulator". Mostly for switching regulators we use a MOSFET or IGBT because of their performance. But their performance also depends where are they being used. You should look very carefully for this.

Eshal said:

Hi
Before going to anywhere else, you said a word a "switching regulator". Mostly for switching regulators we use a MOSFET or IGBT because of their performance. But their performance also depends where are they being used. You should look very carefully for this.

Click to expand...

OP said NOTHING about switching regulator!!!!

Oops,, I mean for switching applications
For switching applications you need to make your switch on and off instantly with loosing of power so MOSFETs and IGBTs are good this purpose. So specific BJTs are also good.

Aduul!
You should get a transistor data book,or check the internet and download the data books if they are downloadable(i am not sure). keep in mind the voltage and current requirements for your specific applications and the speed of switching required,use those models with at least a little higher parameters for safety reasons. It will be best to use a transistor with having 2 times higher voltage and current capabilities as compared to actually required range for the application, if the price and availability are not an issue of concern.
Hfe and switching speeds can be used same as required, may not need to go for higher range of these than actually required.

What is the value Hfe and how can I choose the correct value?

Find out the load resistance RL. Then supply voltage Vs should be preferably little higher than the relay operating voltage but same voltage will do. Take Vsat as about 1 volt. So I = (Vs-Vsat)/RL. Choose the transistor whose rated collector current is 4 times the I. The divide I by the gain (Hfe) of that transistor to get Ib.

How can i determine the right base resistor?

Then find out the driving voltage ,say 3 volt from the microcontroller. Work out the series resistor at the base Rb=3/Ib.

How can i avoid the overheating of a transistor?

Vsat is responsible for heat. Choose a transistor with low Vsat and try to feed a current 2 times the requirement as per gain.

Jibby Benjamen said:

What is the value Hfe and how can I choose the correct value?

Click to expand...

Hfe is NOT used when a transistor is a switch. Hfe is used when a transistor is a linear amplifier that has plenty of collector to emitter voltage.
When a transistor is used as a switch then you want it to saturate when it is turned on so that its collector to emitter voltage is very low. Then the base current should be 1/10th the collector current (but should be more for many power transistors switching a high current).
Use Ohm's Law to calculate the base resistor value.