What will be the first circuit?

[_pike]

i must say that i didn't understand clearly your answer.....    Perhaps my questions were bad?

[kam]

 hfe is the amplification parameter. take for example a magnifying glass. It has for example 4x magnifications. The "hfe" of the glass is 4. A magnifying glass has a fixed "hfe" whereas a transistor varies. The specific transistor can "magnify" the current within a range of 100 to 600. From my experience i can tell beforehand that the specific transistor will have a magnification range of 200.

[_pike]

The specific transistor can "magnify" the current within a range of 100 to 600. From my experience i can tell beforehand that the specific transistor will have a magnification range of 200.

This specific parameter is needed in order to start or continue our calculations....because IB = IC / hfe so if i pick up a transistor that i have in my drawer how would i be able to define the hfe because of my lack of experience?

[kam]

well... you can consult the datasheet, but for a precise value... you have to make tests. Make the circuit, measure base current, measure collector current, do the division (IC/IB) and there you have you hfe.
Remember, hfe (as all hybrid parameters) are not stable. You may have 2 similar transistors in your drawer, and both may have very different hfe.

[_pike]

ok i will test my first circuit on a breadboard and i would post my progress and results......
Something else that i would like to ask.....We calculate the collector current for 35ma....the relay needs 16ma...and our calculations were based on 35ma but finally it proved that the relay consumed 16ma....How did that happen?I mean why the coil didn't consumed much more (because the collector current can give double 35ma) as a result the coil to be burnt?What would might have happened if we had used a led instead of a relay coil?

[kam]

Say you have a 24V power supply able to supply up to 10amperes of current and you connect this 24V relay, will it burn? No, because:

THE CURRENT DEPENDS ON THE LOAD

So, the transistor is able to provide up to 35ma, but the final current depends on the load. Clear?

Now, if you put an LED instead of the relay, you will have fun! But only for a short while...

[_pike]

Sometimes the head stucks!!!!!!!SORRY.........
Also i forgot to mention a very important parameter.I wanted to say what would might have happend if the voltage was 3v instead of 24v.

[kam]

Then the transistor would saturate and all the current (provided by the 3V) would go through the relay, but of course it would not arm as it needs 24V

[_pike]

so far so good......Another intresting calculation might be switching on-off leds with a transistor...Shall we analyze a circuit together?
Transistor as switch again ,but with different kind of load....
LEDS!!!!!!! Controlled by PWM.......    (Corrected..... mistake by rush)

opinions?

[kam]

what do you mean that the current is not stable???