Transistor calculations

[bitem2k]

How do i calculate current through a transistor?
Where can i find voltage drop for each pin of a transistor?

thanks

 

[Analog]

post the schematic of your homework.

 

[bitem2k]

Im not at school. Just a simple hobbyist.

I need to know because im designing a simple circuit to place a light in my car glovebox.
Ive made the circuit and it works fine, but i need to know how to optimize it, as i plan to run it on a 9v battery.

See schematic included
I might, if needed add a LDR if nessercary but just this for the moment.

Im still quite confused about the various voltage drops that occur with different components.
when i make a circuit i can do a few ohms law calcualtions but i dont know how much 'juice' to feed the other components. I also dont know how these components affect the calculations i do.

I know about voltage drops over a led, and can include them in my calcs , but i dont understand the transistor specs from the manufacturer.

thanks

 

[ljcox]

Why do you need transistors? The LED and 200 Ohm resistor could be in series with the switch.

If you really want to use a transistor, one would suffice.

In order to saturate a transistor, you need the base current to be about 1/10th of the collector current.

The circuit you posted will not be thermally stable. The first transistor needs a resistor to 0V.

You don't need the 10 Ohm resistor.

 

[bitem2k]

I need the transistor to invert the input of the switch.
I have got rid of the second transistor and my circuit now looks like this, thanks to the input of people on another thread.

many thanks.

 

[audioguru]

Ask the question in the other thread.

 

[bitem2k]

Dont worry about it , thanks. Sorry for the multithreaded affair.

 

[Roff]

I need the transistor to invert the input of the switch.
I have got rid of the second transistor and my circuit now looks like this, thanks to the input of people on another thread.

many thanks.

The 2mA through the switch will drain a 9V alkaline battery in a couple of weeks. You should use a normally open switch if possible, and ditch the transistor. If that's not possible, use a small MOSFET like 2N7000 or 2N7002, and your pullup resistor can then be large, like 1 Megohm.

 

[bitem2k]

ok thanks;