Electronics Circuit problem.

[paulmdrdo]

Can you please help me solve the problem here. I need to find the current flowing through 100K ohm and 1.7M ohm resistor and the current flowing through the collector, emitter and the base.

please tell me what technique should I use. I know that the circuit is in a voltage-divider configuration but I'am not sure how to go about the problem. Thanks!

 

[Audioguru]

1) Guess that the 100k base resistor turns on the transistor. But I think it is only partly turned on, it is not completely turned on because the collector current is MUCH higher than the base current.
2) Assume that the transistor is partly turned on. You CANNOT calculate the collector current because you do not know the current (color) of the LEDs. Guess that they are 1.8V red LEDs then calculate the collector current.
3) Calculate the typical base current looking at the graph of typical Vbe. Calculate the emitter current.
4) Look at the graph in the datasheet for Vbe vs collector current then calculate the current in the resistors using Ohm's Law.

It is not a voltage divider. Almost all of the current in the 100k resistor goes into the base of the transistor and only a very small part of its current goes into the 1.7M resistor.

 

[paulmdrdo]

what formula do I have to use here? I'm still confused. From what loop in the circuit should I start calculating the currents?

 

[Audioguru]

You need to go to school and learn about how a simple transistor circuit works. The formulas are simple arithmetic.

The 100k resistor value is much too high which causes the transistor to barely turn on. The transistor should saturate (turn on) well.
The datasheet for the BC547 transistor shows that it saturates well when its base current is 1/20th the collector current.

With a saturation voltage (collector voltage to ground) of 0.2V and an LED voltage of 1.8V then the 20.5 ohm resistor and the collector has a current of (6V - 1.8V - 0.2V)/20.5 ohms= 19.5mA. Its base to emitter voltage on a graph shows 0.77V. Then the base current should be 19.5mA/20= 0.98mA but the 100k resistor provides a current of only 6V - 0.77V)/100k= 0.05mA and the 1.7M resistor is stealing some of that tiny current so the transistor barely turns on. The 100k resistor should be (6V - 0.77V)/0.98mA= 5.2k ohms.

See my simple arithmetic?

 

[paulmdrdo]

Yes. I'm kind of getting it already. Where can I find a graph to look for base to emitter voltage?

 

[Audioguru]

The datasheets for transistors have many graphs for a "typical" transistor. The printed text lists the minimum and maximum ratings for many spec's.
Temperature affects the base-emitter voltage a little. The text says at a collector current of 2mA the base-emitter voltage can be from 0.55V to 0.7V and the graph shows the typical voltage is 0.64V.

Here is a graph of the typical base-emitter voltage for a BC547 transistor when it is at 25 degrees C: