circuit analysisresistorsschematics
I am having trouble with a circuit:
simulate this circuit – Schematic created using CircuitLab
I must calculate the voltage across R1.My answer is to first compute the current running across R1 and R4 , the two forming a series circuit:
6V/(1k+2k)ohms=0,002A
Then I calculate the voltage drop across R1:
V1=I*R1=0,002*1000=2V
It seems that I am wrong , but I don't understand how to do it in another way.
How must I compute the voltage across R1?
You can use the Shockley equation, which is another idealization, but a better one than a fixed diode drop. To apply it, you have to know, for the given diode, all the parameters:
$$I=I_\mathrm{S}\left( e^{V_\mathrm{D}/(n V_\mathrm{T})}-1 \right)$$
I would suggest to replace the transistor with an actual diode model, from which you can pull out the parameters and calculate by hand.
In this answer to a question, I was working with the formula, in relation to the behavior of a diode model in CircuitLab. See the question's "appendix" at the bottom.
After step 6, your circuit will be as shown below.
simulate this circuit – Schematic created using CircuitLab
Which can be further reduced as a 16V battery in series with 3k and 5k resistors.
Now, you will have 10V across this 5k resistance, ie, 10V across each parallel 10k. Splitting 10k into 6k+4k, you will have 4V across 4k resistance. From the polarity of v1 marked, V1= -4V.
Best Answer
R1 and R2 are NOT in series.
Here is how you can do it:
R1, R2 and R3 are in parallel. Calculate the parallel resistance of those three resistors so you can see them as one and call it R123.
Now R123 and R4 are in series. They form a voltage divider.
Now you should be able to calculate the volatge accross R123 (which is equal to the volatge across R1).