Need to calculate current flowing through parallel resistors.

[rdhc1330]

Much closer now .



Rt = (R3*R5*R6 / R3+R5+R6) = the total resistance of the resistors which are *not* the resistor in question which you are trying to find the current through ... Correct.

However, Rn = R4 = the resistor in question which you are trying to find the current through.

Therefore Rt + Rn = [ (R3*R5*R6 / R3+R5+R6) + R4 ] and not (R3*R4*R5*R6 / R3+R4+R5+R6).

so -->

(90*70*60 / 90 + 70 + 60)
-------------------------------
(90*70*60 / 90 + 70 + 60) + 80

 

[Zryn]

My apologies, when I read what you said in post #29 I mistakenly thought I read something different to what was actually there, and I have since amended my post #30. Read it again and then comment, but you're on the correct path, just need to get the numbers right.

** An easy way of calculating the total resistance of two resistors in parallel is Rt = Ra*Rb / (Ra + Rb), however this shortcut does not work for any number other than two. When you wrote down (R3*R5*R6 / R3+R5+R6) I had momentarily forgotten that this method will not work for three resistors and advised you that it was correct, when it is not.

You could pair the resistors up progressively, so that Rt = R3||R5||R6 = (R3*R5)/(R3+R5)||R6 = [(R3*R5)/(R3+R5)]*R6 / [(R3*R5)/(R3+R5) + R6], but you can not say that Rt = R3*R5*R6 / R3+R5+R6.

 

[rdhc1330]

My apologies, when I read what you said in post #29 I mistakenly thought I read something different to what was actually there, and I have since amended my post #30. Read it again and then comment, but you're on the correct path, just need to get the numbers right.

hey wait, can't I just go 6.2 / 80? 0.078A

if we do 6.2/90 = 0.069A
6.2/80 = 0.078A
6.2/70 = 0.089A
6.2/60 = 0.103A

== 0.339A

 

[Zryn]

Yes you can, that is the correct answer and the discrepancy is only due to rounding errors.

The current divider that you were going to end up with is as follows:

Ir = Rt / (Rt + Rn) * I

Ir4 = (R3||R5||R6) / (R3||R5||R6 + R4) * I

Ir4 = [ 1 / (1/R3 + 1/R5 + 1/R6) ] / { [ 1 / (1/R3 + 1/R5 + 1/R6) ] + R4 } * I

Ir4 = [ 1 / (1/90 + 1/70 + 1/60) ] / { [ 1 / (1/90 + 1/70 + 1/60) ] + 80 } * 0.337

Ir4 = 23.77 / ( 23.77 + 80 ) * 0.337

Ir4 = 0.229 * 0.337

Ir4 = 0.077

 

[rdhc1330]

Yes you can, that is the correct answer and the discrepancy is only due to rounding errors.

The current divider that you were going to end up with is as follows:

Ir = Rt / (Rt + Rn) * I

Ir4 = (R3||R5||R6) / (R3||R5||R6 + R4) * I

Ir4 = [ 1 / (1/R3 + 1/R5 + 1/R6) ] / { [ 1 / (1/R3 + 1/R5 + 1/R6) ] + R4 } * I

Ir4 = [ 1 / (1/90 + 1/70 + 1/60) ] / { [ 1 / (1/90 + 1/70 + 1/60) ] + 80 } * 0.337

Ir4 = 23.77 / ( 23.77 + 80 ) * 0.337

Ir4 = 0.229 * 0.337

Ir4 = 0.077

Aha! Awesome, I understand this so clearly now. When I realized what I was doing with the voltage it became so clear! Thanks a lot Zryn, so very very much!

 

[rdhc1330]

Yes you can, that is the correct answer and the discrepancy is only due to rounding errors.

The current divider that you were going to end up with is as follows:

Ir = Rt / (Rt + Rn) * I

Ir4 = (R3||R5||R6) / (R3||R5||R6 + R4) * I

Ir4 = [ 1 / (1/R3 + 1/R5 + 1/R6) ] / { [ 1 / (1/R3 + 1/R5 + 1/R6) ] + R4 } * I

Ir4 = [ 1 / (1/90 + 1/70 + 1/60) ] / { [ 1 / (1/90 + 1/70 + 1/60) ] + 80 } * 0.337

Ir4 = 23.77 / ( 23.77 + 80 ) * 0.337

Ir4 = 0.229 * 0.337

Ir4 = 0.077

I have a quick question, how do you calculate Collector emitter resistance?

 

[Zryn]

It's best to start a new thread with any diagrams, equations and workings you have.