Author Topic: Calculate value Resistante through voltage divider  (Read 1679 times)

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Offline JasonbitTopic starter

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Calculate value Resistante through voltage divider
« on: October 18, 2018, 10:45:41 pm »
Hello,

How can I isolate the x, in this fraction? https://postimg.cc/xNV8665j
I am trying calculate the value of R2 through voltage divider (https://www.electronics-tutorials.ws/resistor/res10.gif)

I only need the value of the R2, because I have the value Vout, Vin and R1. This question is relative mathematic, I know, but I spend many hours try search about this theme around the web. (Actually, I do not even know how to search...)

Thanks
 

Offline RandallMcRee

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Re: Calculate value Resistante through voltage divider
« Reply #1 on: October 18, 2018, 11:24:57 pm »
Umm
search for "voltage divider calculator".

Lots to choose from.
 
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Offline Wimberleytech

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Offline pigtwo

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Re: Calculate value Resistante through voltage divider
« Reply #3 on: October 18, 2018, 11:36:35 pm »
I think OP is just asking how to solve the equation.

It is not possible to isolate x in the expression x/(x+y).  This is as simple as that expression can be.  But if you put it into an equation you can solve for x.

You're trying to solve something like 3.3 = x/(x+y) correct?  The first step for solving for x would be to multiply the denominator of the right side(IE x+y) to both sides.  This will remove the fraction and from there you can solve for x using addition/subtraction operations.
 

Offline Zero999

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Re: Calculate value Resistante through voltage divider
« Reply #4 on: October 19, 2018, 08:26:44 am »
Hello,

How can I isolate the x, in this fraction? https://postimg.cc/xNV8665j
I am trying calculate the value of R2 through voltage divider (https://www.electronics-tutorials.ws/resistor/res10.gif)

I only need the value of the R2, because I have the value Vout, Vin and R1. This question is relative mathematic, I know, but I spend many hours try search about this theme around the web. (Actually, I do not even know how to search...)

Thanks
How about solving it from an electronics perspective, rather than an algebraic method?

You know VIN, VOUT and R1, therefore you can calculate the current through R1 using Ohm's law. The voltage across R1 is equal to the difference between VIN and VOUT, since potential differences are relative.

I = (VIN-VOUT)/R1

The current in both of the resistors is equal, since they're in series so R2 can be calculated using Ohm's law.

R2 = VOUT/I

And there's your answer.

If you put both of the above equations together, no doubt you have a similar result to what one would have got using algebraic manipulation.

« Last Edit: October 19, 2018, 08:28:29 am by Hero999 »
 

Offline spec

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Re: Calculate value Resistante through voltage divider
« Reply #5 on: October 19, 2018, 09:51:58 am »
+ Jasonbit

You want to make R2 the subject of:

  Vo = ViR2/(R1+R2) ........ (f1)

∴ Vo(R1+R2) = ViR2 ............ (f2) (this step added 2018_10_20)
∴ VoR1 + VoR2 = ViR2 ......... (f3)
∴ VoR1 = ViR2 - VoR2 .......... (f4)
∴ VoR1 = R2(Vi - Vo) ........... (f5)
∴ VoR1/(Vi - Vo) = R2 .......... (f6)

 ∴ R2 = VoR1/(Vi - Vo) ..... (f7)
« Last Edit: October 20, 2018, 07:42:26 pm by spec »
 
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Offline xavier60

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Re: Calculate value Resistante through voltage divider
« Reply #6 on: October 20, 2018, 06:20:37 pm »
Hello,

How can I isolate the x, in this fraction? https://postimg.cc/xNV8665j
I am trying calculate the value of R2 through voltage divider (https://www.electronics-tutorials.ws/resistor/res10.gif)

I only need the value of the R2, because I have the value Vout, Vin and R1. This question is relative mathematic, I know, but I spend many hours try search about this theme around the web. (Actually, I do not even know how to search...)

Thanks
Voltage across R1,   VR1 = Vin - Vout.
Current is,     I = VR1/R1.
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 
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Offline JasonbitTopic starter

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Re: Calculate value Resistante through voltage divider
« Reply #7 on: October 20, 2018, 06:39:35 pm »
@ Jasonbit

You want to make R2 the subject of:

  Vo = ViR2/(R1+R2) ........ (f1)

∴ VoR1 + VoR2 = ViR2 ......... (f2)
∴ VoR1 = ViR2 - VoR2 .......... (f3)
∴ VoR1 = R2(Vi - Vo) ........... (f4)
∴ VoR1/(Vi-Vo) = R2 ............ (f5)

 ∴ R2 = VoR1/(Vi-Vo) ....... (f6)

After I understand the logic this is appear basic  |O :-DD  ;)
Thanks a lot to your contribute! I would never can discovery this steps! Thanks a lot!
 

Offline spec

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Re: Calculate value Resistante through voltage divider
« Reply #8 on: October 20, 2018, 07:30:18 pm »
No probs. Glad it helped. But I missed out the second step, so will amend my post. :palm:
« Last Edit: October 20, 2018, 07:32:20 pm by spec »
 

Offline JasonbitTopic starter

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Re: Calculate value Resistante through voltage divider
« Reply #9 on: October 20, 2018, 07:39:47 pm »
No probs. Glad it helped. But I missed out the second step, so will amend my post. :palm:

I realized that but it was enough for me realize the logic of algebra, thanks any way
 

Online IanB

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Re: Calculate value Resistante through voltage divider
« Reply #10 on: October 20, 2018, 07:42:39 pm »
After I understand the logic this is appear basic  |O :-DD  ;)
Thanks a lot to your contribute! I would never can discovery this steps! Thanks a lot!

It is indeed basic. You normally would learn this level of algebra when you are 13 to 15 years old at school. Are you younger than that, or did you miss out on some maths lessons?
 

Offline JasonbitTopic starter

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Re: Calculate value Resistante through voltage divider
« Reply #11 on: October 20, 2018, 08:32:45 pm »
After I understand the logic this is appear basic  |O :-DD  ;)
Thanks a lot to your contribute! I would never can discovery this steps! Thanks a lot!

It is indeed basic. You normally would learn this level of algebra when you are 13 to 15 years old at school. Are you younger than that, or did you miss out on some maths lessons?

I got stuck in the basic steps and the math it is not my higher skill!
 

Online IanB

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Re: Calculate value Resistante through voltage divider
« Reply #12 on: October 20, 2018, 08:34:31 pm »
I got stuck in the basic steps and the math it is not my higher skill!

Skills are not something you are born with, you have to learn them and practice to get good at them  :)
 

Offline spec

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Re: Calculate value Resistante through voltage divider
« Reply #13 on: October 20, 2018, 09:04:06 pm »
I got stuck in the basic steps and the math it is not my higher skill!

Don't be put off Jasonbit, the average adult would have trouble with percentages and decimals, let alone algebra, basic or otherwise. ;D  You ask whatever you want.
 
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