Calculate value Resistante through voltage divider - Page 1
Forum section: electronics / beginners

author: Jasonbit
author_rank: Regular Contributor
timestamp: October 18, 2018, 10:45:41 pm
content: Hello,How can I isolate the x, in this fraction?https://postimg.cc/xNV8665jI 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

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 18, 2018, 11:24:57 pm
content: Ummsearch for "voltage divider calculator".Lots to choose from.

author: Wimberleytech
author_rank: Super Contributor
timestamp: October 18, 2018, 11:31:20 pm
content: http://www.ohmslawcalculator.com/voltage-divider-calculator

author: pigtwo
author_rank: Regular Contributor
timestamp: October 18, 2018, 11:36:35 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 19, 2018, 08:26:44 am
content: Quote from: Jasonbit on October 18, 2018, 10:45:41 pmHow about solving it from an electronics perspective, rather than an algebraic method?You know VIN, VOUTand R1, therefore you can calculate the current through R1 using Ohm's law. The voltage across R1 is equal to the difference between VINand VOUT, since potential differences are relative.I = (VIN-VOUT)/R1The current in both of the resistors is equal, since they're in series so R2 can be calculated using Ohm's law.R2 = VOUT/IAnd 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.

author: spec
author_rank: Frequent Contributor
timestamp: October 19, 2018, 09:51:58 am
content: + JasonbitYou 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)

author: xavier60
author_rank: Super Contributor
timestamp: October 20, 2018, 06:20:37 pm
content: Quote from: Jasonbit on October 18, 2018, 10:45:41 pmVoltage across R1, VR1 = Vin - Vout.Current is, I = VR1/R1.

author: Jasonbit
author_rank: Regular Contributor
timestamp: October 20, 2018, 06:39:35 pm
content: Quote from: spec on October 19, 2018, 09:51:58 amAfter I understand the logic this is appear basicThanks a lot to your contribute! I would never can discovery this steps! Thanks a lot!

author: spec
author_rank: Frequent Contributor
timestamp: October 20, 2018, 07:30:18 pm
content: No probs. Glad it helped. But I missed out the second step, so will amend my post.

author: Jasonbit
author_rank: Regular Contributor
timestamp: October 20, 2018, 07:39:47 pm
content: Quote from: spec on October 20, 2018, 07:30:18 pmI realized that but it was enough for me realize the logic of algebra, thanks any way

author: IanB
author_rank: Super Contributor
timestamp: October 20, 2018, 07:42:39 pm
content: Quote from: Jasonbit on October 20, 2018, 06:39:35 pmIt 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?

author: Jasonbit
author_rank: Regular Contributor
timestamp: October 20, 2018, 08:32:45 pm
content: Quote from: IanB on October 20, 2018, 07:42:39 pmI got stuck in the basic steps and the math it is not my higher skill!

author: IanB
author_rank: Super Contributor
timestamp: October 20, 2018, 08:34:31 pm
content: Quote from: Jasonbit on October 20, 2018, 08:32:45 pmSkills are not something you are born with, you have to learn them and practice to get good at them

author: spec
author_rank: Frequent Contributor
timestamp: October 20, 2018, 09:04:06 pm
content: Quote from: Jasonbit on October 20, 2018, 08:32:45 pmDon't be put off Jasonbit, the average adult would have trouble with percentages and decimals, let alone algebra, basic or otherwise.You ask whatever you want.