How to calculate the value of a resistor in a circuit? - Page 1
Forum section: electronics / beginners

author: PedalChase
author_rank: Contributor
timestamp: December 29, 2024, 04:08:12 am
content: I am trying to answer a question given in an electronics self-teaching guide I am going through.I am not having any luck working it out myself with the information given. I must not be applying the correct equations. I am admittedly algebra challenged.The question asks for the value of Resistor 2.The information given is a diagram of a simple battery powered current divider circuit with two resistors (R1 and R2) connected in parallel.Voltage source is 12 V DC battery.Current Total is 300mA.Resistor 1 is 50ohms./--------------------------/---------------------/ Total Current: 300 mA/ / /12V DC R1 50 Ohm R2 ?/ / //-------------------------/--------------------/What equation would you use to determine the value of R2 with the information given?Thanks

author: Simmed
author_rank: Frequent Contributor
timestamp: December 29, 2024, 04:15:09 am
content: i = V/ R0.3 = 12/ RR = 40R// = 1/(1/R1+1/R2)R2 = 1/(1/40 - 1/50)R2 = 200

author: PedalChase
author_rank: Contributor
timestamp: December 29, 2024, 04:22:07 am
content: I had worked out i=V/R. Ohms law.The second part I will examine further.If it was in the material, I had not realized how to apply it in this situation.Thanks for the help.Much appreciated.

author: Konkedout
author_rank: Frequent Contributor
timestamp: December 29, 2024, 04:22:41 am
content: Another way to think about it:R1 will conduct 12VDC/50 ohms = 240 mA.You want another (300 mA - 240 mA) = 60 mAR2 = 12VDC/60 mA = 200 ohms.We can all go ohm now.

author: golden_labels
author_rank: Super Contributor
timestamp: December 29, 2024, 07:35:54 am
content: This is a current divider. The current counterpart of a voltage divider.You may approach the problem by thinking not about resistances, but about conductances. Conductance (G) is inverse of resistance (R):G = 1 / RSpeaking simply: where resistance indicates opposition to current, conductance tells how easy it is for current to pass.Mathematically using both works the same way in equations. But thinking in terms of conductances sometimes makes things easier to wrap your head around and to calculate values.Where with resistances you get “ugly” stacked fractions from parallel resistors, with conductances you have a simple sum:R1 = 50 Ω, so: G1 = 1/50 S = 0.02 S.I = 0.3 AV = 12 VTotal conductance must be: GTOTAL= 0.3 A / 12 V = 0.025 SG2 = GTOTAL- G1 = 0.025 S - 0.02 S = 0.005 SG1 = 0.005 S, so R1 = 1/0.005 Ω =200 Ω“S” above is the unit of Siemens, equal to 1/Ohm.Note that deep inside the equations are identical to parallel resistances. They just put the problem in a different perspective and let you focus on the important part.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: December 29, 2024, 10:57:18 am
content: If you want to work out Voltages and currents in "awkward" chains of resistors, as opposed to the relatively simple question posed earlier, Spice modelling software can provide the answer.My personal favourite is the student version of Tina, which unlike LTSpice, it is not free. The student version is currently $41 for USA buyers:https://www.tina.com/SJ

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2024, 11:22:23 am
content: Quote from: Solder_Junkie on December 29, 2024, 10:57:18 amThere's also TINA-TI, which is free.https://www.ti.com/tool/TINA-TII prefer LTSpice myself. I find it much easier to use.

author: tooki
author_rank: Super Contributor
timestamp: December 29, 2024, 12:24:04 pm
content: Quote from: PedalChase on December 29, 2024, 04:08:12 amI highly suggest reviewing and practicing algebra until you really get the hang of it — algebra is inescapable in electronics. It’s sadly much too common for datasheets to include formulas in forms that aren’t immediately useful. (For example, tons of adjustable voltage regulator datasheets give the formula for calculate the voltage-programming resistors as “Vout = …” (for example, Vout = Vref * (1+R2/R1) ), which is fine if you already have both resistor values and want to figure out what the output voltage is, but more commonly you know what voltage you need, and need to calculate the resistor values to get that voltage. So you need, for example, R2 = R1 * (Vout/Vref - 1). Being able to rearrange formulas to solve for the value you need is an essential skill. Sure, you can use a modern higher-end scientific calculator, or a math program for your computer or phone, that has a “solver” to solve for a particular value. But I think knowing how to do it is a very important skill for electronics.(Note to self: review calculus.)

author: tooki
author_rank: Super Contributor
timestamp: December 29, 2024, 12:32:41 pm
content: Quote from: Zero999 on December 29, 2024, 11:22:23 amHaving used both quite a bit — during my apprenticeship, one instructor required us to use TINA, the other LTspice — I think they both have their places. LTspice’s user interface is… unique. Fast to use once you’re used to it, but very weird. And editing a schematic after the fact is way more fussy than it needs to be.TINA’s UI is more intuitive, but has its quirks too. Editing existing schematics is way easier. Where it totally beats LTspice is in interactive digital (logic) simulation, which LTspice simply does not do.At the end of the day, I ended up preferring LTspice for analog simulations, TINA for digital.But for a beginner — and frankly, for anyone, when facing an analog circuit you don’t understand — I also highly recommend Falstad, the online simulator, because it is interactive (even for analog circuits!) and above all because it actually animates the current flows in a circuit. It’s tremendously instructive to actually see both the magnitude and direction of current flows and voltages visualized.http://falstad.com/circuit/

author: golden_labels
author_rank: Super Contributor
timestamp: December 29, 2024, 02:16:19 pm
content: Simulators are great and indispensable tools. But they aretools.One has to know, how to use a tool. And be the one driving the tool, not being driven by the tool.For electronic simulators the limitation is that they’re nothing more than very fancy calculators. They don’t represent reality. They just package mathematical equations into something that is easy to manipulate and interpret, and automate calculations. Forget about that and you get burned. Same for not learning the models in the first place.Falstad is indeed an amazing simulator. It has many shortcomings, so don’t use it as a proper design or testing tool. But as a tool for education and quick sketching: unbeatable.

author: shapirus
author_rank: Super Contributor
timestamp: December 29, 2024, 02:23:04 pm
content: Quote from: golden_labels on December 29, 2024, 07:35:54 amThis is an advice that should be given way more often than it is. Understanding and using the concept of conductance makes many things much easier to grasp and they start feeling more natural.

author: PedalChase
author_rank: Contributor
timestamp: December 30, 2024, 03:39:14 am
content: Yes, algebra keeps being my hang up as I try to progress. I have not used it in 40 years and need to start from scratch again. I can plug numbers into a formula and do ok but when I need to try and say solve for an unknown, I get locked up.My issue is that I am interested in getting to where I can do some hands-on work and understand what I am seeing rather than just soldering part a to part b. Algebra was never easy for me so I keep trying to slide by with just the minimum interaction with it. Kind of a case of putting my time into algebra which is of little interest or into electronics which is of great interest.On the other hand...I now have an "end use" for algebra which I never had in school. I might absorb it much better now that it means something to me...

author: PedalChase
author_rank: Contributor
timestamp: December 30, 2024, 03:45:21 am
content: I have heard about conductance in a lecture I listened to once but in the reading I do and my areas of interest (guitar pedals/ham radio) I have never once seen it explained or discussed.Thanks for the insights.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 30, 2024, 03:48:00 am
content: Yes, algebra: hard to do those kind of calculations until you once again become comfortable with it.Here's a little taste of it that might help. Start with the one formula you might want to memorize:I = E / R(Imeaning current,Emeaning voltage)Now derive the other two forms of this equation:Multiply both sides of the equation by R:R * I = (E / R ) * RSinceR / Rcancels out (= 1), remove it:R * I = Ereverse it to get it into its usual form:E = IRI'll leave the remaining form as an "exercise for the student".It's really not so bad ...

author: PedalChase
author_rank: Contributor
timestamp: December 30, 2024, 04:33:39 am
content: That example is interesting.I memorized the formula as a triangle years ago and have forgotten the actual steps involved.I can work a problem out using the triangle for any one of the threeE---------I x RBut I can't remember how to do the canceling or reversing.Same with a few other formulasP-------I x EI can fill in a number and get the correct answer (enough to get me by up to now) but can't re-arrange the formula to solve for an unknown.Thanks.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 30, 2024, 04:45:22 am
content: Quote from: PedalChase on December 30, 2024, 04:33:39 amOK, let's see if I can help.If I end up just annoying you and not helping you, let me know.Take that last one:P = I EForget the triangle for now.Let's say you knowPandE(watts and voltage) and need to knowI(current): how do you do that?You want to getIby itself on the left side, right?So we need to hack offEfrom the right side. How do we do that?Well, remember the rule thatanything divided by itself equals 1. Very important.So if you divide both sides byEP / E = (I E) / Ethe twoEs on the right (which areE / E) cancel out and disappear, so we haveP / E = Iwhich you can just flip around so you getI = P / Eand as they say, Bob's your uncle.Does that help?

author: PedalChase
author_rank: Contributor
timestamp: December 30, 2024, 05:06:57 am
content: Yes that helps.I remember the "rule" you mentioned now.I need to do some research and find a good source for a basic algebra course I can review as a refresher.Then again...I am good at procrastinating.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: December 30, 2024, 09:42:35 am
content: You can create a spreadsheet with the various formulas, once done it will be there as your own calculating tool.Daft as it may seem, I have a spreadsheet to calculate the X, Y and Z figures to input into EZNEC, the antenna calculator. The calculations are for an inverted V dipole where you need the length along the ground from the (sloping) wire length. Basic school kid maths, but a spreadsheet with a diagram, etc. is so easy to use and not difficult to setup in the first place.SJ

author: SteveThackery
author_rank: Super Contributor
timestamp: December 30, 2024, 10:50:08 am
content: Quote from: PedalChase on December 30, 2024, 05:06:57 amCheck out Khan Academy - some great courses in beginner maths, including algebra.

author: PGPG
author_rank: Frequent Contributor
timestamp: December 30, 2024, 11:18:29 am
content: Quote from: Solder_Junkie on December 30, 2024, 09:42:35 amI have a spreadsheet to calculate EMC field limits from 10m to other distance.The relationship is not very simple. If you for example calculate limits from 10m to 3m you have two frequencies at which emission changes from near field to far field.Times changes.I was able to do simple calculations (like P=I²R) when I was 10, but to use Internet to find answers to my questions when I was 35. Now the order looks being reversed

author: Analog Kid
author_rank: Super Contributor
timestamp: December 30, 2024, 10:38:52 pm
content: Quote from: Solder_Junkie on December 30, 2024, 09:42:35 amYou could, but for really simple stuff like this (Ohm's Law, etc.), I would recommendnotdoing that.Problem is you'll be leaning on that crutch forever. Better to learn it once and have it at your fingertips forever afterwards.Spreadsheets might be good for more complex stuff, like frequency/reactance calculations. Or you can find lots of calculators online that do all kinds of things, including designing various filters (lowpass, highpass, etc.) for you.