Resistors in parallel - Page 1
Forum section: electronics / beginners

author: wzabala
author_rank: Newbie
timestamp: November 24, 2022, 07:42:11 am
content: Hi Everyone.I'm building a simple circuit but don't have the necessary 22k resistor. I'm waiting for one to be delivered and in the mean time I was thinking about using resistors I do have (in series) to make up the 22k resistance I need. In this case I would require 4 resistors.But here's my question. If I wanted to use resistors in parallel, to achieve the same 22k resistance, what calculation would I use?I have seen calculations to add up resistors in parallel but I need to work backwards, starting with 22k. Hopefully the calculation would give 22k or something close, meaning I could get away with using less than 4 resistors as I would if I connected them in series.Thanks for any helpW

author: mariush
author_rank: Super Contributor
timestamp: November 24, 2022, 08:02:21 am
content: Seehttps://www.electronics-tutorials.ws/resistor/res_4.htmlBasically 1 / R = 1 / R1 + 1 / R2 + 1 / R3 ... + 1 / Rnfor 2 resistors 1/R = 1/ R1 + 1/R2 = (R1+R2) / R1xR2 => R = R1xR2 / (R1+R2)With two identical values the above becomes R = R1 x R1 / 2 R1 = R1/2For 22K, you could do :10K + 10 K + 1K + 1K in series20K + 1K + 1K in series20K + 2.2K in series = 22.2 (could be close enough for your circuit)47K + 47K in parallel = 23.5K68k + 33K in parallel = 22.2K82k+82k+47k in parallel = 21.9kplay around with this "calculator" :https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-parallel-and-series-resistor

author: srb1954
author_rank: Super Contributor
timestamp: November 24, 2022, 08:11:37 am
content: Quote from: wzabala on November 24, 2022, 07:42:11 amDepends on how accurate you want to get and what resistors you have in stock.Using only 2 resistors (from the E24 series) in parallel you can get the following values:R1 R2 R1//R2 Error w.r.t. nominal 22k24k 270k 22k04 0.2%27k 120k 22k04 0.2%30k 82k 21k96 -0.2%33k 68k 22k22 1.0%36k 56k 21k91 -0.4%39k 51k 22k10 0.5%43k 43k 21k50 -2.3%The stated error doesn't include the tolerances of the individual resistors. Any of the above combinations would probably be acceptable accuracy for most purposes except perhaps for the 43k//43k combination.

author: EPAIII
author_rank: Super Contributor
timestamp: November 24, 2022, 09:10:37 am
content: For two resistors in parallel the basic formula is:Rp = Parallel ValueR1 = Resistor 1R2 = Resistor 21/Rp = 1/R1 + 1/R2Now if you have R1 as a given and want a value of Rp then the formula, by subtracting 1/R1 from both sides becomes:1/R2 = 1/Rp - 1/R1In both cases both R1 and R2 will be greater than Rp.Example:You want a 90 Ohm resistor but don't have one. So you start with R1 = 100 Ohms.1/R2 = 1/Rp - 1/R11/R2 = 1/90 - 1/1001/R2 = 0.0111 - 0.011/R2 = 0.00111R2 = 900 OhmsSo a 100 Ohm and a 900 Ohm resistor in parallel will give a value of 90 Ohms. Of course, 900 Ohms is not a standard value so using the closest value, 910 Ohms, the first equation then gives us the actual value of the pair in parallel:1/Rp = 1/100 + 1/9101/Rp = 90.099 OhmsAnd that is pretty close to the desired value. Certainly within 5%.Most scientific calculators will have a 1/x button. And most of them will perform the above calculations by entering the numbers followed by the 1/x key. Then enter either a + or a - sign between them. Follow the second entry with the = sign and then press the 1/x button again. Like this for the Rp calculation:901/x+9001/x=1/xFor the R2 calculation just change the + to a - .

author: EPAIII
author_rank: Super Contributor
timestamp: November 24, 2022, 09:25:34 am
content: When working with 5% resistors I have found that there is a rule of thumb that quickly gives a good approximation of a standard value with no math. It goes like this:Select the next 5% value that is greater than the desired value.Then multiply that value by 10 and put that resistor in parallel with the first.The value of the parallel combination will be very close to the desired value.Example:Desired value = 22K.The next 5% value is 24K.And 10 times 24K is 240K.And 24K in parallel with 240K is 21.8181K. Pretty close to the desired 22K.The 5% resistor values are a power series with a uniform factor (1.100694X) separating the values. This ensures that the above trick works for all the 5% resistor values.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 24, 2022, 02:17:04 pm
content: I think it's best if you list the resistor values you do have. With that we can figure out how best to put them together to get the 22K you want.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 24, 2022, 02:29:14 pm
content: This might help you. Put the available resistors, the target value (22k) and go!https://www.daycounter.com/Calculators/Parallel-Resistor-Combination-Calculator.phtml

author: Swake
author_rank: Frequent Contributor
timestamp: November 24, 2022, 03:31:51 pm
content: A 25kR or 100kR potentiometer (=trimmer =potmeter) set to 22kR can help out too. Or salvage a resistor from an old board (I've got a pile of junk boards just for that purpose)Eventually that 22kR value is not very critical. What is the resistor used for? How close does it have to be?Calculation:The simple trick,onlywhen using all thesamevalue resistors, is to divide the value by the number of resistors you want to put in parallel. example: 3 resistors of 100kR in parallel = 100k/3 = 33k3 times 68kR in parallel = 68k/3 = 22.6kR

author: ledtester
author_rank: Super Contributor
timestamp: November 24, 2022, 03:56:08 pm
content: This is one of the most sophisticated resistor network finder web apps:http://kirr.homeunix.org/electronics/resistor-network-finder/