Voltage divider reisitor calculator - Page 1
Forum section: electronics / beginners

author: sparksflying
author_rank: Contributor
timestamp: August 17, 2013, 12:53:22 am
content: Hello,Does anyone know of an online calculator that will accept the input voltage and output voltage and calculate the required, most efficient *standard* (1% or 5%) resistors to divide the voltage?For example:5v down to 1.86v:R1 = 27k + 12kR2 = 15k + 8.2kIt usually takes me ages plugging different combinations into the formula to get the right output. If there is an easier way, please let me know or point me to a calculator.Cheers

author: ve7xen
author_rank: Super Contributor
timestamp: August 17, 2013, 01:04:40 am
content: It's not online, but if you have an Android device I highly recommend ElectroDroid. It has this among quite a few other calculators and is great for the lazy.

author: free_electron
author_rank: Super Contributor
timestamp: August 17, 2013, 01:13:41 am
content: just use the 1 milliamp trick.it's 1 kiloohm per volt you need.so 12 volt ins : 7.5 volts out :7.5 volts = 7.5k.12-7.5 = 4.5k7k5 is standard value.so is 4.53kdone.by the way, my 'standard' is E96 series ...

author: c4757p
author_rank: Super Contributor
timestamp: August 17, 2013, 01:16:02 am
content: That doesn't work quite as well when you're trying to match E12 or E24 values..... who's got 4.53k resistors just sitting around?I've got a Python script I use, I'll see if I can find it when I get home if nobody suggests anything better.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2013, 01:38:14 am
content: It's not that hard to do by hand. The approach is "take the nearest standard value then adjust it".For instance, you want 1.86 V out of 5 V. So the ratio in the divider is 186:314 (186 + 314 = 500).Nearest 5% values for 186 are 180 or 200. If we start with 180 we could use 18k + 620R = 18.62k. If we start with 200 we could use 20k||270k = 18.62k.Nearest 5% values for 314 are 300 or 330. If we start with 300 we could use 30k + 1.3k = 31.3k. If we start with 330 we could use 33k||620k = 31.3k.Plugging in those results gives V = 18.62/(18.62+31.3) * 5.00 = 1.865 V. Close enough?

author: mariush
author_rank: Super Contributor
timestamp: August 17, 2013, 02:04:49 am
content: Electronics Assistant (free downloadable software) has a potential divider calculator in it, you can download the software from here:http://www.electronics2000.co.uk/free online (and also downloadable) is a java applet which does circuit simulations, and has a ready made voltage divider circuit in the menu (circuits > basics > voltage divider) - you can edit the input voltage and resistor values and see the output voltage in real time :http://www.falstad.com/circuit/(java applet, browser should ask you to accept loading java applets in browser if you have it installed, otherwise you need to install java)

author: jvdb
author_rank: Contributor
timestamp: August 17, 2013, 02:10:55 am
content: This for iOS:Adafruit Circuit Playground

author: free_electron
author_rank: Super Contributor
timestamp: August 17, 2013, 02:37:10 am
content: Quote from: c4757p on August 17, 2013, 01:16:02 amlike i said, my 'standard' is e96 ... 4.53 is standard in that series...

author: bingo600
author_rank: Super Contributor
timestamp: August 17, 2013, 06:56:18 am
content: This one is a nice little utillity (Free)http://www.miscel.dk/MiscEl/miscel.htmlIt does expire sometimes , and one just need to download the latest and reinstall./Bingo

author: DaveW
author_rank: Frequent Contributor
timestamp: August 17, 2013, 12:10:44 pm
content: An online one for you,http://jansson.us/resistors.html

author: jahonen
author_rank: Super Contributor
timestamp: August 17, 2013, 12:46:16 pm
content: Not online, but what I made for myself:http://koti.mbnet.fi/jahonen/Electronics/ResOptimizer/I just recently added a bit of multithreading so one can abort a pending calculation if it turns out too lengthy.Regards,Janne