Voltage divider reisitor calculator

[sparksflying]

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 + 12k
R2 = 15k + 8.2k

It 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

[ve7xen]

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.

[free_electron]

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 = 4.5k

7k5 is standard value.
so is 4.53k

done.

by the way, my 'standard' is E96 series ...

[c4757p]

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.

[IanB]

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?

[mariush]

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)

[jvdb]

This for iOS:

Adafruit Circuit Playground

[free_electron]

who's got 4.53k resistors just sitting around?

like i said, my 'standard' is e96 ... 4.53 is standard in that series...

[bingo600]

This one is a nice little utillity (Free)

http://www.miscel.dk/MiscEl/miscel.html

It does expire sometimes , and one just need to download the latest and reinstall.

/Bingo

[DaveW]

An online one for you,

http://jansson.us/resistors.html

[jahonen]

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