3 resistor voltage divider - Page 1
Forum section: electronics / beginners

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 04:31:34 pm
content: I have 3 resistors in series, tapping off two voltages to the inputs of a high-impedance comparator.If I know the output voltages, input voltage, how is best to calculate the three resistors needed in Excel or similar?This is probably more a question for a Maths forum..Here are my equations:

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 05:15:34 pm
content: You cannot deduce the values of the resistors. You can only find their relative valuesA solution isR2/R3 = (V2-V1)/V1R1/R3 = (Vin-V2)/V1Any value of R3 will work.(edit : to get the absolute values you need the intensity of the current in the circuit).

author: Zero999
author_rank: Super Contributor
timestamp: May 01, 2015, 05:17:38 pm
content: Yes, find the relative values and search for the combination which gives the best results.

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 05:23:58 pm
content: Quote from: JacquesBBB on May 01, 2015, 05:15:34 pmLovely clear formulas. This now makes more sense to me.

author: German_EE
author_rank: Super Contributor
timestamp: May 01, 2015, 05:29:07 pm
content: 1) Start by considering this as a single voltage divider with Vin, V2 as output, R1 and (R2+R3)2) Assign R1 a nominal value (say 1K ohm) and work out (R2+R3) for the voltage you need.3) Do the calculation again but this time for the second voltage divider. Input is V2, output is V1 and make a best guess for R2 before you work out R3So, let's suppose you have an input of 10V and you want your output voltages to be 7,5V and 2,5VR1 is 1K ohms(R2+R3) needs to be 3K ohms for 7,5V outVin for stage 2 is 7,5VR1 is 2K ohms and R2 is 1K ohms (sum 3K ohms)Edit: JacquesBBB got there before me by expressing this as formulae rather than practical examples, c'est la vie

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 05:46:15 pm
content: Great. Thanks. I was getting mixed up in various sim equations earlier which wasn't helping.Now, just say I have two comparators drawing bias current from both V1 and V2. What's the best way to stop the voltage drop across the two resistors? Or will it be so negligible that it doesn't matter?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 01, 2015, 06:30:13 pm
content: QuoteSure they doQuoteMost likely. Unless you choose resistors in the terra-ohm region, you should be fine

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 06:54:38 pm
content: Quote from: PSR B1257 on May 01, 2015, 06:30:13 pmI have a series feed resistor of 27k on my input. With 200nA bias current, that will drop 6mV.Kinda insignificant knowing my input offset maximum is about 6mV, too..

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 01, 2015, 07:17:48 pm
content: QuoteHoly crap, what type of comparator do you use?QuoteRather tight. In this case you also need a stable refevenre voltage. And you should use trimmers in between the resistors, in order to fine tune the voltage.

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 07:34:23 pm
content: Quote from: PSR B1257 on May 01, 2015, 07:17:48 pmLM2903, which has a max bias current of 200nA over temp range & 15mV offset.QuoteI think i will just use trimmers to set the comparator limit & shut them closed with some red Loctite. Works out easier than trying to fit a load of resistors in series, and trimming becomes a whole load easier.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 01, 2015, 09:47:42 pm
content: QuoteWhy did you use this one, if it is supposed to be aprecisionapplicationQuoteYes, but with the resistor-trimmer-assembly you have a smaler range to adjust the voltage, and therefore you can use more of the trimmer-range.

author: Christopher
author_rank: Frequent Contributor
timestamp: May 01, 2015, 09:53:28 pm
content: Quote from: PSR B1257 on May 01, 2015, 09:47:42 pmI can use any I like, this just happens to be the first I have chose for these basic calculations! In my final project I will use a better specced comparator (the only real specs I need to look at are offset voltage, bias current and response time, right?).Any advice on a decent part to use?Quote from: PSR B1257 on May 01, 2015, 09:47:42 pmGood point. I plan on using 10-turn pots and a 5V reference. I can put some series resistance below the pot to decrease the pot range!

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 02, 2015, 06:44:06 am
content: QuoteYou should not use this one for any calculations, if you know, that it is not suited.QuoteYes, these values should be low.QuoteWho knows? We still don't know, what your application looks like.

author: abit
author_rank: Contributor
timestamp: May 09, 2015, 04:55:41 am
content: Quote from: Christopher on May 01, 2015, 04:31:34 pmExcel has a function for permutations, so if you care to run all of E variations (not to mention power ratings and tolerances of 0.1% to 5%) you are going to end up with a DATABASE larger than Earth.. There is a reason why the E-standard was designed as such, and to save your sanity - do not ever think about going parallel more than 2 resistors. Even then, the effort is not going to justify the cost.I have nothing to back this up but I have gone insane before along these lines, and the first approximation of the math in bash script was as expected, it ran for days and gobbled up gigabytes of disk space for a loose result.Do not go beyond two resistors paralleled - and go there only when in dire straights for an odd value. Even then, you are going to loose some precision, since the approximation is a logarithm, and when there IS a PERFECT match you SHOULD have a discrete value of E192 of the same.Now, the question is if you need it in:0.125W, 0.250W, 0.500W, 1W, 2W or higher.0.01%, 0.2%, 0.5%, 1% precision or lower.... You got the ideaDo not think about it, do not go there. The variations of all these man-made objects is larger than Earth itself.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 09, 2015, 12:14:42 pm
content: @abitI am sorry, but I do not understand the purpose of your post.The initial question of this thread is on voltage divider, and it is solved (see above).Then discussion followed on proper resistance values compared to the impedance ofthe opamp that is in the circuit.You seem to answer to a question, but as the answer is confuse, it would at least be nice tostate again which is the question to which your answer corresponds.Apart from that, despite the uncertainty of the question, let me remind you that to solve asimple circuit problem, the best method is not to try all possible values for the components.A simple circuit can be modelized by mathematical formulae that can often be solved,thus providing answer without filling a Earth sized database.

author: Zero999
author_rank: Super Contributor
timestamp: May 09, 2015, 12:34:36 pm
content: It's possible to minimise the effect of the bias current by making the impedances connected to each input as close as possible, that way the offset due to the bias on each input will be the same.