+ Post New Thread
Results 1 to 20 of 22

26th October 2016, 19:19 #1
 Join Date
 Sep 2010
 Posts
 33
 Helped
 0 / 0
 Points
 1,460
 Level
 8

26th October 2016, 19:55 #2
 Join Date
 Jan 2010
 Posts
 148
 Helped
 40 / 40
 Points
 2,400
 Level
 11
Re: Resistor Divider help needed
from resistor divider we have these equations
Code:7.08 = 15 * (R4 / (R4 + R3 + R2 + R1)) 7.1 = 15 * ((R4 + R3) / (R4 + R3 + R2 + R1)) 7.12 = 15 * ((R4 + R3 + R2) / (R4 + R3 + R2 + R1))
for example assume R4 = 7.08 thne you will have R3 = R2 = 0.02
R1 = 7.88

26th October 2016, 21:33 #3
Re: Resistor Divider help needed
Hi,
Quick way is making total resistance equal V+. I do it because I'm not good with numbers...
Code:Rtotal = 150K R1 = 78K8 R2 = 200R R3 = 200R R4 = 70K8 (R2 + R3 + R4)/(R1 + R2 + R3 + R4) * Vin = 0.4746r* 15V = 7.12V (R3 + R4)/(R1 + R2 + R3 + R4) * Vin = 0.473r * 15V = 7.1V (R4/ R1 + R2 + R3 + R4) * Vin = 0.472 * 15V = 7.08V
Sometimes worth doubling/multiplying or dividing resistor values to find ones closer to standard resistor values.
No interest in doing that with op amps to get a more precise and reliable voltage for each tap?
   Updated   
Hi again,
Another, longer way is:
a) Divider voltages/Vtotal
b) Select Rtotal
c) Multiply results from a) by b)
d) Subtract R2 from Rtotal to get R1 value.
e) Subtract intermediate resistor vaules from each other progressively and last from bottom resistor.
e.g.
a) 7.12/15 = 0.47466666666666666666666666666667
7.10/15 = 0.47333333333333333333333333333333
7.08/15 = 0.472
b) Rtotal = 100K
100000 * 0.47466666666666666666666666666667 = 47466.666666666666666666666666667
100000 * 0.473r = 47333.333333333333333333333333333
100000 * 0.472 = 47200
100000  47466.666666666666666666666666667 = 52533.333333333333333333333333333
47466.666666666666666666666666667  47333.333333333333333333333333333 = 133.333333333333333333333333334
47333.333333333333333333333333333  47200 = 133.333333333333333333333333333
Sorry for the lengthy decimals there, just inculded so numbers tally.
So rounding up or down,
R1 is 52K5
R2 is 47K5
R3 is 130R
R4 is 130R
I prefer the other method, it's quicker, but this one has it's uses.

Advertisement

26th October 2016, 22:10 #4
 Join Date
 Nov 2009
 Location
 United States
 Posts
 237
 Helped
 68 / 68
 Points
 3,122
 Level
 13
Re: Resistor Divider help needed
Also note, that the range of values you are trying to create with this divider is only about +/ 2%, so if you use this approach, you must use precision resistors, or simply the normal variations in resistor values will cause you to miss these specifc voltage values.

26th October 2016, 22:33 #5
 Join Date
 Apr 2007
 Location
 Norway
 Posts
 649
 Helped
 119 / 119
 Points
 5,973
 Level
 18
Re: Resistor Divider help needed
Maybe not so much, because there is only 0.04V span between lowest and highest out value.
If I where to use cheapest possible components, I would first decide for a couple of low value resistors for voltage drop over R2 and R3  100Ω for example.
Thereafter I would find the corresponding value for proper voltage drop over R1 and R4.
Also I would allow myself to substitute R1 and R4 with two resistors in parallell to get closer  or better, just use a potmeter.

26th October 2016, 23:17 #6
 Join Date
 Sep 2007
 Location
 Richmond Hill, ON, Canada
 Posts
 6,718
 Helped
 1677 / 1677
 Points
 34,741
 Level
 45
 Blog Entries
 7
Re: Resistor Divider help needed
Your not an idiot but your 15V supply error will quickly dominate your results.
20mV thresholds differences with No Tolerance makes your spec INCOMPLETE.
20mV range even with 10% accuracy is 1mV/15V or 66 parts per million.
TRY AGAIN. write new specs. with tolerances !!
or choose 10 ppm Vref at 7V and pullup to 120mV offset easily with taps..Last edited by SunnySkyguy; 26th October 2016 at 23:28.
A best design is easily achieved with good test specs™
A better question deserves a better answer. ™
... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.
... Tony Stewart EE since 1975
 slightly north of Toronto

26th October 2016, 23:21 #7
Awards:
 Join Date
 Apr 2014
 Posts
 15,623
 Helped
 3560 / 3560
 Points
 77,123
 Level
 67
Re: Resistor Divider help needed
Hi,
0.47466666666666666666666666666667
For a 1% tolerated resistor 3 digits are sufficient, maybe write 4 digits (integer + fractional). Everything above 4 digits makes no sense.
Klaus

27th October 2016, 00:42 #8
 Join Date
 Feb 2012
 Location
 L.A. USA Zulu 8
 Posts
 3,665
 Helped
 869 / 869
 Points
 19,003
 Level
 33
Re: Resistor Divider help needed
On obvious way to get any arbitrary resistor values is to put two 1% resistors in series or parallel of the proper value.
That way the worstcase arbitrary value will be within 1% of the desired.
This website will calculate the optimum values for any desired resistance.
Thus, for example, to get the 70.8k resistor value calculated for R4 in the first part of post #3, you can put 140k in parallel with 143k to give 70.8k within 0.08% (±1% of course).Zapper
Curmudgeon Elektroniker

27th October 2016, 05:50 #9
 Join Date
 Aug 2016
 Posts
 2
 Helped
 0 / 0
 Points
 48
 Level
 1
Re: Resistor Divider help needed
Here is one another such calculator. This one supports configurable stock, which could be useful.
Using this same example, my tool recommends putting 5.9k and 64.9k in series to obtain exactly 70.8k. Of course subject to the tolerance of the resistors used. I think 143k is an E96 value, but both 5.9k and 64.k are from E48 series, so I prefer this combination.

Advertisement

27th October 2016, 07:12 #10
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 45,282
 Helped
 13776 / 13776
 Points
 259,376
 Level
 100
Re: Resistor Divider help needed
Calculation methods and implementation with standard series fixed resistors have been explained.
I wonder if it makes sense to derive exact voltage levels from a 15 V "reference". Precision references are sourcing 10.0 or 10.24 V, industry standard 15 V fixed voltage regulators have +/ 2 % tolerance.

Advertisement

27th October 2016, 09:21 #11

27th October 2016, 15:16 #12
 Join Date
 Sep 2010
 Posts
 33
 Helped
 0 / 0
 Points
 1,460
 Level
 8

27th October 2016, 15:48 #13
Awards:
 Join Date
 Apr 2014
 Posts
 15,623
 Helped
 3560 / 3560
 Points
 77,123
 Level
 67
Re: Resistor Divider help needed
Hi
the best TL431 B grade is rated 0.5% = 5000ppm accuracy.
Would a TL431 accomplish this?
****
No Tolerance makes your spec INCOMPLETE.
Klaus

27th October 2016, 16:03 #14
 Join Date
 Nov 2006
 Location
 Brazil
 Posts
 8,840
 Helped
 1114 / 1114
 Points
 29,583
 Level
 41
 Blog Entries
 9
Re: Resistor Divider help needed

Part of the world that you live in, You are the part that you're giving ( Renaissance )

27th October 2016, 16:27 #15
Awards:
 Join Date
 Apr 2014
 Posts
 15,623
 Helped
 3560 / 3560
 Points
 77,123
 Level
 67
Re: Resistor Divider help needed
Hi,
Is it for a LiIon battery charge status detector?
(if so, then I doubt it will give meaningful results)
Klaus

27th October 2016, 16:46 #16

27th October 2016, 16:54 #17
 Join Date
 Sep 2010
 Posts
 33
 Helped
 0 / 0
 Points
 1,460
 Level
 8

27th October 2016, 17:10 #18
 Join Date
 Sep 2013
 Location
 USA
 Posts
 7,235
 Helped
 1711 / 1711
 Points
 31,335
 Level
 43
Re: Resistor Divider help needed
Looks like you really want something like this: http://www.electronicslab.com/proje...velindicator/

27th October 2016, 18:46 #19
 Join Date
 Jan 2008
 Location
 Toronto area of Canada
 Posts
 8,738
 Helped
 2049 / 2049
 Points
 53,638
 Level
 56
Re: Resistor Divider help needed
I use an LM3915 to drive the LEDs in my sound level indicator that also has a peak detector. The LM3915 has the resistor ladder and 10 comparators that drive the LEDs with an adjustable constant current. Its detection points are 3dB (0.707 times or 1.414 times) apart which is a small but noticeable change in level. Nobody will be able to hear the tiny change in level (less than 1%) with your resistors.

27th October 2016, 18:58 #20
 Join Date
 Sep 2013
 Location
 USA
 Posts
 7,235
 Helped
 1711 / 1711
 Points
 31,335
 Level
 43
Re: Resistor Divider help needed
I didn't even notice that the project I linked to was yours!
I went back to take a look at it as I thought the recommendation for what part to use sounded similar, then I noticed the name of the user that submitted the post.
I'll vouch for you if another moderator thinks you are promoting your own blog ;)

Advertisement
+ Post New Thread
Please login