How to added up a Resistor Value using misc values

[danny davis]

At work today we had to come up with different resistors values but using misc resistors values

1.78K
1.87K
1.91K
1.74K

The problem I had is, how do u know which value to start with and how do u know which value would work in parallel that will give you the result of 1.78K or 1.87K or 1.91K or 1.74K

We had to use resistors in series and in parallel to make up those values which was work for me because it took a lot of trys because I was just guessing

How would I know what to use , using misc resistor values to come up with those values?

I'm not sure where to start and what in the ball park would be good to put in parallel with to make those values

What are some rules of thumbs and how do i know I'm in the ball park?

 

[BobK]

Let me google that for you:

http://www.lmgtfy.com/?q=parallel+and+series+resistor+calculator

Bob

 

[KJ6EAD]

Some problem definition is missing. What resistor values are you allowed to use and how many resistors in combination?

It helps to have a good familiarity with the standard E12, E24, etc. series values.

http://www.logwell.com/tech/components/resistor_values.html

Rules of thumb:

*Use as few resistors as possible.

*Use resistors from the lowest E series possible.

*Try to make the values as similar as possible for good power sharing.

Using E24 series values in combinations of 2 yields the following series circuit solutions (exact matches were not possible with E12 series values nor were there any exact parallel circuit matches):

1600 + 180 = 1780
1100 + 680 = 1780

1600 + 270 = 1870

1800 + 110 = 1910
1000 + 910 = 1910

1500 + 240 = 1740

 

[danny davis]

no, you're not understanding

I already had the total resistor values list which was
1.78K
1.87K
1.91K
1.74K

These are the total resistance that I wanted

The Problem was, which values do I use in parallel to come up with those values? that is the hard part is using misc resistor values to come up with those values

How do I do it, or know where to start?

I was using the internet resistor series and parallel calculator, but how do u know which values to use is the hard part?

 

[KJ6EAD]

I use this calculator. It allows you to enter the desired value, define which E series to work from and displays all matches and near misses in both series and parallel combinations.

http://www.qsl.net/in3otd/parallr.html

By the way, 1.78k and 1.87k are standard E48 values; 1.91k and 1.74k are E96 values.

The basic method if you don't have the calculator is to use the chart of standard values and start at half the desired value for series combinations or double the desired value for parallel combinations.

 

[john monks]

For your 1.78K select the next common highest value, 1.8K. To find the parallel value R use this formula: 1.78K = (1.8K*R)/(1.8K+R), then solve for R.

 

[CocaCola]

For your 1.78K select the next common highest value, 1.8K. To find the parallel value R use this formula: 1.78K = (1.8K*R)/(1.8K+R), then solve for R.

In an example like that (applies to most of the initial list given) it's only a 1.2% variance to start with, so unless you are using 1% or better resistors vs the common 5% adding an additional parallel resistor doesn't really improve accuracy all that much or if at all..

 

[danny davis]

1.78k and 1.87k are standard E48 values; 1.91k and 1.74k are E96 values.

What do u mean by E48 values and E96 values?

I don't know what those are

The basic method if you don't have the calculator is to use the chart of standard values and start at half the desired value for parallel combinations or double the desired value for parallel combinations.

Where do u get this chart? and what is it called?

You start at half the desired value for parallel combination? what u mean if my desire value is 1.78K I start at half the value of it?

You double the desire value for series combination u mean?

 

[CocaCola]

Danny, click on this link and favorite place it... Heck make it your home screen...

www.google.com

Now when you want to know something like what E48 and E96 are you can figure it out, with a few keystrokes...

E48 E96 values

Want a chart? I generally just have a Mouser catalog handy but...

E48 E96 value chart

You start at half the desired value for parallel combination? what u mean if my desire value is 1.78K I start at half the value of it?

Half of 1780Ω in other words somewhere around 890Ω

Remember that link I suggested you favorite place earlier in the post? It helps here as well...

1780/2=

You double the desire value for series combination u mean?

No you would use a fraction of it generally, half works so does any other value less than the original... As long as all the chosen values add up to the desired amount...

 

[danny davis]

E3 50% tolerance (no longer used)
E6 20% tolerance (now seldom used)
E12 10% tolerance
E24 5% tolerance
E48 2% tolerance
E96 1% tolerance
E192 0.5, 0.25, 0.1% and higher tolerances

A Tolerance chart doesn't help me at all find out what resistor values work to get me the desire value

 

[danny davis]

Half of 1780Ω in other words somewhere around 890Ω

Ok if you have 890ohm resistor , what would u use in parallel of 890ohms to get 1.78k?

If the desire value is 1.78K , for a series combination, I take a fraction of 1.78K and then what after?

 

[CocaCola]

A Tolerance chart doesn't help me at all find out what resistor values work to get me the desire value

Read a little further, you have an ongoing tendency to only read the first or second sentence and then jump to conclusions without finishing reading the entire content... Slow down between post, I have suggested this numerous times to you... Spend 15 minutes reading about the E values before your next post...

Example if you are splitting up a 1.1KΩ resistor don't try using a 550Ω as a value because if you look at those charts you will see it's not a value you will find...

 

[CocaCola]

Ok if you have 890ohm resistor , what would u use in parallel of 890ohms to get 1.78k?

Use the calculators that have been provided in this post, or do the math yourself based on the formula provided in those same links... If you can't find the equation on those pages here it is...

If the desire value is 1.78K , for a series combination, I take a fraction of 1.78K and then what after?

This one is so simple you don't need an online calculator, but just the same use it if you must... It's only addition and subtraction...

1780 - (whatever value you pick) = What you need to get to the final value

Say you decide upon 1K as the split value...

1780 - 1000 = 780 thus you need a 780Ω in series with the 1KΩ and you get 1.78KΩ

 

[danny davis]

I'm not sure those E charts don't tell me the Split values and which values to use in parallel to get the desired value

I don't know how to get the split value for parallel resistance totals

If its 1.78K as the desired value than the split value is HALF for parallel Combination?

If its 1.78K as the desired value than the split value is HALF for Series combination?

Which you got the Split value , how do u find out the value of the 2nd resistor you will be piggy backing ontop of when you solder it like a pancake

Is the 2nd resistor the same value as the Split value?

 

[CocaCola]

Danny please go back and re-read this thread, the answers are there and it's just wasteful to keep typing them again and again when they are already here, you just need to read them...

And once again you are looking for that single right answer and once again I'm here to inform you that there isn't one, there are 100s if not 1000s of series and parallel combinations you could use to arrive at a desired value...

 

[donkey]

http://www.1728.org/resistrs.htm
there is a calculator there that will show what 2 resistors in parrallel will obtain. if you can do algebra than you can determine the values by rearranging the equation on the top of the page to accommodate your situation.
there are multiple answers to get the values you require, but finding resistors that match exactly is the fun part lol
http://www.sengpielaudio.com/calculator-paralresist.htm
the second post is better you can input the total and guesstimate 1 value to obtain the second value. the fact it comes up as a negative number seems odd though
http://www.electronics2000.co.uk/calc/series-parallel-resistor-calculator.php
yet another calculator set, this one has down the bottom a calculator that is made for when you know what the total has to be and you know 1 of the values.

 

[danny davis]

there are 100s if not 1000s of series and parallel combinations you could use to arrive at a desired value

yes but what chart of calculator tells you those value arrangements?

If you have the desire value , which is 1.78K, What chart or calculator tells you the 2 parallel resistor values?

I have not found a online calculator that lets you enter in the Desire value and it tells you the Split value and the 2nd resistor value

 

[donkey]

there is no chart and if you make one I'll buy one of ya

 

[CocaCola]

yes but what chart of calculator tells you those value arrangements?

If you have the desire value , which is 1.78K, What chart or calculator tells you the 2 parallel resistor values?

Probably none as it would be a colossal waste of time making one...

I have not found a online calculator that lets you enter in the Desire value and it tells you the Split value and the 2nd resistor value

How hard did you look? Did you Google it? I'm guessing NOT because you would have found one or more if you did...

http://www.changpuak.ch/electronics/calc_06.php

http://www.ti.com/tool/parallelrescalc

 

[donkey]

those are cool calculators. me likey