How to added up a Resistor Value using misc values

[danny davis]

http://www-k.ext.ti.com/srvs/cgi-bin/webcgi.exe?Company={5761bcd8-11f5-4e08-84e0-8167176a4ed9},kb=analog,case=obj%2833635%29,new

This one worked but it only gave me one choice for R1 and R2


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

This one didn't give me values for R1 and R2 when I entered the desired value

 

[CocaCola]

This one worked but it only gave me one choice for R1 and R2

How many choices do you need?

This one didn't give me values for R1 and R2 when I entered the desired value

What value did you enter?

 

[danny davis]

I want alot of choices , but the inventory at my work is misc resistor values

I don't know what R1 or R2 values are , that's the hard part for me to find out

I just know the desired value, but i don't know how to get R1 and R2 values and get different choices

 

[KJ6EAD]

I gave you all the information you'll ever need on this subject in posts #3 and #5.

Do you know what resistance is formed by two 500Ω resistors in series? How about two 2k resistors in parallel?

 

[CocaCola]

And I gave you two calculators to find out the values, with nothing more than the desired final value...

Or use one of the other calculators in this thread and just pick an R1 or R2 value, let your hair down, close your eyes and pull a value out of the air (probably best to pull one off a E value chart) and run with it...

 

[danny davis]

Yes I like this calculator the best
http://www.qsl.net/in3otd/parallr.html

Because it gives you choices of series or parallel combinations for R1 and R2

The E chart is just a tolorance chart, it doesn't tell you the values for R1 or R2 and what they should be

You can't get R1 and R2 values from an E chart

 

[CocaCola]

The E chart is just a tolorance chart, it doesn't tell you the values for R1 or R2 and what they should be

You can't get R1 and R2 values from an E chart

You mis the point, the E charts tells you want values you can get when you go to the distributor and place an order... Manufactures don't make every possible value resistor they make the ones in those E charts, any other value is custom or specialty... In the end it basically tells you what values you are limited to when doing the equations...

 

[BobK]

Why do we keep spoon feeding this troll? If he had bothered to look at the results of the search I did for in the very first response to this thread, he would have found what he was looking for. But apparently that is too much work.

Bob

 

[danny davis]

This is what the tech did at my work, but I don't know how he got R1 and R2 ,
How do u think he calculated the values?

2K in parallel with 15K = 1.76K
47.5K in parallel with 69.8K = 1.87K
16.2k in parallel with 2K = 1.78K
47.5K in parallel with 2K = 1.91K
15.8K in parallel with 1.96K = 1.74K

 

[CocaCola]

How do u think he calculated the values?

Probably by using the same formulas posted here numerous times

It's clear he favored using a 2K as a starting point and then simply working the math, not rocket science...

Except this one, as it's total nonsense...

47.5K in parallel with 69.8K = 1.87K

 

[danny davis]

How would u have done this? the starting point and to get R1 and R2

 

[CocaCola]

How would u have done this? the starting point and to get R1 and R2

I generally wouldn't bother, as I explained in my first post, given the tolerance of the average resistor it's mostly a futile exercise that only applies in extreme cases or as an academic exercise... I'm not saying it's never needed but it's at minimum a very limited case basis.. Once you add in the tolerance of standard resistors you might as well have just picked the next standard value, in most cases...

But, from an academic stand point I would use the equations that you have been shown multiple times now... But, I'm really kinda lazy so I would actually cheat and simply use the online calculators, that again, you have been shown multiple times now...

the starting point and to get R1 and R2

I would take out my on hand resistors, close my eyes, repeat "eeny meeny miny mo" a few times, spin around on one foot, hop up and down on the other and then grab the winning resistor that my finger touched first...

Gotta make work fun sometimes

 

[KJ6EAD]

Using E24 values to get 1.87k, 3.6k paralleled with 3.9k = 1.872k, a 0.107 % error but as I said previously, there are no exact matches in parallel, only in series.

 

[danny davis]

R1 = R2 × Rt / (R2 − Rt)

R2 = R1 X Rt / ( R1 - Rt )

But if i don't know what R1 & R2 values are how do I find them, are you guys just guessing?

 

[KJ6EAD]

I edited a typo in the last paragraph of post #5.

 

[CocaCola]

But if i don't know what R1 & R2 values are how do I find them, are you guys just guessing?

Step 1: "I would take out my on hand resistors, close my eyes, repeat "eeny meeny miny mo" a few times, spin around on one foot, hop up and down on the other and then grab the winning resistor that my finger touched first..." This would be my R1...

Step 2: "I would use the equations that you have been shown multiple times now... But, I'm really kinda lazy so I would actually cheat and simply use the online calculators, that again, you have been shown multiple times now..." This would give me R2...

If values used didn't work well, I would repeat the steps above...

 

[danny davis]

so you're just guessing for R1 and calculate the difference?

 

[CocaCola]

so you're just guessing for R1 and calculate the difference?

It's not a guess, it's just picking a starting point for the calculation...

It works just like this...

If I was to hand you

4 quarters
10 dimes
20 nickels
100 pennies

And I said hand me back 50 cents, what coins would you hand back to me? Is there a single right answer? Is there any right or wrong coin to start with when counting out the 50 cents? The same principles apply here, there is simply no hard set rules or single right way to do it, work the math and get a solution they are all correct as long as you arrive at the same end value...

 

[danny davis]

But do you start below or above the desire value? and what "E list" do u choose from

And I'm really confused about these E lists

Why don't they just have one E list to have all the values?

 

[CocaCola]

But do you start below or above the desire value?

In series you start below, in parallel you start above...

and what "E list" do u choose from

The one that holds the tolerance you require or need...

Why don't they just have one E list to have all the values?

The E list specifies the tolerance... Tolerance will dictate the minimum spread between values without excess overlapping...

Not that you will bother reading this but.... http://www.logwell.com/tech/components/resistor_values.html