So how close does the answer have to be??? and working with LEDs, a bit of difference seldom matters, if they are being used for illumination. LEDs as voltage references is a different issue, tho.

 

Good point. If you are within 20% of the desired value I say that’s good enough.

 

Mostly, within 5% is adequate. 20% could be a stretch, as I see it.

 

I cannot see changing an LED series resistor from 100 Ω to 120 Ω would be noticeable.

 

Is a meter is used then you should be able to get within the tolerances of the meter.

 

Is a meter is used then you should be able to get within the tolerances of the meter.

Why? What is that claim based on?

If you have a collection of resistors available, each choice you make will yield a certain result. There will be gaps between those results. If your desired result is right in the middle of the largest gap, then for your claim to be true, the largest gap can be no larger than the tolerance window of the meter. But as you use better and better meters, that tolerance window shrinks, but that has no effect on the width of the largest gap.

 

Of course it would be useful to understand just what the application of the resistors in question is, since I have learned in my career that some applications are more critical than others.

 

In other words negligible factors frequently aren't. One of my personal memes.

 

In other words negligible factors frequently aren't. One of my personal memes.

Though the reverse is often true, as well. People spend a huge amount of time, effort, and money addressing factors that are negligible.

A major role of the engineer is to identify factors that should be considered and then, in considering them, identify the ones that are critical, those that are relevant, and those that are insignificant.

 

OK, R1 and RT are known values is there a simple formula to calculate for R2?
Rt= 1/R1 = 1 / R2
R1 = ?Rt?T2?

Hi,

The simplest formula appears to be in post #2 of this thread by Papabravo:
1/R2=1/RT-1/R1

and then to get the actual value of R2 you just use the 1/x key of the calculator.

This can be plugged into a calculator step by step as follows...
RT
1/x
- (minus)
R1
1/x
=
1/x
and the value shown will be R2.

Of course if you have a programmable calculator you can use that and program those steps into memory.

 

In other words negligible factors frequently aren't. One of my personal memes.

I for one appreciate that philosophy.