Potential divider

[2camjohn]

Im trying to divide voltage of the ratio of 1:0.1077

Unfortunately neither my maths, nor my knowledge of resistor values is up to the job.

Can anyone suggest two resistors that are close to this ratio?

Many thanks for your time.

 

[samcheetah]

do u really want it to be that accurate????

the ratio 1k:107.7 is the same as 1:0.1077

so u can use the ratio 1k:100 to an approximation.

how did u calculate this value. where do u want to use this voltage divider. u might be able to do what u want by some other method which would be better than finding a 107.7 ohm resistor :!:

 

[laroche73]

ratios

What's the application? I'd also question the need for this much precision, considering that typical resistors will have 1% or 5% tolerances.

Anyways, using a table of standard 5% resistor values from a digi-key catalog,

220/2000 = 0.11
360/3300 = 0.1091
390/3600 = 0.1083

just enter the ratio 1/0.1077 and hit your calculator's "X" key twice (to put it into constant-multiply mode, not all calculators have this function), then enter some standard resistor values followed by "=". Pretty soon you'll get some numbers that come very close to available values.

 

[Roff]

Guys, you're missing something. He wants a voltage divider, not a resistor ratio.

Vout/Vin=R2/(R1+R2)=0.1077

R1/R2=8.285, or conversely,
R2/R1=0.1207

Pick two standard values (in whatever tolerance you require) that come closest to this ratio.

 

[laroche73]

Nice catch, Ron. I think you used too many diodes in the last circuit you posted.

 

[Nigel Goodwin]

A LONGGGGGGGG! time ago I wrote a DOS program using Turbo Pascal, actually on a laptop running a 386SX16 with 4MB of RAM.

The program was for calculating parallel resistors, you simply enter the value you wanted and it gave you the two nearest preferred E12 values, along with the percentage of error.

This could easily be modified to calculate potential dividers, if I can find the original code I'll give it a go!.

But, like everyone else, I'd like to know what it's for?, and why you specified it to such precision?.

 

[Styx]

you might want to look into using a precision (ajustable) reference diode

have a look into the lm4041 it is pretty good

 

[Roff]

Nice catch, Ron. I think you used too many diodes in the last circuit you posted.

I think you're right, Claude. I'm supposed to be . :lol:

 

[2camjohn]

I got 180 and 20k, thats as close as I need to be.

Cheers for all the help fellas

 

[Roff]

I got 180 and 20k, thats as close as I need to be.

Cheers for all the help fellas

Actually, with 5% resistors, your best match is
8.2, 68
82, 680
820, 6.8k
8.2k, 68k
etc., etc.

You pick te`he impedance level depending on the load on this divider.

 

[laroche73]

Precision resistors

Years ago, I came across an article that described a way to make precision resistors out of common (at the time) 1/2W 10-20% tolerance carbon resistors. You would start by choosing a value below the desired resistance, then carefully notch the resistor with a small triangular file while measuring the result with an ohmmeter. As material was removed, resistance increased. The final step was to place a small blob of epoxy over the notch. I can't speak for the long term reliability, but it was an intertesting idea. Tried this once with SMD, it didn't work out very well. I think the file was too large :roll:.