Re: Prefered resistor range

From: The Phantom (phantom_at_aol.com)
Date: 03/16/05 

Date: 15 Mar 2005 23:45:02 -0600

On Tue, 15 Mar 2005 14:22:00 -0800, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

>"R.Lewis" <h.lewis@connect-2.co.uk> wrote in message
> news:39p455F649cduU1@individual.net...
>> "Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in message
>> news:Y6IZd.49$gd7.892@news.uswest.net...
>...
>>> Now, if I want a 63.54k resistor, I enter:
>>> stdvals 1% 63.54k
>> <<snip>>
>>
>> If you need 63.54K and you have the complete range of 1% tolerance resistors
>> available you have a problem that no elementary, or sophistacated,
>> calculator is going to solve
>
>You appear to have assumed that the resistors
>to be used would be 1% tolerance. It happens
>that the 0.1% tolerance parts come in the same
>values, (or more, for more money), so there is a
>use for calculations such as the above example.

You don't have to be using .1 % resistors to get a benefit.

Imagine that you have designed a filter and need a 134.985k resistor.
The nearest E96 values are 133k and 137k. As it happens, 134.985k is
about 1.5% greater than 133k and 1.5% less than 137k, so you can't
find a standard 1% value that is actually guaranteed to be within 1%
of 134.985k. In fact, you can't get any closer than about 1.5%.

There are gaps like that between nearly every pair of adjacent E96
standard values where, in that gap, you are further than 1% away from
either of the nearest standard values (all but 4 adjacent pairs are
like this).

But if you parallel the two standard values 255k and 287k, you are in
effect synthesizing a 135.027675k resistor that is guaranteed to be
within 1% of that value. Since this is within .03% of 134.985k, we
have a resistance that is guaranteed to be within just about 1% of the
134.985k we want, instead of only 1.5%.

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