Re: Prefered resistor range

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

Date: 16 Mar 2005 02:57:06 -0600

On Wed, 16 Mar 2005 14:39:46 +1300, Terry Given <my_name@ieee.org>
wrote:

>Larry Brasfield wrote:
>> "Glenn Gundlach" <stratus46@yahoo.com> wrote in message
>> news:1110918855.695485.18390@z14g2000cwz.googlegroups.com...
>>
>>>Rich Grise wrote:
>>>
>>>>On Tue, 15 Mar 2005 14:23:06 +0800, Rheilly Phoull wrote:
>>>>
>>>>>I seem to have lost the formula that calculates the 1% 'preferred'
>>>>>range of resitors. Can anyone help out please ??
>>>>
>>>>Sure!
>>>>
>>>>http://www.google.com/search?q=standard+resistor+values
>>>>
>>>>Who needs "formulas"? :-)
>>>
>>>Gee, I just had a copy of the page in the Digi-Key catalog. You have to
>>>be able to buy the value you spec.
>>
>>
>> I can appreciate the humor here, abstractly.
>> But after designing the Nth precision active filter,
>> countless dividers, and other precise circuits, I
>> got tired of table lookups. To lookup just one
>> value is not overly hard, just tedious. You start
>> with what you want, look at the table, and do a
>> little simple arithmetic (often by eye) to decide
>> which value is closest. Maybe you adjust that
>> choice according to which way it went for some
>> nearby resistor(s).
>>
>> Now, if I want a 63.54k resistor, I enter:
>> stdvals 1% 63.54k
>> and get back 63400. This saves just a little time,
>> so I cannot argue any clear superiority there.
>>
>> Here is where an algorithmic approach really pays.
>> I really want something closer, and I am willing to
>> parallel two parts to get it. I enter:
>> stdvals 1% 63.54k 2p
>> and quickly get a useful result:
>> Approximate 63540 as 1(1/66500 + 1/1.43e+06),
>> yielding 63544.9 (ratio = 1.0001)
>> This saves some real time. (I can remember when,
>> during my stint as an engineering tech, I was given
>> the task of doing approximately this same search
>> for a short list of resistors. If not for the aid of an
>> HP-35, it would have taken days, not the hours I
>> had to spend on it.)
>>
>> A common task in active RC filter design is to
>> come up with capacitor pairs that deliver a close
>> approximation to the values you really want. Ask:
>> stdvals 5% 588p 2a
>> and ye shall receive:
>> Approximate 5.88e-10 with (5.6e-10 + 2.7e-11),
>> yielding 5.87e-10 (ratio = 1.0017)
>>
>> Any of you who have done this by hand know
>> the lingering doubt that remains when you have
>> not used an exhaustive search to find whatever
>> answer you settled upon. Or you know just
>> how tedious the exhaustive search is. Or both.
>>
>> I guess the natural extension of Rich's joke is:
>> Who needs calculators or computers? I've
>> got tables of values and logarithms, a slide rule,
>> and my trusty pencil and pad. Valves forever!
>>
>
>I do this with Brute Force (ie matlab). I have a simple s-file that
>takes an array of available values, which may or may not be entire
>ranges - smt design optimisation usually involves minimising the number
>of different parts.
>
>Then I calculate every possible 1,2,3,4 resistor combination, using:
>
>1 resistor
>2 series
>2 parallel
>1 paralleled with (2 series)
>2 paralleled with (2 series)
>
>and for each of the 5 cases sort the result in order of increasing error
>
>I specify a required accuracy, then throw away all results which are worse.
>
>then display the first 10 entries in each of the 5 combinations.
>
>Yeah, it makes HAL9000 do a *lot* of calculations, but so what - it
>results in a barely perceptible delay between hitting <enter> and
>getting the answers.

   Exactly. So what? My criterion for acceptable speed has always
been; if the result is there on the monitor, or the display of my
calculator when I have picked up my pencil to write down the results
in my notebook, then that's fast enough!

   I once did some designs for audio, some second order filters. Two
R's and two C's. We were using these low-cost Japanese carbon film
resistors, 5% nominal. I noticed that without fail, they were within
1% of their nominal values. But only values from the E24 series were
available. And the 5% caps we were using were similarly close to
their nominal values. So, rather than specify particular cap values
and then try to find combinations of resistors to get close to the
exact computed values, I generated arrays with a decades worth of
resistor standard values centered on the computed exact values, and
the same with the caps. Then I programmed four nested loops to try
every possible combination of 2 R's and 2 C's, sorting in order of how
closely the desired filter polynomial was realized. This was in the
late 70's, running on Microsoft interpreted Basic. It ran all night,
but there were all the possibilities in the morning! I'm pretty sure
that today it would be fast enough to meet my criterion for acceptable
speed! I guess that's your experience; I just haven't had a need to
do this for a long time.
 
>
>Cheers
>Terry

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