Custom Resistance Calculator

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transistance

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When creating filters, i come up with weird resistor values - i guess that's pretty common. Is there an online website or a program that shows you the combination of commercially available resistors u can use in parallel or series to achieve the resistance u need?

thanks in advance
 
It's easy to write yourself a little computer program that does this for you, I did one years ago under DOS - don't know where it's gone now?.

Store the preferred values in an array, then use a simple loop to parallel all the different values and check the percentage of error for each combination against your required target value, then display the best result.
 
Nigel Goodwin said:
...a simple loop to parallel all the different values...
Click to expand...

can u expand on this part? btw i have no experience programming or compiling code under dos.. oh well, i got ubuntu to solve that problem now...

what would be the logic behind creating a combination of series and parallel connections to achieve perfect results though? create connection in series, get result very close (but less) to the value you are trying to achieve, then connect a parallel connection to achieve the smaller value to get close to the perfect total?

btw where can i obtain a list of commonly commercially available resistor values?
 
transistance said:
can u expand on this part? btw i have no experience programming or compiling code under dos.. oh well, i got ubuntu to solve that problem now...
Click to expand...

I used Turbo Pascal, but any language would be fine.


I just wrote a program to work out the best parallel connection using just two standard resistors, it gave the closest match, and the percentage of error. It could easily be applied to three or more in parallel.

btw where can i obtain a list of commonly commercially available resistor values?
Click to expand...

Look at what values are available, the E12 series has 12 per decade, and the E24 has 24 per decade.

E12 is

1
1.2
1.5
1.8
2.2
2.7
3.3
3.9
4.7
5.6
6.8
8.2
 
Here are two small programs I did in Visual Basic to calculate 1% resistor values and to optimize a parallel combination for a desired resistor value **broken link removed**
 
i just did... i wan wondering if you had something better u wanted to share but couldn't. Looks like that one you posted a link to is gonna work out for me as well though...

thanks for your contribution
 
I wish people would release the source code to these programs - they're not selling them so there's no reason not to.

Another thing that people forget is component tollerences reduce the accuracy more when you start connecting them in series or parallel. For example connect two 5% 100k resistors in parallel for 50k and you end up with 50k +-10% which is

The E12 series has an accuracy of 10% which is good enough for most applications and in the case of the example above you're no better of using two 100k resistors than you are using one 47k resistor.

Connecting resistors in series or parallel is handy when you use a precision resistor for most of the resistance and a less accurate resistor to get that little bit closer to the desired value.

For example, suppose you want 50k.

You have a 0.5% resistor in the e48 value of 48.7k, you can easily get near enough exactly 50k by adding a cheap 5% 1.2k resistor in series.
 
Hero999 said:
Another thing that people forget is component tollerences reduce the accuracy more when you start connecting them in series or parallel. For example connect two 5% 100k resistors in parallel for 50k and you end up with 50k +-10% which is
Click to expand...

Wrong...me thinks????

Worst cases for two 100K 5% resistors:

105K (+5%) paralled with 105K (+5%) = 52.5K (50K +5%)
95K (-5%) paralled with 95K (-5%) = 47.5K (50K -5%)
105K (+5%) paralled with 95K (-5%) = 49.875K (50K -0.2%)

Ken
 
I agree with Nigel's statement from a statistical standpoint. But to follow up on it a little more, does anyone know how the "tolerance" for passive components is defined?

That is, is the "+/-5%" (etc.) related to a coefficient of variation (i.e., the standard deviation divided by the mean as a percentage), with the limit set at 2, 3 or some other number of standard deviations, or is it more like a manufacturing tolerance for which one would expect all (>99.999%) of the components to fall within?

Apropos his comment, "assuming the resistors are from different batches," I have noticed that within a single apparent lot (e.g., a section of cut tape), the values are pretty close to each other, but not randomly distributed with respect to the expected mean. That observation is just anecdotal, but does relate to how one would calculate the expected error for a combination of components from the same lot.

John
 
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As you have found, resistors from the same batch are normally very close in value, but can obviously be anywhere within the stated tolerence. If you need specific accuracy, then use better tolerence resistors, or handpick them from a number of different batches.

Certainly though in recent years, 5% resistors are generally much better than 5%, presumably manufacturing techniques have improved?.
 
You're right, I was talking nonsense. I stupidly took something someone at work told me at face value without thinking about it.

All resistors are much better than the tollerance - ever heard of six sigma?
 
If you want that kind of calibrated precision don't forget to take into account the load condition the resistor will be working under. Thermal variance in a resistor will be significantly more important than generic component tolerance if there is a wide set of temperatures the resistor will be exposed to. You'll get two variance points, the resistors temperature rise from the current going through it and the ambient temperature from the rest of the circuit itself and their thermal linkages (air or heatsink) Heat sinking your resistor could reduce this effect, you could also insulate it with hot glue in a cold environment self heating would keep the temperature constant, the running temperature of the resistor just needs to be as close to constant as possible.
 
I was just migrating all my stuff to a new computer, and ran across a resistance calculator program that I had forgotten I had. Very cool! It's free and available at:

Directory of /~rstevew/Public/Software

Look for Rescad.zip

It works for finding the nearest values, and series, parallel, and divider calculations. And you can choose from E6, E12, E24, E96, or E192 tolerance resistors.


Ken
 
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