### Author Topic: Software for calculating best resistors.  (Read 1655 times)

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#### kripton2035

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##### Re: Software for calculating best resistors.
« Reply #25 on: May 30, 2024, 12:36:46 pm »
I want to have a 117 ohm resistor using standard E12 values and combining max 4 resistors in serie or parallel.

ChatGPT (hallucinating mode ?)
Quote
To achieve a 117 ohm resistor using standard E12 resistor values and combining a maximum of 4 resistors in series or parallel, we can utilize a combination of resistors from the E12 series which includes the following values (in ohms): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. Here's how we can do it:

Let's start by looking at potential combinations:

Series Combination: We can connect resistors in series to increase the total resistance. To get 117 ohms, we need to add up values that sum up to 117.

By inspection, we can see that 82 ohms + 33 ohms + 2 ohms (from parallel combination of 10 and 22 ohms) gets us 117 ohms. However, this exceeds the limit of 4 resistors.

Parallel Combination: We can connect resistors in parallel to decrease the total resistance. Here, we need to find a combination that adds up to 117 ohms or is close to it.

By inspection again, we see that 56 ohms + 33 ohms + 18 ohms + 10 ohms in parallel gets us 117. This is within the limit of 4 resistors.

So, we can use a combination of 56 ohms, 33 ohms, 18 ohms, and 10 ohms resistors connected in parallel to achieve approximately 117 ohms. Let's calculate the exact total resistance:

1 / (1/56 + 1/33 + 1/18 + 1/10) = 1 / (0.01786 + 0.03030 + 0.05556 + 0.1) = 1 / 0.20372 ≈ 4.904 ohms.

Therefore, the combination of these resistors in parallel will yield a resistance close to 117 ohms, and it satisfies the criteria of using standard E12 resistor values and a maximum of 4 resistors.

#### mariush

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##### Re: Software for calculating best resistors.
« Reply #26 on: May 30, 2024, 12:51:29 pm »
142 in parallel with 680 gives you 117.4 ohm    142 can be made with 120 + 22 ohm in series

3300 || 270 || 220 gives you 116.92 ohm

chatGPT doesn't surprise me....

#### kripton2035

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##### Re: Software for calculating best resistors.
« Reply #27 on: May 30, 2024, 02:56:29 pm »
yes it is VERY BAD at anything calculated...

#### Tation

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##### Re: Software for calculating best resistors.
« Reply #28 on: May 30, 2024, 04:56:25 pm »
yes it is VERY BAD at anything calculated...

IMHO, uses to be syntactically correct, semantically absurd.

#### bpiphany

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##### Re: Software for calculating best resistors.
« Reply #29 on: May 31, 2024, 07:08:50 am »
Actually 56+33+18+10=117 in series. Perhaps a coincidence on the hallucinogens.

Edit: and according to the nifty calculator above

Code: [Select]
Target Approximation Network Error Error ≤ 0.1?117 ~117.012 120 || 4.7k 0.011% ✓
« Last Edit: May 31, 2024, 07:12:40 am by bpiphany »

#### Zero999

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##### Re: Software for calculating best resistors.
« Reply #30 on: June 01, 2024, 12:51:58 pm »
1% tolerance resistors can be found in E24 values. There's normally no point in aiming for much less than 0.2% or so, because the resistor tolerance will dominate. Fortunately that should be possible to make a potential divider with three resistors to get a ratio to that tolerance for most ratios, especially the smaller ones.

Awhile ago I made myself some tables with different combinations of E6, E12 and E24 values to make E24 and E96 values. I don't use them much nowadays because there are better online calculators.

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#### SiliconWizard

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##### Re: Software for calculating best resistors.
« Reply #31 on: June 01, 2024, 10:22:02 pm »
For those interested in understanding how the E-series are built instead of just blindly using tables of values: https://en.wikipedia.org/wiki/E_series_of_preferred_numbers

#### golden_labels

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##### Re: Software for calculating best resistors.
« Reply #32 on: June 04, 2024, 07:12:58 am »
Xvr published a divider solver.

It only models a voltage divider, so not exactly the same tool. But certainly worth looking at. An amazing, comprehensive tool.

Instructions for solving a subset of problems from this thread. For all configurations unset Vout and also unset its target status.

2 resistor in series:
Set Rin as target, in “Restricted” tab set the desired value and allowed margins. Run solver.

2 resistor in parallel:
Set Rout as target, in “Restricted” tab set the desired value and allowed margins. Run solver.

« Last Edit: June 04, 2024, 07:34:27 am by golden_labels »
People imagine AI as T1000. What we got so far is glorified T9.

#### Tation

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##### Re: Software for calculating best resistors.
« Reply #33 on: June 04, 2024, 02:29:50 pm »
I'm feeling than that all these calculators do have a little flaw: they do not take into account the tolerance.

I mean, as stated before, E12 value 12 can be achieved as a series of E3 values 10 + 2.2. But E12 values have a tolerance of 10 %, so we want to be inside 12 ± 10 % ⇒ between 10.8 and 13.2. In order for a series of two resistors of 10 and 2.2 to be inside such margin, they must have a tolerance of 5 %, otherwise there is a possibility of their series being outside such interval.

In doing these calculations I'm considering/assuming that:
• (assumption) The resistance of resistors is distributed normally (Gaussian) with mean equal to the nominal value of the resistor and standard deviation proportional to the product of nominal value times tolerance
• Thus, conductance of a given resistor, provided that the tolerance is small, is also distributed normally with mean the inverse of the resistor nominal value and equal tolerance than the resistance
• (assumption) The true resistance values of two resistors are uncorrelated
• Thus, the resistance of a series of two resistors distributes normally with mean the sum of both resistors nominal values and $\sigma=\sqrt{\sigma_1^2 + \sigma_2^2}$
• Thus, the series of two resistors of equal tolerance $T_0$ has tolerance $T=T_0{\sqrt{R_1^2+R_2^2}\over R_1+R_2}$
• The «tolerance» in resistance of a parallel combination of two resistors of tolerance $T_0$ has exactly the same expression as the series case

It is interesting to note that the tolerance of a series/parallel combination of two resistors has a tighter tolerance than the original resistors. In spite of this, there are cases (the 12 ≈ 10 + 2.2 above) when we need resistors of tighter tolerance than that of the target E series. In this example case, if using 10 % resistors, the nominal value of the series is 12.2, and its tolerance 8.4 %, so can expect values between 11.26~13.22, outside by a little of the desired 10.8~13.2. If the resistors are 5 %, the final range would be 11.71~12.71 ⇒ OK.

Maybe I'm a bit picky here, but had a fun time this morning doing these calculations
« Last Edit: June 04, 2024, 02:32:26 pm by Tation »

#### mariush

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##### Re: Software for calculating best resistors.
« Reply #34 on: June 04, 2024, 09:37:29 pm »
I'm feeling than that all these calculators do have a little flaw: they do not take into account the tolerance.

I mean, as stated before, E12 value 12 can be achieved as a series of E3 values 10 + 2.2. But E12 values have a tolerance of 10 %, so we want to be inside 12 ± 10 % ⇒ between 10.8 and 13.2. In order for a series of two resistors of 10 and 2.2 to be inside such margin, they must have a tolerance of 5 %, otherwise there is a possibility of their series being outside such interval.

I don't agree with this... in these days, the manufacturing process of resistors is good enough and the resistors are cheap enough that you would probably have a hard time getting even higher than 5% tolerance.

They most likely bin the resistors automatically and reserve the <1% for the 1% tolerance, <3-4% for the 5% bucket and on 10% you'd probably get anything higher than 3-4% or you'd just get the ones binned for <5%.

Sure, account tolerance, but aim for under 5% at most ... nowadays I just default on 1% resistors because they're cheap enough.

#### Tation

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##### Re: Software for calculating best resistors.
« Reply #35 on: June 05, 2024, 09:36:03 am »
I've found that with only 2 resistors from the E12 series, sometimes in series, others in parallel, one can reach (under the assumptions above):
• all E24 values with 5% tolerance if the E12 resistors are 5%
• all E24 values with 2% tolerance if the E12 resistors are 2%
• all E24 values with 0.2% tolerance if the E12 resistors are 0.1%
• all E48 values with 2% tolerance if the E12 resistors are 1%
• all E48 values with 1% tolerance if the E12 resistors are 0.5%
• all E96 values with 1% tolerance if the E12 resistors are 0.1%
• almost all E192 values (but nine: 1.13, 1.37, 1.91, 2.05, 2.37, 2.43, 4.42, 6.34, 9.31) with 0.5% tolerance if the E12 resistors are 0.1%
• almost all E192 values (but one: 9.31) with 0.5% tolerance if the E12 resistors are 0.01%
« Last Edit: June 05, 2024, 09:54:25 am by Tation »

#### mike449

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##### Re: Software for calculating best resistors.
« Reply #36 on: June 05, 2024, 08:03:19 pm »

Smf