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Old 4th Jun 2020, 4:47 pm   #1
retailer
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Default Parallel Resistor Chart

I came across this parallel resistor calculator chart while internet browsing, I'm sure there are others like me that grab a calculator from time to time to calculate a parallel resistor for some odd value we need, I even contemplated buying a programmable calculator until I came across this chart.

The original was a scan and not great quality and did not have E12 values marked, while they can be located on the scales it is easier to have the E12 vales marked so I have reproduced it in it's original form and also with E12 values marked on the scales, I don't think there are any errors but if there is let me know, it may have been posted here before but in case it hasn't, I have attached it as a printable pdf.

It is so simple to use I don't think it requires an explanation, but just in case - a line drawn across from the R1 scale to the R2scale will cross the Rtotal scale at the parallel value of R1 and R2. I thank Rudolf Graf for the original where ever he is.
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Old 4th Jun 2020, 4:53 pm   #2
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Default Re: Parellel Resistor Chart

You mentioned contemplating a programmable calculator ;

Just about the first program I put in my calculators is \<< INV SWAP INV + INV \>> or its equivalent. But what surprises me is that since 'reciprocal of sum of reciprocals' turns up in many areas (not just electrical engineering) that there has never been (to my knowledge) a calculator with that function built-in.
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Old 4th Jun 2020, 4:54 pm   #3
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Default Re: Parellel Resistor Chart

Thankyou. There is also an online calculator at Sengpiel Audio, which is quite useful.
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Old 4th Jun 2020, 6:10 pm   #4
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Default Re: Parallel Resistor Chart

It's handy, but a couple of improvements come to mind......

Extend the ranges so that you can get a full decade 10-100 on the centre leg.

Add different sized scale marks to make it clear that the centre scale has 1/2 ohm steps (or remove half of the marks).

For E12 or E24, a full house printed table for all possible combinations in order of result magnitude is useful and not too large. Back in the day I had an A4 ring binder with the full E96 set which someone had printed out from the works PDP something or other.
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Old 4th Jun 2020, 9:19 pm   #5
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Default Re: Parallel Resistor Chart

Not having a PDP, I made one using Libreoffice Calc. (I hope it's right!) All figures rounded to 1.

PDF printable with colour background or clear background on A4, or the Calc sheet can be manipulated.
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Old 4th Jun 2020, 10:52 pm   #6
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Default Re: Parallel Resistor Chart

The Libre Office spreadsheet has an interesting fringed appearance viewed at about A4 size for the whole grid! Reminiscent of what used to be called "teleprinter art" or "lineprinter art".


My forgettory has just reminded me that what I had at work wasn't in fact what I said, but was a table of all the possible ratios of E96 resistors. Useful for voltage divider and opamp gain design! It was done on a PDP though
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Old 4th Jun 2020, 11:56 pm   #7
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Default Re: Parallel Resistor Chart

I have to admit to using the Electrodroid andriod app for this since a work colleage introduced it to me, it also got used a lot at work for choosing parallel resistors for select-on-test resistors.

David
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Old 5th Jun 2020, 12:06 am   #8
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Default Re: Parallel Resistor Chart

You might like to try http://81.105.120.101/cgi-bin/resistor-comb
An electronic version of above..... enjoy.
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Old 5th Jun 2020, 6:25 am   #9
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Default Re: Parallel Resistor Chart

Couldn't get my head round that chart, sorry.

I always use the on line calculators, for some reason my sums for || R's always comes out wrong. I use this one to find what twp known R's will give - https://www.allaboutcircuits.com/too...ce-calculator/ and this one to find R||x - https://www.qsl.net/in3otd/parallr.html

Andy
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Old 5th Jun 2020, 8:52 am   #10
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Default Re: Parallel Resistor Chart

You can also work parallel R (and series C) on a calculator with a reciprocal function (1/x).
for example 30 ohms (RA) in parallel with 10 ohms (RB) gives 7.5 ohms.

Key sequence on the calculator 30+10= (gives 40) then press the reciprocal key (gives 0.025) then multiply by 30 (RA) gives 0.75 and again by 10 (RB) gives 7.5
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Old 5th Jun 2020, 9:00 am   #11
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Default Re: Parallel Resistor Chart

why not (AxB) / (A+B). Can usually do it in your head.
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Old 5th Jun 2020, 9:05 am   #12
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Default Re: Parallel Resistor Chart

Quote:
Originally Posted by factory View Post
I have to admit to using the Electrodroid andriod app for this since a work colleage introduced it to me, it also got used a lot at work for choosing parallel resistors for select-on-test resistors.

David
Just downloaded this app, brilliant. Why have I only just heard about it?

Peter
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Old 5th Jun 2020, 9:48 am   #13
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Default Re: Parallel Resistor Chart

Quote:
Originally Posted by Electronpusher0 View Post
Quote:
Originally Posted by factory View Post
I have to admit to using the Electrodroid andriod app for this since a work colleage introduced it to me, it also got used a lot at work for choosing parallel resistors for select-on-test resistors.

David
Just downloaded this app, brilliant. Why have I only just heard about it?

Peter

Another vote for Electrodroid. A colleague introduced me to it a few years ago. It's my "go to" resource for resistor network calculations, potential dividers (etc).
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Old 5th Jun 2020, 10:45 am   #14
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Default Re: Parallel Resistor Chart

Even easier on an RPN calculator.

Press 30 (RA), press 1/x (the reciprocal key)
Press 10 (RB), press 1/x
Press +
Press 1/x
Result shown, 7.5 (RP)
You don’t need to enter the values twice

To calculate RA if you know RB and the required result, you just replace RA with RP, and the + with a -.

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Old 5th Jun 2020, 10:55 am   #15
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Default Re: Parallel Resistor Chart

I think you may be missing the point of the chart.
It's feature is the way you can rapidly assess how various possibilities will work in a way that is clunky with most calculators.

I would "do the maths" at the end to see the exact result.
--
I have a whole book of charts like this. It seems that back in the day this was the quick answer to tricky calculations. Some of the charts work in a really complicated way! But this chart was not in that book - I guess it was regarded as simple enough to do in your head.
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Old 5th Jun 2020, 11:01 am   #16
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Default Re: Parallel Resistor Chart

Once you learn how to use an RPN calculator you want no other sort.

For components (resistors) in series; voltages add, therefore impedances (resistances) add.

For components (resistors) in parallel; currents add, therefore admittances (conductances) add.

Conductance is 1/resistance and resistance is 1/conductance.

So the RPN process given uses the reciprocal key to turn each resistance into the corresponding conductance, the conductances are added, and then the final reciprocal turns the sum of conductances into its corresponding resistance.

Remarkably easy to do and a lot longer in words.

David
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Old 5th Jun 2020, 1:15 pm   #17
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Default Re: Parallel Resistor Chart

Well the original chart is going up on the wall! So easy to use just a ruler needed and I just found a 3 resistor E12 combination to get within 1/4ohm in about 10 seconds...far less than the time it would take me to find my pencil, phone, calculator or browse to an app in my messy office!

Thankyou

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Old 5th Jun 2020, 1:38 pm   #18
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Default Re: Parallel Resistor Chart

There are some useful calculators here. I must admit that I like to fiddle the resistors around in my head as much as possible but where calculators would really come into their own is working out series / parallel combinations from a fixed subset of standard values, rather than actually having to order 50 or 100 just to get a value which would be henceforth little used. Looking at updates to the American Radio History site and into an old annual I saw a quiz question which asked how to get 16 and a half ohms for transformer testing when given four 10 ohm resistors (the answer comes to 16 and two thirds). Although a bit unrealistic for my precise uses it does show the type of problem I often have and the idea I am showing. I like to keep good stock of common values and, especially with surface mount, often series and parallel them. The question does indicate that people in times past made do more with what they had and was readily available rather than having a massive range.
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Old 5th Jun 2020, 2:45 pm   #19
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Default Re: Parallel Resistor Chart

Algebraic vs RPN for calculators is like comparing tricycles to bicycles.

For beginners, tricycles are obviously easier ride than bicycles. They're fine for leisurely riding around, but they do have problems when pushed to the limit, eg cornering at speed. The bicycle "trick" of having to balance the thing is mostly easy to learn, and once you have, the bicycle has many compelling advantages, and you'd never go back to 3 wheels.

Likewise, RPN has a "trick", in this case the fact that all function keys operate immediately. There's very little else to learn, except perhaps which way round the result of rectangular to polar conversion comes out.

With RPN, you can have pending numbers in the stack, but no pending functions.

Some - but not all - algebraic calculators do have pending functions, and you need to know how your calculator handles them. For example if you type "2+3x4=" you get the answer either 14 or 20 depending on which calculator you are using. Some older "smart" phones gave one result if you held the phone horizontally (landscape mode) and the other result if you held in vertically. Doesn't seem that "smart" to me.

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Old 5th Jun 2020, 3:37 pm   #20
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Default Re: Parallel Resistor Chart

I assume you noticed my program for parallel resistors was in RPL (an RPN-based calculator programming language).

To be honest, I am not clever enough to use any other type of calculator. The big plus-point of RPN is that it does what it says, there are no hidden 'gotchas'. You don't have to remember operator precedence because there isn't any. You don't have to wonder how it will do a particular calculation. Once you get used to it you can use it without thinking, without worrying which calculator you've picked up and how it will handle the expresssion. I learnt RPN in an afternoon on an HP41 about 40 years ago and have never looked back (although I must admit I prefer the unlimited stack of an RPL machine like the HP48 or HP49 to the 4 level stack of older machines)
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