# Resistance and Inductance

Discussion in 'General Electronics Discussion' started by tedstruk, Mar 19, 2017.

1. ### tedstruk

475
7
Jan 7, 2012
If say RS was out of 18k resistors and you couldn't find one in your shop...
could you make 18k out of a 22k resistor and an induction coil?

25,448
2,802
Jan 21, 2010
No.

3. ### duke37

5,364
769
Jan 9, 2011
Resistors in series, add the resistances.
Resistors in parallel, add the conductances which are the inverse of the resistances.
1/A + 1/B = 1/C
So 1/22k + 1/B = 1/18k
so 1/B = 1/18k - 1/22k
Using a reverse polish calculator and using ks.
18 1/x
22 1/x -
1/x
gives 99
so put 100k resistor in parallel with 22k, the shop should have one of these.

4. ### tedstruk

475
7
Jan 7, 2012
They didn't
RS have 2 9k's, but I got a pack of 22k's, from what you told me, I can lessen resistance by placing resistors in parallel, where say .04 ohm motor will stop at .04 ohms of resistance! yeah I did that! Same process but in series, with less math! I wonder if that same motor will stop with parallel resistors?(I tried it once and it didn't work. had something to do with the differences and feedbacks of AC-DC source)

Heres one that I remember toying with in my younger years!
"They don't make some of the stuff you will require so you will have to make your own." teach
I wanted a megaohm pot....

5. ### tedstruk

475
7
Jan 7, 2012
Calculating resistors in parallel---
count the number of conductors - "duh... I got like 2 resistors in there"

1 divided by each resistor value and add them together.
(remember to convert all your values to the same value where ohms is king and the polish are itallian)

so
1 / 100000 =.00001
+ 1 / 22000 = 5.45454~
.00001 + 5.45454~ = 5.4545456
1 / 5.4545456 = .18333 (where infinite values of 45 are considered polish and modern calculators do the work for you...) dumps out somewhere around 18k in my case about 17k... On the DDM scale that is.
The POT above is about 5mohms.

6. ### BobK

7,682
1,686
Jan 5, 2010
Uh, 1/22000 is not 5.4545...

Try again.

I get 18033

Bob

7. ### duke37

5,364
769
Jan 9, 2011
I do not understand why polish are italian.

If you have a calculator with a 1/x key, then things can be very easy, you do not need lots of intermediate calculations. I like the reverse polish calculator since I have used it for 30 years and get my numbers in a twist when using a more usual arithmetic calculator.

To calculate the resistance of 100k in parallel with 22k, run in thousands to limit the number of key presses.
Here goes
100 1/x
22 1/x +
1/x
gives 18.033.
Since we were running in thousands, the answer is 18k033 as Bob found.

You could instruct me in how to use an arithmetic calculator.

(*steve*) likes this.
8. ### BobK

7,682
1,686
Jan 5, 2010
1
/
100000
+
1
/
22000
1/x

18032.78689

On my TI calculator

Bob

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,802
Jan 21, 2010
Or for calculators with dubious handling of precidence but having a memory:

1
/
100
=
STO
1
/
22
=
+
RCL
=
STO
1
/
RCL
=

I love my RPN calculators. It makes all this nonsense redundant.

OBW0549 likes this.
10. ### OBW0549

159
118
Jul 5, 2016
Yup.

22000
1/X
100000
1/X
+
1/X
(Display: 18032.78...)

Done.

11. ### BobK

7,682
1,686
Jan 5, 2010
When I was in grad school at Purdue, we had a calculator Olympics. The HP RPN people always won, until TI came out with operator precedence and nested parentheses. Then the TI people always won.

Bob

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,802
Jan 21, 2010
Yes, for any equation you have to mentally perform the infix to postfix notation conversion. And where the calculator can do this quickly, the algebraic I notation calculator will have the lead.

Any calculator using algebraic notation must do the conversion to RPN (or similar) in code (even if they do it very badly). This requires memory and processor cycles that the early calculators did not have in abundance. I remember some AOS calculators literally taking seconds to display a result.

HP eventually switched to AOS for student models because to do otherwise would be to be making a significantly different thing that teachers would have to allow for. At the same time, galloping featuritis pretty much killed the handheld calculator as it tried to emulate a computer or be all things to all people.

I taught my wife to use an HP-28 (AOS) while I still used my HP-41 or occasionally an HP-21 at work. (As she pointed out, what use is a calculator collection of she couldn't use any of them)

It is also interesting to note that the HP calculators seem to have survived the decades far better than the TI models. And that's a shame because they were pretty much fierce competitors at the time, forcing all sorts of technology into the hands of people like myself. It is interesting to note that for a long time the Space Shuttle carried HP-41's along for a number of purposes including very basic deorbit burn calculations. http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv001.cgi?read=2577

13. ### tedstruk

475
7
Jan 7, 2012
Dad used to grill me on fractions... I new already that 1/ a number was not 1, from working on lathe calculations in the shop.
Removal of zeros is part of machinist, and was a small part of pre algebra. I had heard the term "polish calculator" before, but I have to be honest... the first time it didn't work right for me and I still laugh about it today!

14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,802
Jan 21, 2010
The only Polish calculators I can conceive of come from Poland. RPN is never called "Polish" (to my knowledge) and it's connection with Poland is via the nationality of the author of what has become known as Polish notation (a form of notation developed for symbolic logic, and only as an afterthought found to describe arithmetic). Your confusing use of non standard and apparently unique terminology makes you one of the more frustrating people to try to understand.

15. ### WHONOES

993
261
May 20, 2017
I use the following formula:
Where
Ra = required Value
Rx = Known Value
Ry = Unknown Value (to be placed in parallel with Rx)

Ry = (1/(Rx - Ra) / Rx) *Rx
It takes just a few seconds on a hand calculator. I have a spread sheet with all sorts of formulae on it (including this one) which make life a lot easier since I don't have to remember all of them and it takes no time at all to get a result.