Resistors in parallel

[wzabala]

Hi Everyone.
I'm building a simple circuit but don't have the necessary 22k resistor. I'm waiting for one to be delivered and in the mean time I was thinking about using resistors I do have (in series) to make up the 22k resistance I need. In this case I would require 4 resistors.
But here's my question. If I wanted to use resistors in parallel, to achieve the same 22k resistance, what calculation would I use?
I have seen calculations to add up resistors in parallel but I need to work backwards, starting with 22k. Hopefully the calculation would give 22k or something close, meaning I could get away with using less than 4 resistors as I would if I connected them in series.
Thanks for any help
W

[mariush]

See https://www.electronics-tutorials.ws/resistor/res_4.html

Basically 1 / R  =  1 / R1 + 1 / R2 + 1 / R3 ... + 1 / Rn

for 2 resistors   1/R =  1/ R1 + 1/R2  = (R1+R2) / R1xR2  => R = R1xR2 / (R1+R2)


With two identical values the above becomes R = R1 x R1 / 2 R1 = R1/2

For 22K,  you could do :

10K + 10 K + 1K + 1K  in series
20K + 1K + 1K  in series
20K + 2.2K in series = 22.2 (could be close enough for your circuit)

47K + 47K in parallel  = 23.5K
68k + 33K in parallel = 22.2K
82k+82k+47k in parallel = 21.9k

play around with this "calculator" : https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-parallel-and-series-resistor

[srb1954]

Hi Everyone.
I'm building a simple circuit but don't have the necessary 22k resistor. I'm waiting for one to be delivered and in the mean time I was thinking about using resistors I do have (in series) to make up the 22k resistance I need. In this case I would require 4 resistors.
But here's my question. If I wanted to use resistors in parallel, to achieve the same 22k resistance, what calculation would I use?
I have seen calculations to add up resistors in parallel but I need to work backwards, starting with 22k. Hopefully the calculation would give 22k or something close, meaning I could get away with using less than 4 resistors as I would if I connected them in series.
Thanks for any help
W

Depends on how accurate you want to get and what resistors you have in stock.

Using only 2 resistors (from the E24 series) in parallel you can get the following values:
 R1     R2     R1//R2  Error w.r.t. nominal 22k
24k   270k   22k04     0.2%
27k   120k   22k04     0.2%
30k   82k     21k96    -0.2%
33k   68k     22k22     1.0%
36k   56k     21k91    -0.4%
39k   51k     22k10     0.5%
43k   43k     21k50    -2.3%

The stated error doesn't include the tolerances of the individual resistors. Any of the above combinations would probably be acceptable accuracy for most purposes except perhaps for the 43k//43k combination.

[EPAIII]

For two resistors in parallel the basic formula is:

Rp = Parallel Value
R1 = Resistor 1
R2 = Resistor 2

1/Rp = 1/R1 + 1/R2

Now if you have R1 as a given and want a value of Rp then the formula, by subtracting 1/R1 from both sides becomes:

1/R2 = 1/Rp - 1/R1

In both cases both R1 and R2 will be greater than Rp.

Example:

You want a 90 Ohm resistor but don't have one. So you start with R1 = 100 Ohms.

1/R2 = 1/Rp - 1/R1

1/R2 = 1/90 - 1/100

1/R2 = 0.0111 - 0.01

1/R2 = 0.00111

R2 = 900 Ohms

So a 100 Ohm and a 900 Ohm resistor in parallel will give a value of 90 Ohms. Of course, 900 Ohms is not a standard value so using the closest value, 910 Ohms, the first equation then gives us the actual value of the pair in parallel:

1/Rp = 1/100 + 1/910

1/Rp = 90.099 Ohms

And that is pretty close to the desired value. Certainly within 5%.

Most scientific calculators will have a 1/x button. And most of them will perform the above calculations by entering the numbers followed by the 1/x key. Then enter either a + or a - sign between them. Follow the second entry with the = sign and then press the 1/x button again. Like this for the Rp calculation:

90
1/x
+
900
1/x
=
1/x

For the R2 calculation just change the + to a - .

[EPAIII]

When working with 5% resistors I have found that there is a rule of thumb that quickly gives a good approximation of a standard value with no math. It goes like this:

Select the next 5% value that is greater than the desired value.
Then multiply that value by 10 and put that resistor in parallel with the first.
The value of the parallel combination will be very close to the desired value.

Example:

Desired value = 22K.
The next 5% value is 24K.
And 10 times 24K is 240K.

And 24K in parallel with 240K is 21.8181K. Pretty close to the desired 22K.

The 5% resistor values are a power series with a uniform factor (1.100694X) separating the values. This ensures that the above trick works for all the 5% resistor values.

[BeBuLamar]

I think it's best if you list the resistor values you do have. With that we can figure out how best  to put them together to get the 22K you want.

[DavidAlfa]

This might help you. Put the available resistors, the target value (22k) and go!

https://www.daycounter.com/Calculators/Parallel-Resistor-Combination-Calculator.phtml

[Swake]

A 25kR or 100kR potentiometer (=trimmer =potmeter) set to 22kR can help out too. Or salvage a resistor from an old board (I've got a pile of junk boards just for that purpose)

Eventually that 22kR value is not very critical. What is the resistor used for? How close does it have to be?

Calculation:
The simple trick, only when using all the same value resistors, is to divide the value by the number of resistors you want to put in parallel. example: 3 resistors of 100kR in parallel = 100k/3 = 33k
3 times 68kR in parallel = 68k/3 = 22.6kR

[ledtester]

This is one of the most sophisticated resistor network finder web apps:

http://kirr.homeunix.org/electronics/resistor-network-finder/