Introduction: How to Change the Resistance of a Resistor With Another Resistor.

Picture of How to Change the Resistance of a Resistor With Another Resistor.

Sometimes you need a resistor with a value you don't have in your kit. Instead of ordering and wanting for a resistor with a value you need you can change the resistance of a resistor by using another resistor or many. By installing resistors in a parallel or series circuit you can change the value in Ohms.

Parts:

  1. You will need a few resistors
  2. Multimeter
  3. Breadboard

Here is a link to a resistor calculator.

Step 1: Resistors in Parallel.

Picture of Resistors in Parallel.

Resistor in parallel:

Using the calculator

100 Ohm resistor in a parallel circuit with a 100 Ohm resistor you give you a total resistance of 50 Ohm's

470 Ohm resistor in a parallel circuit with a 470 Ohm resistor you give you a total resistance of 235 Ohm's

The two resistors don't need to have the same value. You can also use two resistors with a different value.

100 Ohm resistor in a parallel circuit with a 50 Ohm resistor you give you a total resistance of 33.33 Ohm's

100 Ohm resistor in a parallel circuit with a 25 Ohm resistor you give you a total resistance of 20 Ohm's

You can also use more then two resistors.

100 Ohm resistor in a parallel circuit with a 25 Ohm resistor and a 25 Ohm resistor you give you a total resistance of 11.11 Ohm's

100 Ohm resistor in a parallel circuit with a 25 Ohm resistor and a 20
Ohm resistor you give you a total resistance of 10 Ohm's

Step 2: Resistors in Series.

Picture of Resistors in Series.

Resistor in series:
Using the calculator

100 Ohm resistor in a series circuit with a 100 Ohm resistor you give you a total resistance of 200 Ohm's

470 Ohm resistor in a series circuit with a 470 Ohm resistor you give you a total resistance of 940 Ohm's


The two resistors don't need to have the same value. You can also use two resistors with a different value.

100 Ohm resistor in a series circuit with a 50 Ohm resistor you give you a total resistance of 150 Ohm's

100 Ohm resistor in a series circuit with a 25 Ohm resistor you give you a total resistance of 125 Ohm's

You can also use more then two resistors.

100 Ohm resistor in a series circuit with a 25 Ohm resistor and a 25 Ohm resistor you give you a total resistance of 150 Ohm's

100 Ohm resistor in a series circuit with a 25 Ohm resistor and a 20 Ohm resistor you give you a total resistance of 145 Ohm's

Step 3: Resistors in Parallel and Series.

Picture of Resistors in Parallel and Series.

I don't have a calculator for this how ever you can use the same calculator as before.

Parallel circuit you have a 20 Ohm resistor and a 20 Ohm resistor with a total resistance of 10 Ohm's

Your parallel circuit is in series with a 100 Ohm resistor giving you a total resistance of 110 Ohm's.

Same as above but now your adding another resistor in the series circuit.

Parallel circuit you have a 20 Ohm resistor and a 20 Ohm resistor with a total resistance of 10 Ohm's
Your parallel circuit is in series with a 100 Ohm resistor and 100 Ohm resistor giving you a total resistance of 210 Ohm's.

Step 4: Ohm's Law

Picture of Ohm's Law

Ohm's law defines a linear relationship between the voltage and the current in an electrical circuit.

The resistor's voltage drop and resistance set the DC current flow through the resistor.

With water flow analogy we can imagine the electric current as water current through pipe, the resistor as a thin pipe that limits the water flow, the voltage as height difference of the water that enables the water flow.

R=Resistance(Ω)

I=Amps

V= Volts

Ohm's law definition
The resistor's current I in amps (I) is equal to the resistor's voltage VR=V in volts (V) divided by the resistance 9R) in ohms (Ω):

I=V/R (Amps=Volts/Resistance)

Voltage calculation:
When we know the current and resistance, we can calculate the voltage.

The voltage V in volts (V) is equal to the to the current I in amps (I) times the resistance (R) in ohms (Ω):

V=I*R (volts=Amps*Resistance)

Resistance calculation:
When we know the voltage and the current, we can calculate the resistance.

The resistance (R) in ohms (Ω) is equal to the voltage in volts (V) divided by the current I in amps (I):

R=V/I (Resistance=Volta/Amps)

Since the current is set by the values of the voltage and resistance, the Ohm's law formula can show that:

  • If we increase the voltage, the current will increase.
  • If we increase the resistance, the current will reduce.

Comments

russ_hensel (author)2016-10-12

For another example of ( generalized ) resistance including alternating current, capacitors and inductors see: Incredibility Powerful Resistance Calculator https://www.instructables.com/id/Incredibility-Powerful-Resistance-Calculator/

Kuuran (author)2016-10-07

The first image of series-parallel resistors is weird... The two resistors on the right are shorted.

Kuuran (author)Kuuran2016-10-11

Fixed now.

KISELIN (author)Kuuran2016-10-11

Hi Kuuran. I checked re-checked, triple-checked them images and couldn’t find any wrong about them? Sorry, Yes there’s a mismatch, in the upper picture of step3
the resistance is: R1 + (1/R2 + 1/R3 + 1/R4),
and in the lower picture the resistans is: R1 + (1/R3 + 1/R4) + R2..
But in real life, (unless You are a scientist in electronic’s), you need them ”these” formula’s maybe a couple of time’s during your lifetime, (me 64+years to age and worked with elecrticity take or give for 30+years), thus needing this formulas just sometimes.
Check me at www.teksel.net and find out what we ”INSTUCTABLER’S” can achive

KISELIN (author)2016-10-11

Hi . I checked re-checked, triple-checked them images and couldn’t find any wrong about them? Sorry, Yes there’s a mismatch, in the upper picture of step3
the resistance is: R1 + (1/R2 + 1/R3 + 1/R4),
and in the lower picture the resistans is: R1 + (1/R3 + 1/R4) + R2..
But in real life, (unless You are a scientist in electronic’s), you need them ”these” formula’s maybe a couple of time’s during your lifetime, (me 64+years to age and worked with elecrticity take or give for 30+years), thus needing this formulas just sometimes.
Check me at www.teksel.net and find out what we ”INSTUCTABLER’S” can achive

GOOD INSTR: You have here. kind of a basic to them "newbes"

KISELIN (author)2016-10-11

Actually the unit for voltage in formulas is the capital = U ; capital I = current (Amperes);
capital R = resistans (ohms). Them together produce P = power (watts, Joules, etc.)
The resistans ”ALLWAYS” stands for the work done, and that’s why it’s called
”the Ohm’s, (resistors), law”
So the outcome of this said: P = U x I ; I = U/R; R = U/I etc. etc…

KISELIN (author)KISELIN2016-10-11

WHOOP’s, now some smart-one comes saying of resistanse don’t ”allways” do the job. They’ll reclame to them both capacitive and inductive loads. Halt ! a capacitor when NOT charged is having zero Ohm’s = 0R = shorted, (allmost), resistance. When charged the res. Increases to, theoreticallly, to infinitive, (only the isolationing layer’s between make the brake trough, brakeage). So the capacitor takes its fare share of energy. When charged it will willingly to be decharged…. Geeee… take some & give it back

GianlucaG1 (author)2016-10-10

Parallel formula is wrong: result of your formula is 1/R!

You talk about Ohm's law, but the triangle represents Watt's law! Ohm's law is

V=I*R, not P=I*V.

As said by BasinStreetDesign, I agree you should firstly say that parallel total resistance is lower than the lower resistor (and, for the series, total resistance is higher than the higher resistor...)

JunezRiyaz (author)2016-10-06

Nice tutorial .

gm280 (author)2016-10-06

One good way to make sure you are using the parallel formula correctly is the final resistance will always be less then the lowest resistance value of all the resistors in parallel. Another way to make certain value resistors is you can take a little Dremel cutting disk and cut into a larger value resistor crossways (carbon type) and make a lower value. You can make more precision resistors that way. JMHO

BasinStreetDesign (author)gm2802016-10-06

For parallel resistors, it would be better to say that the final resistance will be less than any one of the resistors in parallel. Also, using a Dremel like that will raise the final value not lower it.

DonCesare (author)2016-10-06

Awesome! Thanks a lot for your explanations. They are really clear, easy to follow and understand. Now, it d be good if you could add what happens to power if using different power ratings. :)

robmawe91 (author)DonCesare2016-10-06

Yea, I could add a something about Ohm's law.

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