The Resistor





Introduction: The Resistor

About: I am a tech lover and much interested in innovations and modifications....!!! ;-)

Resistor is a basic component in electronics.....
Mostly, every circuit in electronics field comprises at least a single resistor.
You can also find them in the circuits lying around you...

It's basically a component which provides resistance between two terminals in a circuit.
It controls the flow of electrons between the two terminals. It allows us to control the current in the circuit as per as our need. For this purpose, various values of resistors are present in the market.

Resistors are also used in series or parallel combination.Any value of resistance can be created by using series or parallel combinations. As resistor blocks the quantity of current, it dissipates power in the form of heat. So, Choosing a proper resistor with proper specs is a must for any application.

This Instructable explains about the basics of a Resistor.

You can also refer the above video for a short and a simple basic explanation about the resistor or check it out here -

Step 1: Understanding the Resistor...

A resistor is a two terminal device and has no polarity. ( No positive or negative terminals )

A Resistor has various color bands on it. The value of a resistor is determined by these color bands.

A Resistor can have 4 to 6 bands, denoting the value of resistance.Refer the color code table for determining the value of a resistor.

The Common types of Resistors available in the market are -

• Carbon Resistors ( commonly used )
• Metal Film Resistors
• Wire wound Resistors

The Resistors are also available in various different sizes. These sizes depend on the use of resistor in a specific application. The size is determined by the value of the voltage that is applied to it and the amount of current flowing through i. We'll will discuss about it more in the further steps.

Step 2: Calculation of Resistance

Resistance Calculation is one of the most important step in the selection of a resistor.

It is calculated using a formula known as the ' OHM's LAW '.

Ohm's Law is given as --

V = I x R


• V is the voltage across the two terminals of the resistor
• I is the current flowing through the resistor
• R is the value of resistance

For eg -

If we connect a resistor of 100ohms Resistor to a 5Volts supply, then we can calculate the current flowing trough it by using Ohm's Law. And this done as follows :-

We have,

• V = 5 Volts
• R = 100 Ohms

So, the current through the resistor is given as -

I = V / R (using ohm's law)

I = 5 / 100

I = 0.05 Amps = 50 mA

Step 3: Calculation of Size of a Resistor

Choosing a proper Resistor is very important for a typical application. Improper size of resistor can lead to over heating of resistor or burning of resistor !!!!!!

Size of a Resistor depends on the power / heat generated by the resistor.

This is done by using a formula to calculate the power / heat dissipation.

It is given as -

P = V x I = ( I^2 ) x R = ( V^2) / R


• P is the power dissipated by the resistor
• V is the voltage across the two terminals of the resistor
• I is the current flowing through the resistor
• R is the value of resistance

For different applications, various different sizes of resistors are available. It can be 0.25Watts , 0.5Watts or 2Watts.

The following example may give you an idea about determining the proper size of resistor.

Example -

We need to determine the proper size of resistor needed, if a 10 Ohms resistor if connected to a 5 Volts DC supply.

So, here we have,

• V = 5 Volts
• R = 10 Ohms

Thus, power dissipated by the resistor is given as -

P = (V^2) / R

P = (5^2) / 10

P = 25 / 10

P = 2.5 Watts

We have got the power dissipated by the resistor now and that is 2.5 Watts !!!

So, for this condition you need a size of resistor of 2.5Watts.

But to be on the safer side, you should always choose a greater one .

A 3W or more resistor would be best suitable for this example stated.

In a similar manner, the proper size of the resistor must be determined for a typical application.

Step 4: The All in 1 Chart

Get All the formulas in one place & also the Standard Values of Resistors available in the market ...!!!

Step 5: Getting Your Value

You can make any required value of Resistance in a very easy way.

Connecting resistance in a series or parallel manner helps to create any required value of resistance.

Connecting in series adds up all the resistance values whereas, connecting them in parallel decreases the total value of resistance.

Resistor in series -

When Resistors are connected in series, then the effective resistance is given by -
R = ( R1 + R2 + R3 +......)

For example -

We have three resistors of the values - 10ohms , 100ohms and 33 ohms.

So if we connect them in series we get the effective resistance as -

R = 10 + 100 + 33

R = 143 ohms

This shows that the value of resistance has increased.

So, in this way you can make any required big value of resistance.

Resistance in Parallel -

When Resistors are connected in parallel, then the effective resistance is given by -
R = 1 / [(1/R1) + (1/R2) + (1/R3) + .....]

For example -

We have three resistors of the values - 10ohms , 5 ohms and 20 ohms.

So if we connect them in parallel we get the effective resistance as -

R = 1/[(1/10) + (1/5) + (1/20)]

R = 1/[( 0.1) + (0.2) + (0.05)]

R = 1/[0.35]

R = 2.86 Ohms

This shows that the value of resistance has decreased.

So, in this way you can make any required small value of resistance.

Step 6: Using a Resistor

Resistor has many uses in Electronics.
Some of the basic uses have been described below.

1) To glow a LED --

- Resistors are used to protect LED from high forward currents.
- The help in limiting the current flowing through the LED, protecting the led.

2) To decrease the current in a Circuit --

- When connected in a branch of any circuit, the current flowing through that branch can be controlled as per as the requirement.

3) In zener - voltage regulator circuits --

- Resistor is placed in series with a reverse-biased zener diode in-order to protect the zener diode from getting damaged by a high current.

3) In Voltage-divider circuits --

- Any voltage can be divided by using a resistor circuit as required.
- This allows us to obtain any value of voltage in a circuit using resistors.

4) As a pull-up & pull-down resistor --

- In digital circuits, any line in the circuit can be pulled-up to high level (5V) by using a pull-up resistor.
- Similarly, any line in the digital circuit can be pulled-down to low level (Ground - 0V) by using a pull-down resistor.
- Check out the images for more info.

5) As a shunt resistor --

- For current measurements, a resistor is used as a shunt resistor.
- The voltage difference across the terminals is measured and the current flowing through it is calculated.

Step 7: Finishing It

Well, that's all about the resistor...

The basics of resistor can be ended-up here.

You can also refer other instructables or the internet for more Info about resistors.

Thanks for checking out this Instructable.......!!!

Check out the other instructables too:

The Diode -

The Capacitor -

The LED -



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Yes, online calculators can be very handy while calculating the value of parallel resistance.....

The BEST calculator of all is KNOWING how to calculate in the first place! for those fortunate enough to have an old HP scientific calculator, it is very fast and simple: suppose you have three reistances in paralell, of 2, 3 and 4 Ohm.

You punch 1 ENTER 2 / 1 ENTER 3 / + 1 ENTER 4 / + 1 ENTER (EXCHANGE X AND Y) / AND THE RESULT IS 0.923 OHMS.

Even simple calculators allow to perfom inverse of a number if you press two times the operation "/" or divide by...

it goes like: 2 / / 1 = 0.5, then 3 / / 1 = .33, then 4 / / 1 = .25, then sum the 3 numbers and lastly obtain the inverse or reciprocal of that sum.


Bulk 'Carbon' resistors are rarely used these days. They had a bad habit of catching fire when overloaded. They were also quite unstable, temperature wise. They can be identified in antique circuits by their smooth cylindrical shape. They do not have the 'lumps' at each end that the modern ones do. (We used them as Temperature Sensors in Low Temp Physics labs 'back in the day'.)

1 reply

most of the circuit's do still consists of carbon film resistors..... as they are available at a cheap price....

Hi, cool video, but there is an error in the power equation:

V=I*R so I=V/R

P=IV, so P = V^2/R

4 replies

Sorted out that error.... it was the video editor's error.....thanks for checking it out..........

Looks cool. May I ask what video editor you are using? I am trying to find a simple editor (and hopefully one that is cheap).

you can try out VideoShow video editor's a free app available on the Play Store....

I am going to download that now, thanks!

"Resistors are used to protect LED from high inrush currents."

This is fundamentally incorrect.

3 replies

Basically, all leds are diodes.... and a diode conducts heavily when forward biased.......So, if such happens in case of a blows out.....
So, to protect it from blowing out....we add a resistor....which limts the current flowing through it....

We do not talk about "inrush" current for a LED or a diode.
"Inrush current" refers to a current that is initially very high and then reduces.
This applies to the initial current due to the presence of an electrolytic.

Oh...I wasn't knowing about it...... however, got that word replaced..!! thanks for the help...!!! :-)

In step 5 under the heading "Resistance in Parallel -", on the second line of your example, you say they're in series. I believe that was a mistake. And one other mistake in your example, 1/20 is not the same as 0.5. I think that should be 0.05. Correct?

1 reply

corrected...!!! thnks for the help....

The Resistors in Parallel example is incorrect. Should be:


2 replies

i got in a big hurry while making this instructable.

Thanks for the support ...!!! :-)

Thanks for this lesson.