Determining Unknown Resistors Values

You might have often come across Resistors and wondered, tried to figure out its value and asked yourself “what is the value of this resistor?” You become frustrated to determining unknown resistors’ values.

Perhaps on a Printed Circuit Boards (PCBs), you might have seen a resistor soldered and with its value written on the board.  You tried to master the colours of that resistor so that anywhere you see it next time you recall its value. But, sorry, the sad news is some of them aren’t exactly the values written on the board.

In this article, you will get the answers and solutions to those problems. Next time you don’t need to ask or get yourself worried trying to know the value of the resistor which is only painted in sorts of colours!

I’ll teach you in two methods how to determine unknown resistors’ values. The first is by using a DIGITAL MULTI-METER and secondly – COLOUR CODING CALCULATIONS

USING DIGITAL MULTI – METER

Before we start determining unknown resistors’ values using this method, we make sure that the resistor isn’t connected to a circuit. This is because in most cases, a resistor’s value varies when connected to a circuit due to some effects of other components in the network. Sometimes they result to be in parallel configuration and thus you may not get the real value. Alright, let’s take the steps below to measure the resistance.

Step1

Turn your meter selector to the Resistor Testing Range. In this range you will notice some figures with or without an alphabet as its coefficient. Something like: “200, 2K, 200K, 2M, 20M,” etc. The “200” means resistance value from 200Ω and below, “2K” 2KΩ (2 Kilo-Ohms) and below, “2M” 2MΩ (2 Mega-Ohms) and below, etc.

Step2

Connect the meter’s testing probes i.e. the cables in this form – Black probe to “COM” and the Red probe to “VΩ” sockets respectively.

Step3

Touch or connect the two probes (red and black) together first. This trick is to measure the resistance of the cables or testing probes (the red and black cables). The value you obtained will be subtracted from the value of the last step ( step 5).

Step4

Use the probes with the resistor to be tested on an insulated and dry surface (not held by you). Meanwhile, you can hold it as shown in the picture below. This is because some low resistance may appear from your fingers. Connect the black probe to one of the two terminals of the resistor and the red to the other terminal.

Step5

Read the value displayed on the screen. Subtract the value you obtained in (step3) above from this result. The answer you get is the resistance.

COLOUR CODING CALCULATION

This is my favourite method of determining unknown resistors’ values. This method is a straight forward method. This is because you don’t need to disconnect the resistor from the circuit board if it was on the board. But, you will only look at the colours, do some simple mathematics and get the exact value!

Once you get conversant with the colour code of resistors, this will be your simple and effective way of determining unknown resistors’ values.

Let’s briefly learn about the colour codes table (the table is below or skip to it HERE. There are eleven (11) colours use in resistor colour coding. On the table below, black comes first followed by brown, red, orange, yellow, green, blue, violet, grey, white, gold and lastly, silver. They are in order of “1 – 11”

There are also five (5) columns on the table i.e. the Colour. First Band, Second Band, Third Band, and Tolerance. The first band is the first colour on the resistor (note that these colours are not the gold or silver), second band is the second colour on the resistor. Third band is the third colour after the second colour on the resistor.

READING THE COLOURS ON THE RESISTOR AND USING THE COLOUR CODES TABLE

1. Don’t start from the Gold or Silver colour
2. The digit for the first colour on the resistor is found from the colour codes table under the first band column of that colour. E.g. if the first colour on the resistor is Orange, we write “3”
3. The digit for the second colour on the resistor is the number of zeros under the second band of that colour if the colour is not the same as the first band colour. E.g. if the second colour on the resistor is Yellow, we write 0000 after the first digit of the first band
4. If both the 1st and 2nd band colours are the same, the first digit is written for both of them i.e. in two places. For example if 1st and 2nd band colours of the resistor are green, we will write “55” for the first and second band digits.
5. The third band colour is the multiplier of the first two band digits. For fast calculations, just take the number of zeros under second band of the third colour and write them after the 1st and 2nd digits. For example if the 3rd colour band of the resistor is red, we will write 00 after the digits gotten from the 1st and 2nd band colours. If the third band colour is Gold, we multiply the 1st and 2nd digit by 0.1 or 0.01 if the band is Silver.
6. The final value obtained is the resistance in Ohms (Ω). Divide the value by 1000 if it is more than 3 digits for your answer in Kilo Ohms (KΩ).
7. The fourth band colour is the tolerance.

Resistor Colour Codes Table

Example 1

A resistor below has BROWN colour first, BLACK second, YELLOW third, and SILVER fourth, what is its value?

We then use the logic

1. On the resistor colour code table under the column FIRST BAND row of BROWN = 1, we then write or note 1 down.
2. Then black is the second colour we then look on the column SECOND BAND row of BLACK = 0. We then write 0 after the 1, we will have two digits now i.e. “10”
3. Then the third colour which is YELLOW, we then look on the column THIRD BAND (the multiplier) =X10⁴ (the “X” stands for the digits gotten from the first and second band i.e. “10”). Therefore we have 10 times the multiples of 10 in 4 places 10(10×10×10×10)

=10(10,000)

=100,000Ω let’s convert it into Kilo Ohms (KΩ) by dividing it by 1000

=100KΩ (100Kilo-Ohms)

Note that, the value of the third colour on a resistor is just the number of zeros under the SECOND BAND of the third colour of the resistor. As in this case YELLOW under second band has 4 zeros (0000).

The shortcut of this method

As stated earlier – write down the digits of the first and the second band colours followed by the zeros of the third colour under its second band. Then, divide by 1000 (if the final figure is up to a thousand i.e. four digits) for your answer in KΩ.

In the case of the resistor above;

1. 1st digit (Brown under first band) is 1,
2. 2nd digit (Black under second band) is 0, we then write “10”
3. 3rd values (Yellow under second band) is 0000

Finally we have 100000Ω when divided by 1000 we obtain 100KΩ

Example 2

What is the value of the Resistor below (BROWN, BLACK, BROWN, and SILVER)

1. The 1st digit (brown under first band) is 1,
2. 2nd digit (black under second band) is 0, we now write “10”
3. The 3rd digit (brown under second band) is 0.

Finally, we now have 100Ω which its value.

If the third band colour of the above resistor was red. We would write 10 and followed by 00. (Red under second band) and our result would be 1000Ω which is equal to 1KΩ

Example 3

What is the value of a resistor with these colour codes RED, RED, ORANGE, and SILVER

1. The 1st and 2nd band colours of the resistor above are the same (RED, RED). Therefore we will write the digit of Red under first band in two places for them. The digit of Red under first band is 2, therefore we will write the 2 in two places i.e. 22.
2. Orange under second band (the zeros) is 000.

Finally we have 22000Ω =22KΩ (divided by 1000)

READ ALSO THESE

Resistors and Types of Resistors / Resistors in Series and Parallel / Ohm’s Law and its Limitations / How to Measure Resistor’s Value using Multimeter / How to Read Resistor Colour Codes