MINI LESSON 3 – RESISTORS

GENERAL INFORMATION

· Here is an image of a resistor:

• In circuit diagrams, resistors are identified by the following symbol:
  
• Resistors are used to resist electric charge. Adding a resistor to a path increases the resistance in that path and hence lowers the current that will go through that path.

• Resistors convert electrical energy into heat.
  
  
• The purpose of a resistor is to lower the electrical power that goes to some loads.
  
  
• For example, LEDs generally need to be placed in series with a resistor to stop them from burning out due to high current.
  
  
• The resistance of a resistor is measured in ohms (Ω).

MEASURING RESISTANCE

· To measure resistance, we use an ohmmeter. Nowadays, an ohmmeter is part of a multimeter. The image below shows a typical multimeter (left) and the symbol for an ohmmeter (right).

· To measure the resistance of a resistor, we must follow these steps:

o set the multimeter dial to the ohm symbol

o disconnect the resistor from any circuit

o connect each probe to one side of the resistor (preferably using alligator clips)

o read the meter to get the resistance in ohms

CALCULATING RESISTANCE

· Most resistors have colored stripes on them. We will work with resistors that have 4 stripes on them (though some have 5 stripes).

· The stripes allow us to approximate the resistance of that resistor. To do this approximation, we need to use a resistor chart (below).

· Here are the steps to follow to calculate the resistance of a resistor:

o Find the silver or gold band. That is the 4^th band and allows you to orient the resistor correctly.

o Using the above chart, the colour of the 1^st band gives you the first number of the resistance.

o The colour of the 2^nd band gives you the second number of the resistance.

o The third band gives you the multiplier by which you multiply the first two numbers to get the resistance value.

EXAMPLE

Consider the following resistor and calculate the resistance.

STEP 1

The gold band is the 4^th band. So we need to look at it from the other direction.

STEP 2

Band #1 is green. So our first number is 5.

STEP 3

Band #2 is blue. So our second number is 6.

STEP 4

Band #3 is orange. So the multiplier is 10³ or 1000.

STEP 5

We multiply. 56 x 1000 = 56 000 ohms

The resistor’s resistance is about 56 000 ohms.

STEP 6 (EXTRA)

Recall the 4^th band is gold. Gold means that the precision of our calculated resistance is within ±5%.

5% of 56 000 is 2800

So, we expect our resistor to be 56 000 ± 2800 ohms. Or, we can also say, we expect the resistance to be between 53 200 ohms to 58 800 ohms.