Using Ohm’s Law to Build a Voltage Divider

Student Worksheet:
Step by Step Procedures

Step 1: Reading Resistor Values
Before the appropriate resistors can be chosen, their values must be determined. One way, of course, is to measure every resistor. Obviously, this is not practical. Fortunately, every resistor comes with a color-coded value printed on it.

Review the handout on Resistor Color Codes.

Determine the color codes for the following resistors. Find the resistor in the lesson kit. Measure and record the resistance. Is the value within the tolerance?

Resistor	First Band	Second Band	Multiplier	Tolerance	Highest Resistance	Lowest Resistance	Measured	Within Tolerance?	
820 ohm	

								
470 ohm	

								
1K ohm

								

Using Ohm’s Law to Build a Voltage Divider

Student Worksheet (continued)

Step 2: Understanding Breadboards
Engineers often use a tool called a Breadboard to build prototype circuits. Find a white plastic board full of holes and a box of pre-cut wires in the lesson kit. This is the breadboard.

Breadboards are made so that some of the holes are electrically connected, so that circuits can be built. Using the multimeter and wires from the kit of pre-cut wires, draw a diagram of some of the connections on the breadboard.

	

Next, draw a diagram showing how a voltage divider circuit might be built with the breadboard.

	

Using Ohm’s Law to Build a Voltage Divider

Student Worksheet (continued)

Step 3: Building a Voltage Divider
Using the diagram and the resistors in the lesson kit, build a voltage divider using the 9V battery, R1=820ohm, and R2=470ohm.

What voltage would you expect?

	

=

	
*	
	
			R2	
			
(

	
+	
)

	
V2	Vbattery		(R1+R2)	

What voltage did you measure?

	

Step 4: Building a Voltage Divider for a Desired Output
Using the voltage divider circuit, select R1 and R2 from the lesson kit to produce the following output voltages

V2	Vbattery	R1	R2	V2 calculated	V2 measured	
2.0V

					
3.0V

					
5.0V

					
7.0V

					

Using Ohm’s Law to Build a Voltage Divider

Student Worksheet (continued)

Step 5: Building a Light Emitting Diode circuit
A circuit to illuminate a Light Emitting Diode (LED) is very similar to a Voltage Divider circuit. An LED circuit replaces R1 with the LED. For an LED in this configuration, V1 will be a constant voltage, regardless of the total current. Therefore, for these circuits, V2 is also known. LED’s require a certain amount of current for optimal operation. This is called the bias current. Therefore, if V2 and bias current are known, a value for R2 can be calculated.

As the name implies, LED’s are diodes and they will only illuminate when installed with the proper polarity. For this circuit, the flatted side should point towards R2

If R1 is replaced with an LED that has a voltage V1 of 2.0V and requiring a bias current of 20mA, (0.020A) determine the following

What is the value of V2?

	
=		
-	
2.0	
V2		Vbattery		V1	

What is the value of Itotal

	
=	
0.020	
=	
	
Itotal		I1		I2	

What is the resistor required for proper bias.

	
=	
	
/	
0.020	
R2		V2		I2	

Try a few resistors near the calculated value to find an optimal LED brightness.  Measure V2 and compute the bias current. 

V2 measured	Vbattery	R2	I2 calculated	

				

				

				

				
Using Ohm’s Law to Build a Voltage Divider

Student Worksheet (continued)

Which resistor is the best choice? Why? What happens if the LED is installed backwards?