Exercise 1: LED calculations and Power Connections

 

Equipment Provided

 

1.  Breadboard

2.  Pre Cut Wire

3.  Pasco SF-9584A Low Voltage Power Supply

4.  Fluke 73 Multimeter

5.  2 LED, different colors

6. Resistors as needed (based on your calculations)

 

Set Up

 

1.  Make the power supply connections as demonstrated last class (not the ones we have

     been using with batteries) - we will be using these for most if not all labs

2.  Set up the low voltage AC/DC power supply (demonstrated in class) - do not plug in and

     turn on until told to do so

      a.  Black cable goes in the black (-) slot on the left  of the power supply, red cable

           (+) goes into the adjacent red slot on the power supply

      b.  Place the other end of the red cable in the red slot on the breadboard.

      c.  Place the other end of the black cable in the green (ground) slot on the breadboard.

      d.  Locate the voltage adjustment know. When told to do so, set this to 6 volts DC (on the

           left)

      e.  When told to do so, turn on by pressing the red switch on the right

3. Set the Fluke 73 multimeter to DC volts (second V with a dottet line under a solid line) - the

    V with the  ~ symbol is for AC - for a later lab

 

Calculations

 

The requirement is to use the two LEDs in series.

Calculate the required resistances

Set up the circuit

 

Turn in

 

    Calculations

 

    Circuit diagram using the software provided

    Note: You do not need to submit this lab in the typical format (hypothesis, etc) - just submit

    the above 2 items and (see below)

 

Demonstrate the finished product

Cathode is negative, anode is positive

 

n When wiring an LED you have three options

 

       ð wire each LED single with its own resistor

       ð wire multiple LED's in Series with a single resistor

       ð wire multiple LED's in Parallel with a single resistor

 

n Example Calculations

 

       ð Assumptions

 

               Ø A supply voltage of 5V

               Ø A single bright blue LED

               Ø The LED has a voltage drop of 3.3V
               Ø The LED should run at 20mA

 

       ð Calculations

 

               Ø Subtract the LED voltage drop from the supply voltage: 5 = 3.3 = 1.7 V

               Ø Divide this voltage by the LED current (V = IR): 1.7 V/.020A = 85 ohm

 

               Ø Resistance required = 85 ohm

               Ø Next highest standard resistor is 100 Ohm (within 10%)

       ð Wiring

 

               Ø Find the long leg of the LED which is positive and attach the proper

                      resistor to it.

               Ø Connect the ground to the short negative lead.

 

n Rule of thumb values to use for calculating unknown LED's

 

       ð 20mA for all colors
       ð Blue, White, Green, Pink, Purple, 3.3V voltage drop.
       ð Yellow, Red, Orange, 2.0V voltage drop.