[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
Lab 3: LEDs and transistors
1
Introduction
The goal of this lab is to introduce nonlinear devices based on semiconductor materials such as silicon. These
can be thought of as resistors whose resistance, rather than being a constant, depends on a voltage on or current
through the device.
For your convenience, questions and lab procedures are introduced in a unique unique color.
2
Components and Tools Required
• From Your Kit:
Breadboard
Wires
Wire Cutters and Pliers
Red LED
1N4148 diode
PN2222A transistor
Various resistors
Extra credit: LEDs of other colors
A Cat
• On The Lab Bench:
Variable DC Power Supply
Function Generator
Oscilloscope
3
3.1
Working with LEDs
Identify the Anode and Cathode
Identify the anode and cathode of the red LED in your kit using the diagram in the notes. What is diffrent about
the cathode when compared to the Anode? Remember that current goes iN the aNode and ouT the caThode.
Figure 1: Description of Anode and Cathode of LED
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[ECEN 1400]
3.2
Introduction to Digital and Analog Electronics
R. McLeod
Confirm with your power supply
Limit your power supply current to 30 mA. Verify your identification of the leads by connecting the diode to your
power supply and turning the voltage up slowly to 2-2.5 V. Be careful above 2V since the current will increase
rapidly above turn-on. Reverse the direction and repeat. The diode should light up when you have the anode
connected to the positive terminal. What happens when the positive terminal is connected to the cathode?
3.3
Confirm using your multimeter.
Your multimeter can also identify the forward bias direction of diodes. As shown in Figure 2, you use the same
terminals as when measuring voltage and resistance. The meter beeps when you have the diode forward biased,
as shown on the diagram. Try both orientations of your LED. The meter should not beep on either arrangement.
Find the test voltage in the multimeter manual (it is on the class website) and explain why the test failed. In
general, multimeter diode testers will fail with LEDs for this reason.
Now try your 1N4148 silicon diode. Confirm that the meter agrees with your identification of the anode and
cathode using the diagrams in the notes.
Figure 2: Diode test using the HP multimeter. Source: HP 34401A multimeter manual.
4
4.1
Limit the current
Find the turn on voltage
Since the resistance of the diode is very small after turn-on, if you hook an LED across a typical source voltage
(e.g. 5V) you will draw too much current and damage the LED. First, determine the turn-on voltage for your
LED, as shown in Figure 3.
Figure 3: Determine the turn-on voltage of your LED by SLOWLY increasing the forward voltage until the LED
starts to emitting a reasonable amount of light.
4.2
Limit the current
Using this voltage, calculate the resistance of a series resistor that will limit the current to 10 mA when using a 5V
source.Record the resistance you caluclate. Construct the circuit and measure the current. If it is not quite right
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[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
(within 0.1 mA), measure the voltage drop across the LED in this configuration and recalculate the resistance.
Explain why this was needed using the LED I/V relationship.
Figure 4: Circuit configuration for a current-liming resistor.
5
Transistor Controlled LED
When building logic circuits, which we will be doing in the future, providing 5 volts is not a problem. If you have
a device that can provide 5 volts and enough current, you can simply replace the 5 volt power supply in Figure 5
with your device and now you can control the LED.
Unfortunately, there are many devices that cannot provide enough current to power an LED. For such devices
we need a slightly different approach. Instead of controlling the LED directly, a device can indirectly control the
LED with a transistor. The circuit for such a scheme is shown in Figure 5. Use the current-limiting resistor R2
you found previously to keep the LED current at 10 mA.
This is an example of a circuit that needs two independent voltages with a common reference. Your Agilent
power supply can provide this. Connect the two COM terminals (black) from the +6 and +25 supplies. Adjust
one to 5V and use it for V2. Set the other supply to zero volts and use it for V1 you can then adjust V1 and
measure the current through the LED with your multimeter. Adjust your current limits to protect your circuit.
Ask your TA if you are not sure about your settings or your circuit.
Figure 5: Transistor-controlled LED circuit.
In this circuit, there is a PN2222A transistor below the LED. The symbol you see is a standard symbol for
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[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
an NPN bipolar transistor. For this class you dont have to worry about what is meant by NPN or bipolar, just
associate the symbol with a transistor. For our purposes, the transistor behaves like a type of switch. The three
leads (wires) coming out of the transistor are named, from top to bottom, collector, base and emitter. This is
shown in Figure 7.
Figure 6: Symbol and package of an NPN bipolar transistor. The package view is top-down, so the wires shown
emerge from the bottom of the device.
Before turning on the power, lets discuss what the transistor will be doing in this circuit. The main current
flow through the resistor is between the collector and emitter. As long as the voltage on the base wire is below a
threshold voltage, no current will flow between the collector and the emitter. If the voltage at the base goes above
a particular threshold, current will start to flow. The transistor thus acts like a switch. When the switch is on
or closed, collector-emitter current flows. When the switch is off or open, no current flows. Whether the switch
is opened or closed depends on the input to the base. Below a particular threshold, the switch is off/open, above
the threshold, the switch is on/closed.
You have a couple of these transistors in your lab kit, go ahead and get one of those out and be sure it is
the correct number of PN2222A. Then go ahead and wire the circuit as shown above. You should take the 1.7k
resistor from your lab kit. (It turns out that any resistor close to this value works.) Remember that you should
not turn on the power until you have everything wired, and the variable DC power supply V1 on the left should
be initially set to 0 Volts.
5.1
Investigate the circuit operation
Wait! Did you read the previous section? Yes? Good.
Slowly increase V1 until the LED turns on. Record the voltage where this happens. Measure the diode current
at several voltages V1 above and below the turn-on point and confirm the predicted behavior from the pre-lab.
Move the ammeter to measure the base current (that is, the current through R1) and again confirm the predicted
performance. Note that the base-emitter circuit is operating just like a forward-biased diode. How much current
gain (collector current over base current) are you getting just after the LED turns on? The transistor can provide
up to a factor of 100, although its not typically used with this much gain.
6
6.1
Extra Credit
Different Colors
Repeat the first steps (finding the turn-on voltage and limiting the current) with some different color LEDs.
The turn-on voltage (in joules per coulomb, youll remember) times the charge of a single electron (in coulombs)
represents the energy released (in Joules) when the electrons meet the holes at the PN junction. The majority of
this energy is released as a single photon. Since photon energy relates directly to color, the color and the turn-on
voltage of an LED are directly related by quantum physics.
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[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
Convert the energy drop of the electrons in the diode (which is the charge on a single electron times the turn-on
voltage) into photon wavelength via the equation
λ[nm] =
1.239
[V ]
VT
The origin of the equation is explained at http://en.wikipedia.org/wiki/Electronvolt. You should find
moderate accuracy in predicting the color from the voltage. In general the wavelength predicted from your voltage
measurement will be smaller (shorter) than the actual emission. Can you figure out why?
6.2
A Two LED Circuit
Construct the circuit below using two nominally identical (same part number) LEDs and the current-limiting
resistor you found previously for these LEDs at 5 V. Have your TA show you how to use the function generator
and set it up for a 5V peak (10V peak to peak) sinusoidal wave with a 2 Hz frequency. Before turning the power
on, predict what this circuit will do. You should use the concept of current division on the parallel LEDs. How
would you modify this circuit if you wanted to use two different color LEDs that had different turn-on voltages?
Figure 7: Circuit layout. Note that the LEDs are in opposite directions.
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