CAPACITORS AND TRANSISTORS:– Pre-lab Questions
1. How are capacitors able to store energy?
2. In Experiment 2, how are the capacitors in the circuit connected, series or parallel? ______________________
3. In Experiment 3, how are the capacitors in the circuit connected, series or parallel? _____________________
4. What is the minimum potential difference (or voltage drop) needed to generate sufficient base -current flow in
the bipolar junction transistors used this lab activity?
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INTRODUCTION TO CAPACITORS
Capacitors: Capacitors are electrical components that can store electrical energy for periods of time, by causing a
charge separation between the two plates. This creates a potential difference between the plates. When this occurs, the
capacitor is said to be charged. A capacitor is charged by having a current flow through it. E v e n t u a l l y, wh e n t h e
v o l t a g e b e c o m es e q u a l t o t h a t o f t h e e n e rg y s o u r c e , D . C c u rr en t wi l l n o l o n g e r f l o w t h r o u g h t h e
c a p a c i t o r . T h e c a p a c it o r can be discharged by letting a current flow in the opposite direction out of the capacitor.
To discharge a capacitor rapidly and/or completely, briefly connect both sides with a wire.
Capacitors are described by their capacity for holding electric charge, called their Capacitance. Capacitance is
expressed in farads (F), or more commonly in microfarads (F = 10-6 F ) or picofarads (pF, = 10-9 F). Almost all
capacitors used in electronics vary from 1pF to 1,000F.
Although there are many different types of capacitors, their basic construction is the same. The leads connect to two or
more metal plates that are separated by high resistance materials called dielectrics. Two types used in this and future
labs are electrolytic capacitors and disc capacitors. Electrolytic capacitors are cylindrical and have “+” and “–” polarity
(orientation) markings. The lead marked “+” should always be connected to a higher voltage than the “–” lead (all of your
Wiring Diagrams account for this). Disc capacitors don’t have polarity markings (they can be hooked up either way). Their
value is usually marked in pF with a 3 digit code similar to the stripes used on resistors. The first 2 digits are the first 2
digits of the capacitor’s value and the third digit tells the power of 10 to multiply by (or the number of zeros to add). For
example, the 0.005F (5,000pF) and .047F (47,000pF) disc capacitors are marked 502 and 473, respectively.
Capacitors have symbols as follows:
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EXPERIMENT #1: MAKE YOUR OWN BATTERY
Parts Needed
breadboard
switch
a 9V battery
one 470Ωresistor
one LED
220Fcapacitor
short and long wires
Wiring Checklist
Insert switch into f3 and f4.
Insert red battery wire into j3 and black battery wire (ground) into any negative (–).
Insert the 470 Ωresistor into i4 and i11.
Insert an LED into h11 and h15 (“flat” side goes into h15).
Insert the positive side of the 220F capacitor into g15.
Insert the negative side of the 220F capacitor into g23.
Insert a wire from f23 to the same negative row as ground.
Q1: This circuit contains a switch, resistor, LED and capacitor in series with the 9V battery. Draw the circuit diagram for this
circuit:
Q2: Follow conventional current from the positive terminal of the battery, through the circuit to the negative terminal. Which
way does the current flow, clockwise or counterclockwise (circle correct answer)?
Q3: Press the switch. You should see the LED light up briefly. Based on your knowledge of capacitors, explain why it lit up
and then went out while the switch was pressed:
If a capacitor acts like a battery we should be able to use it alone to light up the bulb. Remove the battery.
Q4: In what hole should you move the wire end that is presently in the negative row so that it completes the circuit_______?
Q5: Follow conventional current from the positive terminal of the capacitor, through the circuit to the negative terminal.
Which way does the current flow, clockwise or counterclockwise (circle correct answer)?
If you press the switch now, it will not light the LED. This is current will only pass through from the positive side (long wire) to
the negative (flat) side. Turn the LED so that it is in the proper orientation for counterclockwise current.
Q6: Press the switch. You should see the LED light up briefly. Based on your knowledge of capacitors, explain why it lit up
and then went out while the switch was pressed:
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Q7: Draw the circuit diagram for the discharging capacitor.
EXPERIMENT #2 - CAPACITORS IN SERIES
Parts Needed
breadboard
switch
a 9V battery
one 3.3.kΩresistor
one 10kΩresistor
one LED
one 100Fcapacitor
one 10Fcapacitor
short and long wires
Q1: Draw the circuit diagram for this circuit:
Connect the circuit according to your circuit diagram and the wiring diagram and press the switch to determine if the
circuit is connected correctly.
Q2: What wiring feature makes these capacitors connected in series?
Q3: The equivalent capacitance (Ceq) of capacitors in series is calculated in the same manner as equivalent resistance
for resistors in parallel. Should Ceq be greater or less than 10μF?
Q4: Calculate Ceq:
-
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EXPERIMENT #3: - CAPACITORS IN PARALLEL
Note: Parts needed are same as for previous experiment.
Connect the circuit according to the circuit
diagram and the wiring diagram and press the
switch.
Q1: What wiring feature makes these capacitors
connected in parallel?
Q2: Calculate the equivalent capacitance for the
circuit, noting that the procedure will be the same
as that of calculating the equivalent resistance of
resistors in parallel:
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INTRODUCTION TO TRANSISTORS
The Transistor: The transistor was first developed in 1949 at Bell Telephone Laboratories, the name being derived from
“transfer resistor”. It has since transformed the world. There are many different families of transistors but we will only
use the type called the NPN Bipolar Junction Transistor or BJT. It is made from the semiconductor silicon. It has three
connection points, called the emitter, base, and collector.
In our water pipe analogy the BJT may be thought of as the lever pivot shown here:
base current
check valve
Notice that it includes a check valve that is connected to a lever arm. A small amount of “base current” pushes on the check
valve which turns and opens the lever arm. But before this base current can start to flow though it must have enough water
pressure to overcome the spring in the check valve (between 0.6 and 0.7V). If the base pipe is much smaller than the
collector and emitter pipes, then a small base current IB flowing in will cause a large collector current IC to flow in,
these will combine
and exit the device as emitter current IE.
In transistors the emitter, base, and collector are different regions of permanent electrical charge, producing the effects
described above for the lever pivot. The properties and uses of transistors may seem confusing at first but will become
clear as you proceed through the experiments. All but one of the remaining experiments will use the transistor, so its
importance to electronics should be apparent.
A key advantage of semiconductors is that several transistors can be manufactured on a single piece of silicon. This led
to the development of Integrated Circuit (IC) technology, in which careful control of complex manufacturing processes has
enabled entire circuits consisting of transistors, diodes, resistors, and capacitors to be constructed on a silicon base. Some
ICs used in computers now have more than a million transistors on them. Spectacular improvements in cost, size, and
reliability have been achieved as a result.
The schematic symbol for a transistor is shown below:
TRANSISTOR
Symbol for NPN TRANSISTOR
COLLECTOR
BASE
Flat
EMITTER
Note the arrow in the emitter, this indicates which direction the current will flow through the device.
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EXPERIMENT #4: THE ELECTRONIC SWITCH
Part One – Build a Series Circuit
Parts Needed
breadboard
switch
a 9V battery
one 10kΩresistor
one 1kΩresistor
one LED
one Bipolar Junction Transistor
short and long wires
Build a series circuit with the following elements: 9V battery, 1kΩ resistor, and LED. Make sure the LED lights to show
that you were successful.
Q1: Draw the circuit diagram for your circuit below:
Now add the transistor in series with the other elements of the circuit. Note it has 3 pins. If turned so the flat side is facing
outwards (toward you), the 3 pins are, from left to right, emitter (E), base (B), collector (C). Connect the transistor using
the two outside pins (C and E). The emitter (E) should be closer to ground than the collector (C).
Q2: Draw the circuit diagram for this circuit, below.
Q3: Why doesn’t the LED light this time when the circuit is completed?
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Part Two – Using the Transistor as a Switch
Q4: There is a continuous connection between the positive and negative terminals of the battery, if one ignores the branch
with the switch and the 10KΩ resistor. Based on your last circuit, do you think LED will light without pressing the switch?
Why or why not?
Build the circuit as shown above and press the switch. Now a small amount of current passes through the larger 10KΩ
resistor to the base (middle pin) of the transistor. This is referred to as the base current.
Q5: The base current does not flow through the LED, instead passing through the transistor to the emitter (E) and then to
ground. Why then, does the LED light?
EXPERIMENT #5: MOVIE THEATER LIGHTING
Parts Needed
breadboard
switch
a 9V battery
one 3.3.kΩresistor
one 10kΩresistor
one 1kΩresistor
one LED
one 100Fcapacitor
one 10Fcapacitor
short and long wires
Connect the circuit according to the schematic and wiring diagram. Press the switch and hold it for a second or two.
Then release the switch
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Q1: Why does it take time less time for the LED to turn on than to turn off?
Q2: Replace the 3.3kΩresistor with a 100kΩresistor. Before pressing the switch, fully discharge the capacitor with a
short piece of wire. What happened to the charge time? What happened to the brightness?
In the next experiment, we will investigate whether using a transistor along with a capacitor allows us to obtain both a long
charge time AND a bright LED. For this next experiment, you will work in pairs.
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EXPERIMENT #6: BETTER MOVIE THEATER LIGHTING
Parts Needed
breadboard
switch
a 9V battery
one 3.3.kΩresistor
one 100kΩresistor
one 470Ωresistor
one 100Fcapacitor
one LED
one transistor
short and long wires
Find a partner for this part of the experiment. One of you will keep the circuit from the previous experiment. The other will
connect the circuit shown above. Make sure the capacitor is fully discharged in both circuits. Note that both circuits
charge through the 100k Ω resistor.
Q1: Compare the 2 circuits. Do they both take about the same time for the LED to light? Are the LEDs both the same
brightness?
When you first press the switch, all of the current flowing through the 100kΩresistor goes to charge up the capacitor, the
transistor and LED will be off. As the capacitor charges, the voltage drop across it increases, in turn causing a voltage drop
between base and collector. When this latter voltage drop becomes greater than the threshold voltage, the transistor will
first turn on and close the main circuit loop. The LED will light. As the capacitor voltage continues to rise the current flow
through the 470 Ωresistor and into the transistor base will increase. The current through the LED will then rise rapidly
due to the transistor’s current gain.
When the switch is released the capacitor will discharge through the 470Ωresistor and the transistor base, the LED will
dim as this discharge current decreases. When the voltage difference drops below 0.7V the transistor will turn off.
By adding a transistor you can use a large resistor for a slow charge time and still have a bright LED.
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