Download Basic Circuits

Experiment 1
Basic Circuits
Equipment in the lab
123L Lab Manual 8/06
1
Low voltage power supply
2
2 light bulbs w/ attached wires
3
Digital multimeter
4
Assorted wire leads
Experiment #1
Basic Circuits
Laboratory Objectives:
To understand series and parallel circuit connections and how to connect meters for
reading current and voltage.
Introduction:
In a series circuit there is only one path for the flow of charge, or current, as it leaves the power
supply and travels through the circuit. Therefore all of the current from the power supply travels
through each circuit component (lamps, carbon resistors, etc.).
In a parallel circuit there are two (or more) paths for the flow of charge. When the current reaches
the branch in the circuit, it splits. Some current travels through one branch and the remaining
travels through the other branch.
An ammeter is used to measure the amount of current (in amps or milliamps) flowing in a circuit at
the point of insertion of the meter. An ammeter is always connected in series with the circuit
component you are measuring.
A voltmeter is used to measure potential energy difference, voltage (in volts or millivolts) across a
circuit component. A voltmeter is always connected in parallel with the circuit component you
are measuring.
Procedure:
As you change circuits throughout this experiment, physically follow your connections to verify the
current direction is correct. Part of the purpose of this lab is learning to check circuits.
If the bulbs don’t light up or the meter doesn’t display a non-zero value, visually follow the circuit
path. Try to determine what could be plugged into the circuit incorrectly.
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Experiment #1
Basic Circuits
PART A. SERIES CIRCUIT
Schematic diagram
Real circuit
There are 3 types of ends on the leads you will be using this quarter to connect circuits:
This is called a banana end. Use this type
to plug into meters, power supplies, etc.
This is called an alligator end. Use this
type to clamp onto a post or wire end.
This is called a spade end. Use this type
to clamp under a binding post on power
supplies, etc.
1. Using 2 bulbs, the power supply, and 3 leads, connect a series circuit. Connect the leads going into the
power supply to the bottom DC terminals (NOT the AC). Turn on the power supply and turn the
voltage adjust knob until the lights glow. Draw both a schematic diagram and a real circuit sketch.
2. Unscrew one of the bulbs from the socket.
QUESTION: What happens?
3. Reconnect the bulb and turn off the power supply.
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Experiment #1
Basic Circuits
CONNECTING AN AMMETER
An ammeter is always connected in series with the circuit component you are measuring.
Schematic diagram
Real circuit
Notice how the leads are connected into the ammeter—
the negative side of the circuit ALWAYS into common
[COM] and the positive side this time into 10A. The
units will be amps, A, in this connection.
4. To prepare it for reading current--the rotary switch is
moved to the setting for DC current.
5. Connect an ammeter into your circuit before both
resistors to determine the current through the bulb.
Turn on the power supply.
6. Record the 3 current values (before the resistors, between the resistors, and after the resistors)
obtained by the ammeter. (never forget units).
QUESTION: Does it matter which bulb you measure?
7. Turn off the power supply, remove the ammeter and reconnect the circuit so the bulbs glow again
when the power is on.
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Experiment #1
Basic Circuits
CONNECTING AN VOLTMETER
A voltmeter is always connected in parallel with the circuit component you are measuring. (2 leads may be
connected at the same point in a circuit, as shown with the voltmeter in the sketch below.)
Schematic diagram
Real circuit
Notice how the voltmeter leads are connected to the meter
(again, the negative side of the circuit ALWAYS into COM,
the positive side into V, voltage, for this measurement).
8. The rotary switch is moved to the setting for DC
voltage.
9. Connect a voltmeter across your circuit to read the
voltage drop across one of the bulbs. Record this
voltage.
10. Connect the voltmeter and record the voltage across
the other bulb.
11. Connect the voltmeter
across both bulbs as
shown.
Record the voltage.
(As always include units.)
12. Turn off the power supply and dismantle your circuit.
QUESTION: How does this voltage drop across both bulbs relate to the voltage across each? (Be specific)
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Experiment #1
Basic Circuits
PART B. PARALLEL CIRCUITS
In a parallel circuit there is more than one path for the current to take. The current splits, part going one
path, part going through the other path.
Schematic diagram
Real circuit
Leads can be “stacked” to allow several leads
to connect to the same point within a circuit:
1. Using 2 bulbs, the power supply, and 4 leads, connect a parallel circuit. Connect the leads going
into the power supply to the bottom DC terminals (NOT the AC).
2. Turn on the power supply and turn the voltage knob until the lights glow.
3. Unscrew one of the bulbs from the socket.
QUESTION: What happens?
(For the following instructions turn off the power supply each time before you reconnect the circuit.)
4. Connect an ammeter in series between the power supply and
one of the bulbs to read the current flowing into that bulb.
(Again take care in how the leads are connected to the
ammeter and the setting on the meter to prepare it for
reading current.) Record the current through the first bulb.
Real circuit
5. Draw a schematic diagram for the circuit to the right.
Real circuit
6. Reconnect the ammeter so that it reads the current through
only the other bulb. Check your circuit for accuracy before
turning on the power supply. Record the current through
the second bulb.
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Experiment #1
Basic Circuits
7. Reconnect the ammeter so that it reads the current going into BOTH bulbs. Record the current
through both bulbs.
QUESTION: How does the current through both bulbs compare to the current through only one bulb
(be specific)?
8. Draw a schematic diagram for the circuit where the current is going into BOTH bulbs.
9. Turn off the power supply, remove the ammeter and reconnect the circuit so the bulbs glow again
when the power is on.
10. Connect a voltmeter across the combination of the two bulbs.
(Again notice how the voltmeter leads are connected to the meter
and the appropriate setting on the meter.)
Record the voltage drop across both bulbs.
Real circuit
11. Draw a schematic diagram for the circuit to the right.
12. Connect the voltmeter across just one bulb.
Record the voltage drop across one bulb.
13. Draw a schematic diagram for voltmeter across just one bulb.
QUESTION: How does the voltage drop across the 2 bulbs in parallel compare to the voltage drop
across only one (be specific)?
14. Dismantle your circuit.
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