CurrentVoltage Characteristics

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Department of Physics
Current-Voltage Characteristics
Lei wei-hua
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Contents

• Objectives
• Pre-lab Questions
• Introduction Apparatus
• Procedure Experiment
• Problems

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Objectives

• Study the current-voltage characteristics of
  different electrical devices

• Learn how to use the voltmeter and ammeter

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Pre-lab Questions

• Sketch the I-V characteristics of resistor, diode
  and transistor.
• What is Ohms law?
• What are ohmic and nonohmic devices?
• What are voltmeter and ammeter?

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Pre-lab Questions

• Sketch the I-V characteristics of resistor, diode
  and transistor.

• The I-V curve of resistor is a straight line. The
  slope is 1/R.
• The I-V curves of diode and transistor are
  nonlinear and asymmetric.

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Pre-lab Questions

• What is Ohms law?

• The resistance remains constant over a wide range
  of applied voltages or currents.

Ohm, a high school teacher in Cologne and
later a professor at Munich. Ohm's most important
discovery (of 1826) now bears his name Ohm's Law
describes the relationship between voltage,
current, and resistance. (The unit of electrical
resistance, the ohm, is named after him.).
Ohm's importance was not recognized through most
of his lifetime, but in 1852 he became professor
of physics at the University of Munich.
GERG SIMON OHM (1787-1854)
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Pre-lab Questions

• What are ohmic and nonohmic decvies?

• Materials that obey Ohms law are said to be
  ohmic, e.g.. resistor.
• Materials whose resistance changes with voltage
  or current are nonohmic, e.g., diode and
  transistor.

resistor
diode
transistor
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Pre-lab Questions

• What are voltmeter and ammeter?

voltmeter
ammeter

• Voltmeters measure the electrical potential
  difference between two points in an electric
  circuit.
• Ammeters measure the electrical current through
  two points in an electric circuit.

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Introduction Apparatus

• The Diode

Diode is a typical nonlinear device, it is
constructed simply by joining P type and N type
silicon
At the junction, free electrons from the N-type
material fill holes from the P-type material.
This creates an insulating layer in the middle of
the diode called the depletion zone.
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Introduction Apparatus

• How do diodes work

When the negative end of the circuit is hooked up
to the N-type layer and the positive end is
hooked up to P-type layer, electrons and holes
start moving and the depletion zone disappears.
When the positive end of the circuit is hooked up
to the N-type layer and the negative end is
hooked up to the P-type layer, free electrons
collect on one end of the diode and holes collect
on the other. The depletion zone gets bigger.
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Introduction Apparatus

• The Transistor

A transistor is another commonly used nonohmic
device. Comparing with diode, it adds a third
Silicon part to the diode. These parts are called
the Emitter, the Base, and the Collector.
PNP transistors and NPN transistors
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Introduction Apparatus

• How do transistors work

The illustration shows pipework with three
openings B, C, and E. The reservoir of water at C
is the supply voltage which is prevented from
getting though to E by a plunger. If water is
poured into B, it pushes up the plunger letting
lots of water flow from C to E. If even more
water is poured into B, the plunger moves higher,
and the flow of water from C to E increases.
Therefore, a small input current of electricity
to the Base leads to a large flow of electricity
from the Collector to the Emitter.
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Introduction Apparatus

• The I-V characteristics test box

Circuit 1
Resistor Diode
Circuit 2
Transistor
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Introduction Apparatus

• The I-V characteristics test box (back view)

Circuit 1
Circuit 2
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Introduction Apparatus

• The meters

microammeter
voltmeter
milliammeter
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Procedure Experiment

• CAUTION
• Always turn off the power of the box before
  connecting a circuit.
• Always begin by turning the Rw knob to the
  counter-clockwise stop so that voltage begins at
  0V.
• Measure the Resistance of Resistor
• Measure the I-V Characteristics of Diode
• Measure the I-V Characteristics of Transistor
  (Optional)

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Procedure Experiment

• Measure the Resistance of Resistor (Circuit 1)

1. Setting
Circuit 1 left panel

• Set K2 to left (Resistor).
• Connect the voltmeter and milliammeter to the
  test box. Turn the knob on the voltmeter to the
  2.5V scale, and the milliammer to 30mA.

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Procedure Experiment

• Measure the Resistance of Resistor (Circuit 1)

2. Measure the resistance use Circuit A
Circuit A K1 to left

• Turn the Rw knob to change voltage. Do 10
  measurements

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Procedure Experiment

• Measure the Resistance of Resistor (Circuit 1)

2. Measure the resistance use Circuit B
Circuit B K1 to right

• Turn the Rw knob and do 10 measurements

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Procedure Experiment

• Measure the I-V Characteristics of Diode (Circuit
  1)

1. Setting
Circuit 1 right panel

• Set K2 to right (Diode).

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Procedure Experiment

• Measure the I-V Characteristics of Diode (Circuit
  1)

2. Measurement (Forward-biased)
Forward-bias K3 to right

• Turn the knob on the voltmeter to the 1V scale,
  and the milliammeter to 15mA.Turn on the power of
  the test box. Turn the Rw knob to change
  voltage. Record 10 (V, I) pairs.

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Procedure Experiment

• Measure the I-V Characteristics of Diode (Circuit
  1)

2. Measurement (Reverse-biased)
Reverse-bias K3 to left

• Set K3 to left (Reverse-bias). Turn the knob on
  the voltmeter to the 5V scale, and the
  milliammeter to 7.5mA, and repeat the above
  instructions.

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Procedure Experiment

• Measure the I-V Characteristics of Transistor
  (Circuit 2)

1. Setting
Circuit 2

• Connect the voltmeter, milliammeter and
  micro-ammeter to the test box.
• Turn the knob on the voltmeter to the 1V scale,
  the milliammeter to 7.5mA and the microammeter to
  100uA.

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Procedure Experiment

• Measure the I-V Characteristics of Transistor
  (Circuit 2)

2. Measurements

• Adjust the knob Rw1 to set the base current to
  40uA. Turn the Rw2 knob to change collector
  voltage. Record 10 (Vce, Ic) pairs.
• Set the base current to 60uA, and repeat the
  above instructions.

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Problems

• How to determine the anode and cathode of a diode
  by a multimeter?
• Does the Ohms law hold for the diode and
  transistor? Does the diode conduct equally when
  the current flows in the forward or the reverse
  directions?

End