SIMPLE EXPERIMENTS with SEMICONDUCTORS and LEDS 1 THE CHANGE IN - - PowerPoint PPT Presentation

SIMPLE EXPERIMENTS

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SEMICONDUCTORS LEDS

and with

THE CHANGE IN RESISTANCE OF Ge-CRYSTAL

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We need:

• multimeter
• slice of Ge
• an insulating underlay,
• two joiners through which we link

the Ge-crystal with the multimeter

• tweezers

• You place the slice of Ge on

the insulating underlay then you fix the joiners carefully.

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• The multimeter is switched into

resistance measurement mode and the wires are connected accordingly.

• The multimeter measurement limit is set

in the resistance measurement range of 20 MΩ-inch measuring range.

• At room temperature the instrument

indicates a 14.51 MΩ resistor.

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• When you heat the crystal with a

hairdryer the resistance value decreases from 14.51 MΩ to 8.93 MΩ.

THE CHANGE IN THE Ge-CRYSTAL RESISTANCE DUE TO TEMPERATURE

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• The slice of Ge is illuminated with a 25-

watt bulb.

• The Ge resistance decreases due to the

light, the instrument displays 5.25 MΩ resistor.

THE CHANGE IN THE RESISTANCE OF THE Ge-CRYSTAL UNDER LIGHT

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THE LED AS A CIRCUIT ELEMENT

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• Forward direction
• The anode is connected to the

positive potential,

• the cathode is connected to the

negative potential.

• The current flows through the

diode

• The LED lights

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• Reverse direction
• the anode is connected to the

negative potential,.

• the cathode is connected to the

positive potential

• in the photo we changed the

polarity of the voltage source.

• There is no current in the circuit
• The LED does not light up

THE LED WILL CONDUCT ELECTRICITY IN ONLY ONE DIRECTION

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• The LED will be connect to the

direct current power supply with the help of a long wire.

• The LED lights up only in case
• f forward direction.

• The LED rotated on the

wire draws a continuous "circle of light".

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• The LED is connected to

alternating power supply with a long wire.

• Apparently the LED lights

well.

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• THE LED ROTATED ON THE WIRE DRAWS A DASHED

LIGHT CIRCLE

On the move it is perceptible that the LED in fact lights only in the half period of the alternating voltage of forward direction.

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THE ELECTRIC FIELD CAN BE SCANNED BY THE LED

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• Devices needed:
• Slightly wet sand in a bowl
• Two metal electrodes
• LED

Its legs are bent in 2 cm

• Direct current power supply
• Voltage meter
• Rulers
• Wires

2 cm

THE EXPERIMENT

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• On the top of the sand U ≈ 30 V

direct current voltage is switched on electrodes placed in parallel in a distance of 10 cm.

• In the wet sand surface layer nearly

a homogeneous electric field is formed.

• The LED stuck in the sand lights

depending on how much and what kind of polarity voltage there is between its bent legs.

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2 cm

p

+

n

• Forward direction

polarity U ≈ 30 V 10 cm ∙ 2 cm ≈ 6 V

• The LED lights brightly.

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• n

p

+

1,4 cm

• Forward direction polarity

U ≈ 30 V 10 cm ∙ 1,4 cm ≈ 4,2 V

• The LED lights well.

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0,7 cm

• Forward direction polarity

U ≈ 30 V 10 cm ∙ 0,7 cm ≈ 2,1 V

• The LED barely lights.

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• Legs of LED are standing on

an equipotent line U ≈ 0 V

• The LED does not light up.

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• Reverse direction polarity

U < 0 V

• The LED does not light up.

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• Reverse direction polarity

U ≈ −6 V

• The LED does not light up.

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ILLUSTRATION OF THE OPERATION OF THE GRAETZ RECTIFIER CIRCUIT

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• The bar magnet

moved through the coil induces alternating voltage

• In the circuit diodes

are lit, for which the current tension is in forward direction.

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PRACTICAL APPLICATION OF GRAETZ CIRCUIT

The motor always turns in the same direction owing to rectification

Comment: We use a 3-volt motor

THE LED AS A VOLTAGE SOURCE

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• The LED terminals are switched to a

voltmeter

• The multimeter measures in the DC voltage

range of 20 V at the measuring range.

• In daytime lights the instrument shows 0.05

volts.

• The LED is illuminated by another

LED from above.

• Among the electrodes of the

illuminated LED voltage increases.

• The measured voltage by varying

the intensity of illumination (approaching and removing LEDs) changes. The LED and the solar cell work similarly.

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