RESISTORS

1
RESISTORS

• Always remember
• V IR
• (V voltage in volts I current in amps R
  resistance in ohms)

2
Special types of resistors
3
Diodes

• A diode is a device which allows current to flow
  only in one direction.
• It is also known as a rectifier
• This is what a diode looks like

4
Diodes

• This is the circuit symbol for a diode
• Draw this symbol in your books
• The arrow on the symbol shows the direction the
  current can flow.

5
Diode

• If the diode is reversed it acts like a very high
  value resistor and prevents current flow.

6
Diode

• Uses of diodes
• Protecting equipment should the power be
  connected the wrong way round,
• Converting a.c. into d.c.

7
Light dependent resistor

• The LDR

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LDR

• This is a resistor whose resistance depends on
  the amount of light falling on it.

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LDR

• This is the circuit symbol for a diode
• Draw this symbol in your books
• The arrows represent light falling on the LDR

10
LDR

• When light falls on an LDR its resistance falls
  rapidly.
• In low light or the dark they act as high
  resistors.
• In bright light they offer very little resistance

Resistance of an LDR
Light level
11
LDR

• LDRs can be used to switch on street lights when
  light falls.

12
Thermistors

• Temperature-dependent resistors

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Thermistor

• This is a resistor whose resistance depends on
  the temperature.

14
Thermistors

• This is the circuit symbol for a thermistor
• Draw this symbol in your books

15
Thermisitors

• Thermistors are used in circuits which are
  designed to be controlled by temperature, such as
  a sprinkler system.

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Investigating thermistors
Temp Volts Amps Resistance oC
V A ohms
Calculate the resistance at 5 different
temperatures
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• Plot a graph of resistance against temperature
  for your thermistor.