RESISTORS Always remember…… V = IR (V= voltage in volts; I = current in amps; R = resistance in ohms)

Special types of resistors

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

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.

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

Diode  Uses of diodes 1. Protecting equipment should the power be connected the wrong way round, 2. Converting a.c. into d.c.

Light dependent resistor The LDR

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

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

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 Light level Resistance of an LDR

LDR  LDR’s can be used to switch on street lights when light falls.

Thermistors Temperature-dependent resistors

Thermistor  This is a resistor whose resistance depends on the temperature.

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

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

Investigating thermistors Temp Volts Amps Resistance o C V A ohms Calculate the resistance at 5 different temperatures

 Plot a graph of resistance against temperature for your thermistor.