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 1. 2. Uses of diodes Protecting equipment should the power be connected the wrong way round, 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 Resistance of an LDR Light level 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 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 oC Volts V Amps A Resistance ohms Calculate the resistance at 5 different temperatures Plot a graph of resistance against temperature for your thermistor.