# GCSE Science/Three useful components

This page looks at three components that are used in electrical circuits. The three components in question are: The diode, the thermistor, and the light dependent resistor. You will see how the components work and how they are used.

## The Diode A diode is a device that allows current to flow in one direction but not in the reverse direction. Its circuit symbol consists of a triangle pointing in the direction that current is allowed to flow with a line at the point, inside a circle. Diodes are useful for stopping current flowing in the "wrong" direction.

Above is a current voltage graph for a typical diode. Note that the diode needs a small voltage (about 0.6V) to work. Below 0.6V in the forward direction very little current flows. At or above 0.6V the resistance drops dramatically and a huge current can flow. In the reverse direction, virtually no current flows no matter how large the voltage is. (Eventually, when the voltage gets high enough, the diode "breaks down" and current is able to flow in the reverse direction. For normal diodes however this "break down voltage" is very high and can be ignored. A diode that is connected in reverse to use the break down effect is called a Zener diode. It is used to provide voltage control in power supplies (called the Zener voltage).)

Diodes are useful for turning AC current into DC current (AC/DC converters are called rectifiers). In AC the electricity flows back and forth in cycles. Mains electricity is AC. It cycles 50 times every second. This is fine for something like a lightbulb, where the bulb does not care in which direction the electricity flows. Some devices however can only work off DC. A toy electric train for example would not go with AC. The current would be saying "go forwards", "go backwards" over and over again 50 times a second! With a diode in the circuit the negative part of the cycle is stopped.

The graph on the left shows AC current. Note that the current cycles positive, negative, positive, and so on. The electricity is flowing to and fro. Putting the current through a diode has the result seen on the right. The positive part of the cycle in unaffected, but the negative is wiped out. The current goes positive, zero, positive.

## The Thermistor

A thermistor is a just a resistor whose resistance depends on temperature. Most thermistors have lower resistance when the temperature is high. They are used in circuits which are temperature dependent. For example, fire alarms, the temperature gets too high; the alarm sounds.

## The LDR

A light dependent resistor is a resistor whose resistance depends on the amount of light shining on it. Its resistance gets less the more light shines on it. It can be used in circuits that are light dependent. For example, switching circuits for street lights. The ambient light gets too low; the lamp turns on. It is also known as a photoresistor from photo, the Latin for light.

## Questions

 Summary A Diode allows electricity to flow in one direction, but not the otherA thermistor is a resistor that has a lower resistance the higher the temperature A Light dependent resistor has a lower resistance the higher the light level shining on it