P3 - Cicuit Devices

Question 1

What is the special characteristic r a diode?

Answer 1

It only allows current to flow in one direction

Question 2

Give one use for a diode?

Answer 2

To convert an alternating supply (a.c.) to a direct supply (d.c.)
Or
In a radio receiver.

Question 3

What does LDR stand for?

Answer 3

Light Dependant Resistor

Question 4

Explain the characteristics of a LDR

Answer 4

As the amount of ambient light increases resistance of the LDR decreases.

Question 5

Where are LDRs often used?

Answer 5

In the photocell of a street lamppost

Question 6

What is a Thermistor?

Answer 6

A temperature dependant resistor, it’s resistance increases as the temperature drops

Question 7

Where are thermistors used?

Answer 7

Electronic thermostats, e.g. irons

Question 8

LDRs and Thermistors can be used in sensing circuits describe a sensing circuit that will increase a fan speed as the room gets hotter and explain why this happens

Answer 8

A thermistor is in line with a fixed value resistor, a fan is connected in parallel to the fixed value resistor, a p.d. is applied to the circuit.

As the temperature in the room INCREASES the resistance of the thermistor DECREASES, as resistance is INVERSELY proportional to the resistance the p.d. a high p.d. reaches the fixed value resistor.
As the fixed value resistor will only allow a fixed value of p.d. though, a high p.d. value is directed to the fan, thus increasing the fan speed.
As the fan cools the room the temperature decreases and the thermistors resistance increases, therefore slowing the fan and helping to keep the room at a constant temperature