Electronics - AS Module 1 & 2

Power

Power is the energy transferred in each second.

Electrical power is the electrical energy transferred each second.

Power= Energy Transferred ÷ Time Taken

P = E/ t

The Units of Power are Joules per Second ( J/s)

1 Joule per second is called a Watt (W)

1 J/s = 1 W

But also

Energy Transferred = Charge x Voltage

E = Q x V

And so

Electrical Power = (Charge x Voltage)÷ Time Taken

P = (Q x V) / t

But

Charge / time taken = Electric Current

Q/t = I

And so the expression for Electrical Power becomes

Power = Current x Voltage

P = I x V

Resistance

Ohm’s Law

As charged particles try to make their way round a circuit they encounter resistance to their flow eg. they collide with atoms in the conductor. More resistance means more energy is needed to push the same number of electrons through part of the circuit.

This resistance is measured in ohms, .

Definition -“If it takes 1 volt (1 joule per coulomb) to push a current of 1amp through a resistor, it has a resistance of 1 ohm”

In equation form, that says

R=V/I

Otherwise written as the more familiar

V=IR

This equation summarises Ohm’s law. It suggests that any value of voltage you put across a resistor divided by the current it produces in the resistor, will always give the same value of resistance. So, if you plotted a graph you would get:

Any resistor that does this is called an ohmic resistor. Any resistor that doesn’t do this is therefore called a non-ohmic resistor.

A filament lamp (non-ohmic).

A diode (non-ohmic).

Combination of Resistors in Circuits, Parallel and Series

Series

If you have more than one resistor in a circuit it is often useful to be able to calculate a value of a single resistor which would be the same as the actual combination of resistors

Current used in all 3 will be the same (current doesn’t get used up) but energy used per coulomb (i.e. pd) will depend on value of resistance

So,

Vtotal = V1 + V2 + V3

and as V = IR

IRtotal = IR1 + IR2 + IR3

canel the I's

Rtotal = R1 + R2 + R3

Parallel

Here the voltage across all three will be the same but current through each depends on resistance of each.

So

Itotal = I1+ I2 + I3

and as I=V/R

V/Rtotal = V/R1 + V/R2 + V/R3

Cancel the V’s to get:

1/Rtotal = 1/R1 + 1/R2 + 1/R3

Diodes

A diode is a semiconductor device which allows current to flow through it in only one direction. Although a transistor is also a semiconductor device, it does not operate the way a diode does. A diode is specifically made to allow current to flow through it in only one direction.There are a number of different electronic devices which tend to be called diodes. Although they're made differently they all have three things in common.

• They have two leads like a resistor.
• The current they pass depends upon the voltage between the leads.
• They do not obey Ohm's law!
  

The following are some examples of common diodes:

Voltage regulation diode (Zener Diode)

The circuit symbol is .

It is used to regulate voltage, by taking advantage of the fact that Zener diodes tend to stabilize at a certain voltage when that voltage is applied in the opposite direction.

Light emitting diode

The circuit symbol is .

This type of diode emits light when current flows through it in the forward direction. (Forward biased.)

Variable capacitance diode

The circuit symbol is .

The current does not flow when applying the voltage of the opposite direction to the diode. In this condition, the diode has a capacitance like the capacitor. It is a very small capacitance. The capacitance of the diode changes when changing voltage. With the change of this capacitance, the frequency of the oscillator can be changed.

Characteristics of a LED

• Typically, a LED needs a forward current of 10mA to operate.
  
• The voltage drop across the LED is then typically 2 Volts.
  
• The LED must have an external resistor in series with it to limit the current to 10 mA.
  

The value of the external resistor is given by

R = (Supply Voltage - 2.0 ) / (10 x 10-3)

The advantage of LEDs are

• small size
  
• reliability
  
• long life
  
• high operating speed
  

The graph below the electrical characteristics of a typical diode.

When a small voltage is applied to the diode in the forward direction, current flows easily.
Because the diode has a certain amount of resistance, the voltage will drop slightly as current flows through the diode. A typical diode causes a voltage drop of about 0.6 - 1V (VF) (In the case of silicon diode, almost 0.6V)
This voltage drop needs to be taken into consideration in a circuit which uses many diodes in series. Also, the amount of current passing through the diodes must be considered.

When voltage is applied in the reverse direction through a diode, the diode will have a great resistance to current flow.
Different diodes have different characteristics when reverse-biased. A given diode should be selected depending on how it will be used in the circuit.
The current that will flow through a diode biased in the reverse direction will vary from several mA to just µA, which is very small.

Uses of Diodes

Diodes are used in many electronic circuits.

Diodes are used to change Alternating Current (a.c.) into Direct Current (d.c.).

Light Dependent Resistor (LDR)

Circuit Symbol

Characteristics of the LDR

• The resistance of an LDR decreases as the intensity (brightness) of the light falling upon it increases.
  
• In BRIGHT light an LDR has a low resistance.
  
• In the DARK an LDR has a high resistance.
  

Uses of an LDR

• Measuring Light Intensity (Light Meter)
  
• Controlling lights.
  
• Burglar detectors.
  
  

LDR as arranged in a potential divider

Vout/Vin = Rldr / R + Rldr

Thermistor

Circuit Symbol

Characteristics of a Thermistor

The resistance of a Thermistor decreases as the temperature goes up.

At LOW temperatures a thermistor has a high resistance.

At HIGH temperatures a Thermistor has a low resistance.

Uses of a Thermistor

Measuring temperature - thermometer.

Controlling temperature - thermostat.

Thermistor as arranged in a potential divider

Vout/Vin = R/ Rth+ R