**Capacitors**

A capacitor is a charge storage device. It comprises two conducting plates separated by an insulator. The

*charge*(Q) stored is related to the voltage across the capacitor (V) by:
Q = C x V

Where C is the “Capacitance”. To change the voltage on a capacitor, charge must flow into or out of the capacitor. In a circuit the charge flow rate is limited by any resistance present, so the time taken for the voltage to change is given by T=RC, the ‘time constant’ of the circuit.

**Capacitors in parallel are added together – capacitance increases**.

**In series capacitance is reduced.**

**Resistor**

Resistor is an electronic component which opposes the flow of current.

R=V/I V=Voltage

R=Resistance

I=Current.

Unit of resistance is ohm.

**Resistors in series are added together – resistance increases**

**.**

**In parallel resistance is reduced.**

In particular if R1 = R2 then R will be half of R1 (or R2).

**Inductors**

An inductor is a device that tries to keep a constant current flowing through it.

As electronic components inductors are bulky and much rarer than resistors or capacitors. However the property of inductance is real and some appreciation of it is needed.

Many electrical components are highly inductive:

• Motors

• Transformers

• Relay drivers

The impedance of an inductor is given by:

ZL = ω.L

i.e. zero at D.C. and increasing with frequency

Everything (e.g. wires) has some inherent inductance.

These are three main semiconductor devices used to design any electronic circuit. These are used for the purpose of restrict or allow current or voltage in a required amount.

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