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Variable Capacitors:- These capacitors consist of two banks of blades with a sufficient gap to allow the second bank mounted on a shaft to pass through it and provide a dielectric. In some cases a dielectric sheet of polystyrene or mica is interposed. This enables the clearances to be reduced. Variable capacitors are used in tuned circuits ie transistor radio's, television etc.

Charge and Discharge Characteristics

When the switch S is closed current i charges capacitor C via R1. The voltage across the capacitor rises until it reaches Vout (see Fig 1 ). Vout is calculated from the equation:

Vo = Vin * R2/(R1 + R2)

(see section on voltage dividers)

The time taken to achieve full output voltage (Vout) is calculated as follows:

T (sec) = C * R1

When the switch S is opened capacitance C discharges through resistor R2, (see fig 2). The time taken to fully discharge T is calculated as follows:

This circuit is especially useful in generating voltage ramps and timing functions, ie delay circuits when used in conjunction with an amplifier. This enables only the near linear portion of the curve to be used. The equation assumes that the ramp is linear and in practice when this circuit is used with a CMOS buffer which will switch before the circuit has timed out, a correction factor of 0.7 is used, ie T = 0.7CR.

Capacitors in Power Supplies.

Electrolytic Capacitors are used in power supplies to smooth the output DC voltage. The drawing below shows the output from a diode bridge rectifier feeding a smoothing capacitor C. The effectiveness of the smoothing capacitor depends on the amount of current fed into the circuit load R, the larger the current requirements the greater the capacitance value must be to maintain an acceptable voltage ripple on the output voltage.

To calculate the value of the capacitance:

CV = IT

C = IT/V

ie If Vin = 12V (rms) 50 Hz

Vout = 1.414 * 12 - 1.2 = 15.8V

Where 1.414 gives the peak value of the waveform, and 1.2v is the voltage drop across the diode bridge.

If R = 20 ohms then I = 15.8/20 = 790mA

If maximum acceptable ripple 1V then

C = 0.79/ 2 * 50 * 1

C = 0.0079F = 7900æF

Nearest preferred value 10,000mF

Note:- This would be perfectly acceptable when feeding a voltage regulator but in many applications this amount of ripple would be unacceptable.

Inductors

Various Chokes

Transformer (PCB Mounting)

Inductors are employed in many high frequency circuits, ie radio, television, video recorders, etc. Included in this section are transformers and chokes which can be considered to be inductors, and are used in almost every ac appliance. Inductance (L) is measured in Henries (H).

Inductors are available in all types ranging from small air cored coils used in radio frequency tuned circuits, having an inductance of a fraction of a micro-Henry (µH) to the iron cored coils used in passive filters, where the inductance may be as large as 100 Henries.

At high power and low frequency iron laminates are used, ie chokes and transformers (this is necessary to reduce heat build up in the core which would result in high temperature and poor efficiency). At high frequency magnetic powder cores are used (ie Ferrite). Ferrite consists of ferromagnetic particles coated with insulating material, compacted to form a solid mass (used in RF chokes, IF transformers and ferrite rod aerials).

Coils or inductors are generally made from copper wire either solid or in the form of twisted and insulated filaments. As the frequency increases a higher proportion of the current is carried on the outside of the conductor, this is called the "skin effect" which is even noticeable at 50 Hz. By increasing the outside area of the conductor economies can be made in size and weight. At ultra high frequencies (GHz range) wave guides are used which resemble square section pipes the size dependant upon the frequency.

Direct current relays use a solid core where the requirement is to produce a magnetic field capable of operating relay contacts.