ADVANCEDTEMPLATE.MZT

ADVANCED LESSON 17

LEARNING OBJECTIVES and NOTES
Capacitance 1.
Introduction Capacitors allow AC to pass through, but block DC. They can be used to "drain away" any AC in a DC circuit - decoupling capacitors. They hold a charge and can be used in a variety of circuits e.g. timing circuits, Smoothing DC output of a power supply after the rectification stage.. They come in a variety of types for use in different circuits.
Understand the factors influencing the capacitance of a capacitor; area and separation of the plates, permittivity of dielectrics and formula C=KA/d. A capacitor is made up of two or more flat parallel metal plates separated by a space filled with an electrical insulating material called a dielectric. This could be air, a vacuum, plastic, ceramic, oxide, plastic. • One lead is connected to one plate and one to the other. CAPACITANCE AND CHARGE •The Unit of Capacitance is the FARAD. •A capacitance of 1 FARAD is when 1 Coulomb of charge is stored with 1 Volt PD. There are 6.241506×1018 electrons in a coulomb, but don’t bother learning this!. [Q - charge in coulombs (C); C – capacitance in farads (F); V - potential difference in volts (V)] •Q=CV or Q / C = V or Q / V = C •One Farad is rather large so uF, nF, pF are more commonly used.
The formula for calculating the capacitance of a parallel plate capacitor is: where k= ε_oε_r Where, k=dialectric constant of the material between the plates (ε_r) x 8.854x10^-12 (ε_o) Where C=capacitance in Farads, k=dialectric constant ( between plates) A is the area of the plates in metres and d is the distance between plates in metres. The factors influencing the capacitance of a capacitor For the exam the key things to remember are. As the distance between the plates decreases, the capacitance increases The higher k, the larger the capacitance The bigger the area, the higher the capacitance	Example 1
Features of electrolytic capacitors: Electrolytic capacitors hold more charge for a smaller size Electrolytic capacitors have to be connected with the correct polarity. Positive to the positive rail and the negative side to the Earthy side.If they are connected the wrong way round they may explode. Non-electrolytic capacitors can be connected either way round.
Recall that different dielectrics are used for different purposes, e.g. air, ceramic, mica and polyester; and that with some dielectrics, losses increase with increasing frequency. Air - is used for variable capacitors for example in VFOs and ATUs Ceramic – There are different types. NPO/COG ceramic capacitors have high stability and are used in bandpass filters and VFOs. 2E6 typically 1 nF to 10 µF, 20%. Good for bypass, coupling applications. Low price and small size Mica - Advantages of mica capacitors arise from the fact that the dielectric material (mica) is unreactive. It does not change physically or chemically with age and it has good temperature stability. Very resistant to corona damage. Silver mica capacitors are very stable and used in high power low pass filters. Polyester - Smaller in size when compared to paper or polypropylene capacitors of comparable specifications. May use plates of foil, metalized film, or a combination. capacitors have almost completely replaced paper capacitors for most electronic applications. Operating voltages up to 60,000 V DC and up to 125 °C. Low moisture absorption. Some dialectrics show an increasing loss with increasing frequency.	Dialectrics Some common dialectrics constances: Vacuum= 1 Ceramic in NPO capacitor= 15-100 Mica = 4-9 Polyester resin =2.8-4.5
Breakdown voltage Understand that capacitors have a breakdown voltage and that they need to be used within that voltage. Capacitors are rated at a certain voltage. Above this voltage the dielectric will allow current to flow. In an air spaced capacitor you will see a blue light/arc as the electrons flow through the air. In a capacitor with a solid dielectric it will probably melt. The capacitor is then useless. Make sure the rated voltage of the capacitor is higher than that of the circuit. Remember that in RF amplifiers that have a 12v power supply the RF voltage could be 5000V.