Chelmsford Amateur Radio Society Advanced Course (3) Technical Aspects Part-2 - Resistors and Capacitors

1. Chelmsford Amateur Radio Society Advanced Course(3) Technical AspectsPart-2 - Resistors and Capacitors

2. Voltage Drop • In any closed circuit if current is flowing there must be a voltage drop across circuit elements. • This follows OHMS LAW which is recalled as:- • R = V / I V = I . R I = V /R

3. Source Resistance • There is NO ideal source. • Every source has some internal resistance. • This is termed Source Resistance/Impedance. • When current is drawn there is a voltage drop across this internal resistance/impedance • Source resistance limits the amount of power that can be drawn. • Example: Lead Acid cells have very low internal resistance

4. Internal Resistance, r EMF Source, V ElectroMotive Force - EMF • EMF is the unloaded potential difference between two points. • The electromotive force of a source is that force which tends to cause a movement of electricity in a circuit. • It has an unloaded voltage V which can be transposed to the output terminals provided no current flows.

5. Internal Resistance, r Potential Difference Load, R Source, V Potential Difference • The loaded voltage is the Potential Difference between two points when current is flowing in the circuit. • There is now a potential difference across the load as current is flowing. The value of which follows Ohm's Law.

6. R1 R2 R3 R1 R2 R3 Resistors in Series/Parallel • Resistors in Series:- Rtotal = R1+ R2+ R3 . . . • Resistors in Parallel:- • 1/Rtotal = 1/R1+1/R2+1/R3 . . .

7. Internal Resistance, r Power, P in Load = V x I Source DC Power • Power is measured in WATTS. • The calculation for power in a circuit is Volts multiplied by Amps • P = V . I • and also • P = I2 x R P = V2 / Rfrom using Ohms Law for V or I

8. R1 VIN 10v VOUT R2 Potential Dividers • By connecting resistors in series a potential divider network can be constructed. This is the basis of Volume controls, trimmers etc • VOUT can be calculated by proportion without worrying about the current through the resistors if load is high impedance at VOUT. • VOUT = VIN x R2 / ( R1 + R2 )

9. Bleed Resistor Value greater than 100K Safety Leakage Paths • Capacitors can hold a lethal charge when open circuit. • A parallel resistor can provide a safety leakage path to discharge large value capacitors. • High value resistors provide a very low current drain across a capacitor. For example it can make a PSU safer without affecting operation. • In a circuit this is known as a BLEED RESISTOR.

10. A + - B A + - B Capacitance • Potential difference between A & B. Charge is set in motion therefore current flows. • The amount of current is determined by the nature of the conductor - its Resistance • Also a potential difference between A & B, but other than the short time at switch-on NO current will flow. • However the negative plate will contain an excess of electrons and the positive plate a deficit. • The plates are said to be charged and the name given to the ability to store charge is Capacitance.

11. Dielectric Metal Plates, Area “A” Spacing "d" Capacitors • Instead of two ends of wire a capacitor is made up of two or more flat parallel plates. The capacitance can be calculated. • The plates have area "A" and are separated by distance "d". • The space between the plates is filled with an insulating material known as the DIELECTRIC • Dielectric can be any insulator - typically Oxides, Plastics, or Ceramic

12. Capacitance & Charge • The Unit of Capacitance is the FARAD [after Faraday] • A capacitance of 1 FARAD is when 1 Coulomb of charge is stored with 1 Volt PD. • Q = C.V • or • Q / C = V • or • Q / V = C • One Farad is rather large so uF, nF, pF are more common

13. Capacitors • CAPACITANCE, C is : • Proportional to the plate area "A". • Inversely proportional to the distance between the plates "d” • Depends upon the material used for the dielectric. • This can be written as C = ε0 εr A / d where C is in Farads A is area of each plate in square metres d is distance between plates in metres. • ε0 - A constant known as the permittivity of free space [or vacuum, dry air] having a value of 8.854 x 10-12 . • εr - [or “k”] Relative permittivity of the dielectric, chosen for its TEMPERATURE STABILITY, BREAKDOWN VOLTAGE

14. Capacitance Formula & Dielectric Materials The formula C = ε0εr A / d is also written as C = K . A / d where K = ε0εr • The dielectric type in a capacitor will depend upon the function the capacitor is to perform in a circuit. • As frequency increases dielectric loss increases, • Each dielectric also has different breakdown voltages. • Each material will suffer a change in properties due to temperature, this will affect the capacity. • Typical εr - Air=1, PTFE=2, Mica=5, ceramics ~10, Hi-k ceramics ~10000

15. C1 C2 C3 V Capacitors in Parallel • Capacitors connected in parallel all have the same voltage • The charge on them is dependent upon the capacitor value. Ctotal = C1 + C2 + C3

16. C1 C2 C3 V Capacitors in Series • When capacitors are in series the charge is the same in all of the circuit • The voltage is divided between capacitors. 1/Ctotal = 1/C1 + 1/C2 + 1/C3

17. 1 R 2 VC C Voltage, V I Discharging VC VC Charging t t I I Charge & Discharge of a Capacitor • Time = C.R Secs. • Where C = FaradsR = Ohms