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

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

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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

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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.

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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.

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Resistors in Series/Parallel

• Resistors in Series-
• Rtotal R1 R2 R3 . . .

• Resistors in Parallel-
• 1/Rtotal 1/R11/R21/R3 . . .

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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

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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 )

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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.

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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.

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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

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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

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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 e0 er A / d where
• C is in Farads
• A is area of each plate in square metres
• d is distance between plates in metres.
• e0 - A constant known as the permittivity of free
  space or vacuum, dry air having a value of
  8.854 x 10-12 .
• er - or k Relative permittivity of the
  dielectric, chosen for its TEMPERATURE
  STABILITY, BREAKDOWN VOLTAGE

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Capacitance Formula Dielectric Materials
The formula C e0 er A / d is also written
as C K . A / d where K e0 er

• 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 er - Air1, PTFE2, Mica5, ceramics
  10, Hi-k ceramics 10000

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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
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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
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Charge Discharge of a Capacitor
Time C.R Secs. Where C FaradsR Ohms