Capacitors

Introduction:

  • They can maintain an electrical charge for a period of time and can be used to smooth out the flow of electrical current.
    • They are often found in computer power supplies.
  • In essence, capacitors are small batteries which can store electrical charge for release later on. They can be used in timer circuits, monostable or astable, as well as for a variety of other functions.
  • When power is supplied to a circuit containing a capacitor it charges up and when power is turned off, the capacitor discharges slowly.

Types:

  • Capacitors are split into two different types: electrolytic (polarized and made of conducting material as well as an electrolyte) and non-electrolytic (non-polarized and made of ceramics or polyester).
  • Electrolytic capacitors also come in radial and axial format.
    • They generally have a lower tolerance.
    • They are more expensive.
    • They have an electrolyte as one of their plates:
      • Allows them to achieve a larger capacitance per unit volume than other capacitors.

Structure:

  • Composed of two conductors separated by an insulating material known as the dielectric:
    • This could be paper, plastic film, ceramic, air or a vacuum.
  • The conductors could be:
    • Aluminium discs, aluminium foil, or even thin films of metal applied to either side of a solid dielectric.
  • This sandwich can then be rolled up into a cylinder or left flat.

Measurements:

  • Capacitance is measured in farads. A one farad capacitor will hold one coulomb of charge and have a PD of 1v across its terminals.
    • Capacitance (EQ)

      • F is capacitance (farads).
      • C is charge (coulombs).
        • Remember that:
          • Charge
      • V is voltage (volts).
      • I is current (amperes).
      • T is time (sec).
  • Capacitors in series (just two):

 Total Capacitance in Series

  • Capacitors in series (more than two):¬†

Total Capacitance in Series M

  • Capacitors in parallel:

Total Capacitance in Parallele

  • Time Constant of Capacitor

    • Please note that the time constant mentioned here is the time needed for a capacitor to charge or to discharge 2/3 of its total charge.
    • The resistor and the capacitor must be connected in series for this to happen with no other components except for a power supply connected to them:
      • The resistor means that the capacitor charges and discharges more slowly.

Properties:

  • All capacitors have a breakdown voltage:
    • This is the minimum voltage which causes a portion of the insulator to become conductive.
      • In capacitors, this insulator is the dielectric which lies between the two electrodes.
  • As capacitors charge up the resistance across them increases accordingly.
    • The reason for this is probably because as capacitors charge up their anodes and cathodes slowly gather charge:
      • When this charge saturates there will be no potential difference between the positively charged pole of the capacitor and the power supply as such no current will be allowed to flow.
        • Notice I use the term positively charged pole and not anode because a charged non-electrolytic capacitor does not have an anode but can have one positively charged pole and one negatively charged pole.
        • The capacitor can then be discharged by disconnecting the original power supply and letting the capacitor temporary take its place.
          • As such the cathode of the capacitor would become the 0V rail and the anode of the capacitor would become the voltage supply rail.
    • For instance, in the circuit below, as the capacitor charges up the bulb dims because the current being supplied to the transistor’s base terminal is decreasing.

Circuit Capacitor 1

Circuit Capacitor 2

  • The greater the capacitance of the capacitor the longer it takes to charge up and so the slower resistance across it grows as it charges up.
    • Therefore it is possible to alter the speed at which the bulb dims by altering the capacitance of the capacitor:
      • It is also possible to do this by adding more capacitors in parallel to each other or by altering the resistor in series to the capacitor:
        • Speculation.
      • However, and this is especially evident with larger capacitors, the bulb will shut off before the capacitors finish charging:
        • This is because the resistance decreases perpetually across capacitors as they charge up and so, the larger the transistor is the more time it spends with resistances too great to allow the transistor enough current to shine.
  • Such a circuit can be used both as a demonstration for capacitors and in circuits controlling night lights.

Uses:

  • One may find tuning capacitors used in radios etc.
  • Can be used as temporary batteries:
    • Thus can maintain power supply whilst batteries are being charged to prevent information loss in volatile memory.
  • To store high voltage for use in flashtubes (i.e. those found in cameras).
  • Capacitor banks can be used to supply pulsed power and to power weapons.
  • Power conditioning:
    • Capacitors are attached in parallel to the power circuits of most electronic devices and even of things like factories:
      • This shunts away and conceals current fluctuations from the power source to provide a clean power supply.
  • Signal coupling:
    • Capacitors block DC when charged to the voltage of the power source but do not block AC:
      • This allows them to filter a pure AC signal from a signal comprised of both DC and AC.
  • Decoupling:
    • Capacitors can be used to block noise or transients:
      • This reduces the effect that such phenomena have on the other parts of the circuit.
  • Can be used to filter out interference in inductive circuits.
  • Smooth current:
    • To remove power ‘spikes’.
    • They can take a varying input and then output a relatively smooth output.
    • This prevents the false triggering of components such as relays.
    • Non-electrolytic capacitors are generally employed for current smoothing.
  • Motor starter:
    • The primary winding in single phase squirrel cage motors is not capable of starting the motor:
      • To start the motor a second winding has a non-polarized capacitor connected in series.
      • This is able to generate enough force to start the motor.
      • This capacitor is then disconnected by a centrifugal switch or some other such component and the primary winding comes into action and sustains the motion.
    • Used in motors with high starting torque.
  • Signal processing.
  • Sensing.
  • Timer circuits:
    • Such as in the 555.
  • Capacitors can also be used to provide time delays because they charge up quickly but discharge more slowly:
    • As such they can help computers detect very quick switch presses.

Timer Circuits:

  • In this timer circuit when the switch is pressed the capacitor charges up instantly to 9 volts:

Timer Circuit Capacitor

  • Please note that in the case of the circuit diagram above you would normally need some kind of negative voltage supply for the op-amp.
  • After the switch is released¬† the capacitor discharges through the resistor and the signal voltage drops.
    • The rate at which it drops reduces as the voltage gets lower.
    • As a result of this drop the LED turns ON the moment the input voltage drops below the reference voltage.
      • It is possible to reverse this by switching the inverting pole and the non-inverting pole around.

Hazards:

  • Capacitors can retain a charge for a long time even after being removed from a circuit:
    • This can lead to fatal shocks, fires, or damage to circuitry:
      • i.e. the capacitor powering the flash unit in a 1.5V disposable camera can be charged up to over 300V.
    • To discharge them safely one can use a Brinkley stick.
  • Pre-1975 oil-filmed or plastic film capacitors may contain polychlorinated biphenyls which can leak into groundwater under landfills:
    • This can cause serious health problems for humans and animals.
  • Capacitors may fail catastrophically when they near the ends of their lifetimes, when subjected to voltages or currents beyond their rating or when subjected to a reverse current that is too powerful:
    • Interconnection failures in the dielectric or metal may result in arcing. This could vaporise the dielectric fluid causing bulging, rupturing or even an explosion:
      • This problem is lessened by using semi-cut caps on radial capacitors that allow for a certain amount of expansion and so prevent explosions from occurring.
  • High voltage vacuum capacitors can emit soft x-rays even during normal operation:
    • Such capacitors are often pre-charged which limits the in-rush of current during charge up:
      • This increases the lifespan of the capacitor and may mitigate high voltage disasters.
    • Proper containment, fusing, and preventive maintenance can help to minimize these last two hazards.
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