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									    DXARTS 472



Lecture 2: Power Supplies
                              Solar Power
•   Solar (photovoltaic) cells produce electric
    power from sunlight.
•   Solar cells are typically made from
    semiconductors. When photons of light
    strike their surface, it agitates the
    movement of electrons, which allows
    current to flow.
•   Solar cells are fairly inefficient, meaning
    that they generate fairly little power. In
    order to improve their capacity, we
    cascade them in series or in parallel.
     •   Place in series to increase voltage
     •   Place in parallel to increase current
     •   Right hand cell is 3.5V, 6mA, $4
     •   http://www.solarbotics.com
                          The Solar Engine
•   Solar energy is especially useful for small, lightweight, mobile
    systems.
•   When used as a direct power source for electronic systems,
    typically we use a common circuit known as The Solar
    Engine.
     •   Simple circuit consisting of solar cell, capacitor, transistors and a
         trigger component.
•   The capacitor charges and when it has reached a sufficient
    voltage threshold, the trigger activates the NPN transistor,
    which allows the motor to turn. This also activates the PNP
    transistor, which then removes the trigger from the circuit.
    Once the capacitor is drained, everything resets.
                                Batteries
•   Again batteries are useful for low power, lightweight, mobile
    systems.
•   Batteries are measured in amp hours (Ah)
     •   A 2 Ah battery can supply 2A for 1 hr, or 1A for 2hrs, etc.
•   Primary batteries are non-rechargeable and tend to be have
    greater power density:
     •   Carbon zinc (cheap and inefficient)
     •   Alkaline-manganese (general purpose)
     •   Lithium (efficient, expensive)
•   Secondary batteries are rechargeable. When a battery
    recharges, it can do so when fully discharged or not. Batteries
    that must recharge from empty are called ‘deep cycle’.
     •   NiCd (recharge at 10%, 14 hours)
     •   NiMh (faster recharge, less memory effect)
     •   Lead-Acid (larger, eg car batteries)
                            Battery Chargers
•   When building a battery-powered system, it is often a good idea to
    incorporate a battery charger to prevent the need to remove the
    batteries each time.
•   Battery chargers are simple systems, basically you need to:
     •   detect the current battery level
     •   Be able to limit a power supply to 10% of the battery’s capacity for
         charging
     •   Be able to limit a 1/30 supply for trickle charging
•   This can be achieved with resistors, voltage regulators and a trigger
    such as an Silicon Controlled Rectifier (SCR)
     •   An SCR is a rectifier (diode) that conducts electricity when a signal is
         received from a gate.
     •   Within a circuit, this is a good component for checking the amount of
         charge in a battery.
•   Make your charger as automated as possible.
•   Incorporate a solar cell to make an autonomous system that can
    charge itself…
                      Direct Current Sources
•   Typically the electronic circuits we work with require a DC
    power supply.
     •   Fixed voltage
     •   Sufficient current for the circuit’s load
•   Batteries provide DC power, but at low current. To provide
    higher current, we convert AC voltage to DC.
                                AC to DC

•    Converting AC to DC involves 4 stages:

1.   Stepping down the AC voltage to a usable
     level with a transformer
2.   Using rectifer diodes to convert the AC
     into variable DC
3.   Smoothing out the variable DC into a
     fairly constant DC output with a capacitor.
4.   Inserting a voltage regulator to eliminate
     any output ‘ripple’.
                                Transformers

•   Either ‘step down’ or ‘step up’ AC voltage.
     •   As voltage is decreased, current increases.
•   Consist of two coils: a ‘primary’ input and a
    ‘secondary’ output.
•   The coils are linked via a magnetic field.
•   The number of turns on each coil determines the
    ratio of input:output voltages.
•   I.e. a large number of turns on the primary and a
    small number of turns on the secondary would
    result in a step down transformer.
•   Transformers are very efficient, with power in
    being approximately equal to power out.
     •   V p x Ip = V s x Is
                         Diode Recap
•   A diode is a one-way gate for current flow, and a very simple
    semiconductor device.
•   When the anode is more positive in voltage than the cathode,
    current can flow (forward-biased); when the cathode is more
    positive than the anode, current is blocked (reverse-biased).
                         Rectifiers

• A rectifier is a semiconductor
  device, such as a diode, that
  converts AC to DC.
• A single diode rectifier will block
  the negative portion of the AC
  waveform, leaving half-wave,
  variable DC.
• Using four diodes, we can construct
  a ‘full wave’ bridge rectifier.
• If our transformer is center-tapped,
  we can use two diodes instead.
                       Smoothing
•   Capacitors are used for storing charge, and by making them
    discharge when the DC waveform falls, we can smooth out
    the DC output.
•   For this application, we need to use high-value electrolytic
    capacitors, which will quickly store charge near the peak of
    the DC waveform.
•   As soon as the waveform starts to dip, the capacitor
    discharges and supplies current to the DC output.
•   Note that a small amount of ripple remains, due to the
    voltage falling as the capacitor discharges.
                       Voltage Regulators
•   When we use DC power supplies, they are typically described as
    regulated or unregulated.
•   In an unregulated supply, any voltage spikes from the AC source will
    be passed through our conversion circuit and cause a similar spike in
    the DC output.
•   For precision DC circuits, this can cause problems.
•   Further, unregulated supplies cannot maintain a constant voltage
    when its load changes.
     •   i.e. a high resistance load will result in a higher voltage output than a low
         resistance load (V=IR)
•   This problem is easily fixed by using a voltage regulator such as the
    7805.
         Putting it all together: 120V AC to 5V DC
•   In the circuit below, we use a 120VAC to 25.2 V AC center tapped
    step down transformer. Note the current rating for your
    transformer.
•   The 1000µF capacitor acts to smooth the DC output from the two
    rectifier diodes.
•   Once the voltage has been regulated by the 7805, it should be a
    smooth 5V signal. However, there is still potential for fluctuation if
    the load conditions change. For that reason we add two capacitors:
    one to filter out low frequency variations (10µF), and one for high
    frequency (0.01µF).
                          Negative Voltage

•   In some circuits, we need to work with negative voltages.
•   As you know from transistors and diodes, charge moves through
    a conductor based on either a surplus of free electrons, or an
    absence of electrons, known as ‘holes’.
•   Depending on which direction the electrons are moving in then,
    we get a positive or a negative voltage.
•   So for instance, when we connect a multimeter to a 9V battery,
    when we have the leads one way round, we read 9V, and when
    the other way round we read -9V.
             Optional LAB exercise: DIY power

•   Build the below power supply.
•   Rectifier diodes are 1N4004

								
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