DXARTS 472

1
DXARTS 472

• Lecture 2 Power Supplies

2
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

3
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.

4
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)

5
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
  batterys 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

6
Direct Current Sources

• Typically the electronic circuits we work with
  require a DC power supply.
• Fixed voltage
• Sufficient current for the circuits load
• Batteries provide DC power, but at low current.
  To provide higher current, we convert AC voltage
  to DC.

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AC to DC

• Converting AC to DC involves 4 stages
• Stepping down the AC voltage to a usable level
  with a transformer
• Using rectifer diodes to convert the AC into
  variable DC
• Smoothing out the variable DC into a fairly
  constant DC output with a capacitor.
• Inserting a voltage regulator to eliminate any
  output ripple.

8
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 inputoutput 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.
• Vp x Ip Vs x Is

9
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).

10
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.

11
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.

12
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
  (VIR)
• This problem is easily fixed by using a voltage
  regulator such as the 7805.

13
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).

14
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.

15
Optional LAB exercise DIY power

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