Autotronics - Power Source


Introduction


- Source of electrical power.

- Device or system that supplies electrical or other types of energy to an output load or group of loads

- It is called a power supply unit or PSU.


Primary or Secondary Power Supplies

There are a lot of types of power supplies which may include a power distribution system as well as primary or secondary of energy such as:

- A converter which converts one form of electrical power to other desired form like converting AC voltage to a well regulated lower voltage DC for electronic devices. Low voltage or low power DC power supply units are normally integrated into electronic devices such as computers and household electronics.

- Batteries

- Fuel cells

- Solar power

- Generators or alternators.


There are some factors that should be considered when using a power supply:

- The amount of the power that can be supplied

- The duration of supply without needing some kind of recharging or refuelling

- The stability of the output voltage or current under varying load conditions

- Whether they provide continuous power or pulses.


Power Supply types

A) Linear Power Supply

- Simple design which becomes bulky and heavy for high current devices.

- Voltage regulation in a linear supply can result in low efficiency.

Battery Power Supply

- linear power supply which is mobile, portable and reliable.

- Constructed using multiple electrochemical cells connected to get the voltage desired


(i) Dry-Cell Battery

- The most commonly used dry-cell battery is the carbon-zinc dry cell battery.

- Made of carbon plate, electrolyte paste and a zinc plate which are stacked together.

- The chemical process which happened inside the battery produce voltage desired.

- The voltage is produced during the discharge of a carbon-zinc battery

- The zinc metal is converted to a zinc salt in the electrolyte

- Magnesium dioxide is reduced at the carbon electrode.

- It produces a voltage at about 1.5V.


(ii) Lead –acid battery

- Rechargeable

- Consists of lead and lead/dioxide electrodes which are immersed in sulphuric acid

- When fully charged it can produce about 2.06-2.14V voltage

- During discharge process, the lead is converted to lead sulphate and the sulphuric acid is converted to water

- But during charging process, the lead sulphate is converted back to lead and lead dioxide.


(iii) Nickel-cadmium battery

- become more popular.

- Completely sealed and rechargeable.

- Has longer life span because electrolyte is not involved in the electrode reaction

- During the charging process nickel oxide is oxidized to its higher oxidization state and cadmium oxide is reduced.

- It can be stored both charged and uncharged

- Has a long service life, high current availabilities, constant voltage and ability to be recharged.



AC Linear Power Supply

- Uses transformer to convert the voltage from the socket outlet (mains) to lower voltage.

- If it used to produce DC voltage so a rectifier will be used.

- A capacitor is used to smooth the pulsating current from the rectifier.

- The voltage produced by unregulated power supply will vary depending on the load and the variations in the AC power supply.

- Linear regulator is normally used by critical electronics applications to stabilize and adjust the voltage.

- It normally provides current limiting, protecting the power supply and attached circuit from the over current.

- The simplest dc power supply circuit consists of a single diode and resistor in series with the AC supply.

- The circuit is common in rechargeable flashlights.


AC/DC Supply

- Formerly mains electricity was supplied as DC and AC separately.

- A simple power supply would run directly from the either AC or DC mains often without using transformer.

- Consisted of a rectifier and a capacitor filter.



B) Switching power supply.
- A switched mode supply of the same rating as a linear supply will be smaller
- Usually more efficient but will be more complex.



Switch Mode power supply (SMPS)

- A SMPS work differently.

- AC, mains input is directly rectified without the use of a transformer, to obtain DC voltage.

- This voltage is then sliced into small pieces by a high-speed electronic switch.

- The size of these slices grows larger as power output requirement increase.

- The input power slicing occurs at a very high speed.

- High frequency and high voltages in this first stage permit much smaller step down transformers than are in a linear power supply.

- After transformer secondary, the AC is again rectified to DC.

- To keep output voltage constant the power supply needs a sophisticated feedback controller to monitor current draw by the load.

- Nowadays it is equipped by the crowbar circuit as the safety features to help protect the device form the harm.

- When the abnormal high current power is detected, it can assume this is a direct short and will shut down before it is damaged.

- Normally used in PC to provide a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.


Programmable power supply

- Can supply output which can be varied remotely.

- Can be controlled digitally using a computer interface.

- Composed of a processor, voltage/current programming circuits, current shunt, and voltage/current read-back circuits.

- Used in many applications such as automated equipment testing, crystal growth monitoring, and differential thermal analysis.


Uninterruptable power supply (UPS)

- It takes its power from two or more sources simultaneously.

- Usually powered by AC (mains), while simultaneously charge the storage battery.

- Should there be a dropout or failure of the mains, the battery will take over so that the load never experiences an interruption.

- Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data.

- Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains.

- The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption.

- Such a scheme might be found in hospitals or telephone central offices.


High-voltage power supply

- It refers to an output on the order of hundreds or thousands.

- High voltage supplies use a linear setup to produce voltage in this range.

- It is able to reverse the output polarity along with the use of circuit breakers and special connectors to minimize arcing and accidental contact with human hands.


Voltage multipliers

- It is actually a circuit designed to multiply the input voltage.

- The input voltage may be doubled (voltage doubler), quadrupled (voltage quadrupled).

- It is also actually a power converter.

- An AC input inside converted to a higher DC output.

- It allows high voltages to be obtained using a much lower voltage AC source.

- It can be found in televisions, photocopier, cathode ray tubes, and oscilloscopes.


Applications


Computer power supply

A modern power supply is a switched mode supply designed to convert 110-240V AC power from mains supply to several output both positive DC voltages.


Welding power supply

Arc welding uses electricity to melt the surfaces of the metals in order to join them together. The electricity is provided by a welding power supply and can be either AC or DC. It requires high current between 100 and 350 amps.


AC Adapter

A linear or switched mode power supply that is built in the top of the plug is known as power adapter. They are actually either the same kind of DC plug offering different voltage or polarity or different plug offering the same voltage.

The least expensive AC units consists of a small transformer while DC adapters include a few additional diodes. Whether or not a load is connected to the power adapter, the transformer has a magnetic field continuously present and normally cannot be completely turned off unless unplugged.