Ohms Law

V = I R

I = V / R

R = V / I

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Component Theory - Power Supply

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​​​​​​​​​​​​Electricity is created when negatively charged electrons are forced to move along a material from one point to another to a positive source. Metals, which have a high number of loosely bound electrons orbiting the nucleus are conductive as they allow for easier movement of electrons along the material. Primarily, Copper is used for this reason.


  • V- The amount of electron charge between the two points is known as Voltage and measured in Volts (V). Also referred to as 'potential difference'. 
  • I- How fast those electrons flow through the material is know as Current and measured in Amps (A). The higher the current the more energy that is being transferred  
  • R - To what extent the electrons are slowed down is known as Resistance which is measured in 'Ohms' (Ω). It is the friction created by a materials limiting of electron flow


The 'Water Analogy' is used to visualise the behaviour of electricity as the two are very similar and electrons can't be observed.









If you imagine a tank of water suspended up high, this would be the Voltage, the more water/electrons in the tank the more pressure to force the water out. Current is the rate at which the water flows from the tank and is measured over time. Resistance is therefore the size of the pipe which would restrict the flow of current and reduce the voltage at the end

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All three measurements are linked together by a mathematical rule known as Ohms Law. It states that the current through a conductor between two points is directly proportional to the potential difference across the two points


So if the voltage was 1V and the resistance than the current is  1A


Power is a measure of the energy created from the Voltage (V) & Current (A) and is measured in Watts (W). A Watt is defined as 1 joule per second. With the water analogy in mind, a water wheel would represent the Power created. The higher the current and voltage the more energy created. A 60W lightbulb at 240V requires (60/240) 0.25A







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A Power Supply (PSU) is used to control the AC voltage (240V) from UK mains plugs in a product. Their primary function is convert one form of electricity energy to another. They can be regulated so that they maintain a constant output voltage/current or unregulated 


In a DC 12V PSU a number of process need to happen before the voltage is outputted.


  1. Electricity enters the plug at 240V AC from the mains throughLive, Neutral, and Earth wires and goes through a fuse. The fuse wire is rated as to break the connection if a power surge occurs.
  2. Next the 240V is reduced to 12V using a step down transformer. This is essentially two coils of copper wire with a gap in the middle for the electricty to be induced by a magnetic field
  3. The AC is then converted to DC using a bridge rectifier array of diodes. The diodes control the flow of current and restrict the negative side of the wave and rectify the positive
  4. The rectified wave is then smoothed using capacitors which reduce the peaks of the wave and ensure a consistent voltage
  5. Finally a Low Voltage Regulator is used to maintain a constant 12V output to the connected device by loosing excess energy through heat and an internal cirucit

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Copyright © A.F.Billington 2018. All rights reserved.

​​​​​​Mr BILLINGTON

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