29-03-2011, 11:25 AM
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Transformers
What is a Transformer?
• A Transformer is a device that is used to transfer electrical energy from one circuit to another through electromagnetic induction.
• Typically transformers are used to step up or down voltage between two circuits (though there are other uses).
• Electromagnetic Induction
• When a conductor passes through, or is passed by, a magnetic field, a current is produced in the conductor.
How Transformers Work
• Alternating current is passed through the primary coil (the input) which creates a changing magnetic field in the iron core.
• The changing magnetic field then induces alternating current of the same frequency in the secondary coil (the output).
Step Up Transformer
• A step up transformer has more turns of wire on the secondary coil, which makes a larger induced voltage in the secondary coil. It is called a step up transformer because the voltage output is larger than the voltage input.
• If the secondary coil has twice as many turns of wire then the output voltage will be twice the input voltage.
Step Down Transformer
• In a step down transformer the voltage output is smaller than the voltage input. If the secondary coil has half as many turns of wire then the output voltage will be half the input voltage.
• (Decreasing the voltage does not decrease the power. As the voltage goes down, the current goes up.
What is the Turn Ratio?
• The turn ratio is all about the windings in the primary coil vs. the windings in the secondary coil. If there are more windings on the primary than the Secondary the transformer will be a Step down. If there are more windings on the secondary then the transformer will step down voltage.
• The Ratio of one set of turns to another will help you identify how much your voltage and current will change. If a Transformer is said to have a 2:1 Ratio it means for every 2 turns of wire on the primary there is 1 on the secondary.
Real World Uses of Transformers
• Transformer Schematic Symbol
What is a Center Tap?
• Voltage Primary Vs. Voltage Secondary
• Voltage Primary of Ep is the voltage across the primary coil.
• Voltage Secondary of Es is the voltage across the secondary coil.
• In a Step up transformer Es will be larger than Ep and in a step down it will be the exact opposite.
Turn Ratio and Voltage
• The transformer equation relates the number of turns of wire to the difference in voltage between the primary and secondary coils.
Ep/Es = Np/Ns
• Ep is the voltage in the primary coil.
• Es is the voltage in the secondary coil.
• Np is the number of turns of wire on the primary coil.
• Ns is the number of turns of wire on the secondary coil.
Calculating Turn Ratio
• The Turn Ratio can be calculated by dividing the number of Secondary turns by the number of Primary turns.
Turn Ratio = --------------
N = The number of turns in the coil.
Current Primary vs. Current secondary
• Current Primary or Ip is the current that flows through the primary coil.
• Current Secondary or Is is the current that flows through the secondary coil.
• The current is inversely proportional to the turns ratio:
• Is/Ip = Np/Ns
Power Primary vs. Power Secondary
• Power, which is the work that can be done by a circuit is measured in Watts. It is calculated by multiplying the voltage by the current. – P = IxE
• In a transformer the Pp is equal to the Ps.
Transformer Formulas
• There are two points to remember
1. Transformers only work with alternating current.
Using direct current will create a magnetic field in the core but it will not be a changing magnetic field
and so no voltage will be induced in the secondary coil.
2. Using a step up transformer to increase the voltage
does not give you something for nothing. As the voltage goes up, the current goes down by the same proportion.
The power equation shows that the overall power remains the same, P = V x I Power = Voltage x Current.
Oscilloscope
• An electronic measuring instrument that creates a visible two-dimensional graph, on a screen, of one or more continuously varying voltages or currents.