How to Figure the Transformer Ratios for Primary to Secondary
A transformer conveys electrical energy from an energized alternating circuit to a circuit that otherwise would have no current. The energized, or primary, circuit passes its electrical energy to the transformer magnetically by coiling around it. The secondary circuit siphons off the energy magnetically by also coiling around the transformer. The number of coils that each circuit makes determines the difference in voltage and current between the two circuits. Transformers therefore primarily function to change alternating current voltage.
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Instructions
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1
Determine the current ratios from the coil turn counts using the equation n1*i1 = n2*i2, where the n's denote the two circuits' coil turn counts and the i's denote the currents through the two circuits. The asterisks [*] imply multiplication.
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2
Determine the voltage imparted to the secondary circuit from the voltage drop across the primary coil using the equation V1/V2 = n1/n2. V1 denotes the primary circuit's voltage drop across its coil and V2 denotes the voltage imparted to the secondary circuit through its coil.
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3
Determine the currents through the two circuits from the voltages using the equation V1/V2 = i2/i1.
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Tips & Warnings
The voltage drop across the primary coil is not from resistance, as in resistor elements of the circuit. The resistance in the coil is relatively small for that. Instead the voltage drop comes from an induced electromotive force (emf) that opposes the voltage source of the primary circuit.
The equations above follow from the magnetic field equation for a current-carrying solenoid (coil) and from the principle of energy conservation. If there is minimal energy loss through the transformer, then the power going in, P1=i1*V1, must equal the power going out, P2=i2*V2.
References
- Photo Credit transformer image by palms from Fotolia.com
