A learning project by Huy · AI-assisted content · Updated
Power13 min read
Transformers
Connect changing magnetic flux, turns ratio, isolation, current ratio, and real transformer losses.
Mental model
A transformer is like a magnetic gearbox. More secondary turns produce more voltage but proportionally less current for approximately the same transferred power.
The same changing core flux links both windings; their turn counts set the ideal voltage ratio.UNDERSTAND THE BEHAVIOR
01
Alternating primary current creates changing magnetic flux in the core. That flux links the secondary winding and induces a voltage. The induced voltage per turn is approximately equal for windings sharing the same core.
02
The turns ratio sets voltage ratio in the ideal model. If the secondary has fewer turns, voltage steps down. Conservation of energy means the available secondary current rises by the inverse ratio, but only within the winding, core, and thermal ratings.
03
A real transformer draws magnetizing current even with no load. Copper resistance causes voltage drop and heating, while core hysteresis and eddy currents cause frequency-dependent loss. Isolation and insulation ratings are safety constraints, not optional details.
Why it matters
Transformers provide voltage conversion, galvanic isolation, impedance transformation, and signal coupling in power supplies and communication circuits.
Core ideas
A transformer requires changing flux, so steady DC cannot continuously transfer energy through an ordinary transformer.
Voltage follows the turns ratio, while ideal current changes by the inverse ratio.
Real designs must consider copper loss, core loss, leakage inductance, regulation, insulation, and VA rating.
1A primary has 1,150 turns and is connected to 230 V AC.
2For 12 V, Ns = 1150 × 12/230 = 60 turns.
3A 24 VA transformer can ideally supply 24/12 = 2 A on the secondary.
4The measured loaded voltage will be slightly lower because winding resistance and losses are not zero.
Common mistake
Never connect an ordinary mains transformer primary to DC. Flux will stop changing, induced secondary voltage disappears, and excessive primary current can overheat the winding.
RememberA transformer requires changing flux, so steady DC cannot continuously transfer energy through an ordinary transformer.
DEEPER EXPLANATION
Why do winding turns set the voltage ratio?
A changing current in the primary creates changing magnetic flux in the core. That shared changing flux induces voltage in each winding. In the ideal model, each turn experiences the same induced voltage, so Vp/Np = Vs/Ns. Both voltages must be measured using the same convention, for example RMS for both.
A secondary with half as many turns has about half the primary voltage. In the ideal lossless model, power in equals power out, so the secondary current capacity changes in the opposite direction. A lower voltage does not mean free energy: more output current corresponds to more input current than it would at no load, scaled by the ratios.
Real transformers have copper loss, core loss, leakage inductance, temperature limits and a specified operating frequency. The ratio does not tell you whether a winding has adequate insulation or whether a core will saturate. Work with a low-voltage isolated AC example while learning; do not use this equation as a mains construction instruction.
Walk through another example
Feed a suitable low-voltage transformer primary with 12 V AC RMS. Assume Np = 120 turns and Ns = 60 turns.
The ideal ratio is 60/120 = 0.5; Vs = 12 × 0.5 = 6 V AC RMS.
If the load draws 0.4 A at 6 V in the lossless model, output power is 2.4 W and input current would be 2.4/12 = 0.2 A. Real input current and losses differ.
Check your intuition
Swap the winding turn counts conceptually: with 60 turns on the input and 120 on the output, the ideal secondary voltage doubles instead of halving.
Where this model can mislead you
This magnetic induction relationship requires changing flux. Ordinary DC applied continuously to a transformer does not produce a steady transformed secondary voltage and can overheat a winding.
TEST YOUR UNDERSTANDING
Three quick checks
Choose one answer per question, then check your work. Each answer includes an explanation.
Practice and check your understanding
Try each question before opening the worked answer.
1. An ideal transformer has Np = 200, Ns = 50, and Vp = 12 V RMS AC. Find Vs.
Show answer and reasoning
Vs = Vp × Ns/Np = 12 × 50/200 = 3 V RMS AC. The winding ratio is 4:1. This is a low-voltage theoretical example, not an instruction to connect to mains.
2. Can a conventional transformer continuously step down a steady DC input?
Show answer and reasoning
No. Transformer action requires changing magnetic flux. A steady DC connection can drive excessive winding current and saturation; switched converters deliberately create a changing waveform.
A related calculation tool, not a complete simulation of the circuit. Check its model and units.
Continue learning
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Frequency & Phase — Connect period, frequency and phase to what you actually see on an oscilloscope.