• What is the purpose of faraday shields in a transformer?
  • What is the interest of insering a Faraday shield between the primary winding and the secondary winding in a halve of transformer windings ?
  • Is it for reducing leakage inductance ?

Compare this picture (where we only see half of the total transformer winding) from "Eddy Current Losses in Transformer Windings and Circuit Wiring" by Lloyd H. Dixon, Jr:

enter image description here


1 Answer 1


One of the common causes of "ground loops" is the coupling between transformer primary and secondary. Assume 100 picoFarads, using C = Eo * Er * Area/Distance.

For pure sin (no spikes from motors or flourescent lights), the dV/dT will be Vpeak * radians/second or

160 volts * 377 ~~ 56,000 Volts per second.

That dV/dT * 100pF gives the "displacement current" coupled thru the capacitor as charges rush in and out of the two plates (the 2 windings). Our math is

50,000 volts/second * 100pF = 5e+4 * 1e-10 = 5e-6 = 5 microamps.

That level, 5uA, is well below heart-fibrillation threshold, but enough to interfere with audio vinyl record playback levels, so the turntables have a FIFTH WIRE, a bare braided copper wire, between the turntable chassis and the audio preamp/poweramp chassis. That 5th wire eliminates (most of) the current that demands a path back home to mother, the 117VAC power.

Is this 5th wire magic? no. The 5th wire merely provides an alternative path for electrons that would otherwise capacitively couple onto the phono cartridge; those electrons would take both wires from the cartridge to the Preamp, and cause 60Hz hum.

The 5th wire merely shunts the capacitive coupling onto the cartridge, and greatly reduces the level of hum.

Notice there are TWO capacitors in series:

1) primary---to---secondary of transformer

2) chassis/metal-tonearm--to--the tiny wires running inside tonearm pipe

What does a Faraday Shield buy you? You still have to tie that shield to some node that provides a return path............

.......thus design of RETURN PATHS is a big part of successful system thinking.

  • \$\begingroup\$ Why Vpeak is equal to 160V ? Does the voltage across the capacitor is 160 V ?Does it depends on the turn ratio ? I have to review what are ground loops for better understanding what you say :) Thanks ! \$\endgroup\$
    – Jess
    Mar 23, 2020 at 8:33
  • \$\begingroup\$ I m back with more knowledge about EMI ;) Thank you for this comment :D \$\endgroup\$
    – Jess
    Apr 22, 2020 at 7:10

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