An Oudin coil, also called an Oudin oscillator or Oudin resonator, is a resonant transformer circuit that generates very high voltage, high frequency alternating current (AC) electricity at low current levels, used in the obsolete forms of electrotherapy around the turn of the 20th century. It is very similar to the Tesla coil, with the difference being that the Oudin coil was connected as an autotransformer. It was invented in 1893 by French physician Paul Marie Oudin as a modification of physician Jacques Arsene d'Arsonval's electrotherapy equipment and used in medical diathermy therapy as well as quack medicine until perhaps 1940. The high voltage output terminal of the coil was connected to an insulated handheld electrode which produced luminous brush discharges, which were applied to the patient's body to treat various medical conditions in electrotherapy.
How it works Oudin and Tesla coils are spark-excited air-core double-tuned transformer circuits that use resonance to generate very high voltages at low currents. They produce alternating current in the radio frequency (RF) range. The medical coils of the early 20th century produced potentials of 50,000 up to a million volts, at frequencies in the range 200 kHz to 5 MHz. The primary circuit of the coil has Leyden jar capacitors (C) which in combination with the primary winding of the coil (L1) make a resonant circuit (tuned circuit). In medical coils usually two capacitors were used for safety, one in each side of the primary circuit, to isolate the patient completely from the potentially lethal low frequency primary current. The primary circuit also has a spark gap (SG) that acts as a switch to excite oscillations in the primary. The primary circuit is powered by a high voltage transformer or induction coil (T) at a potential of 2 - 15 kV. The transformer repeatedly charges the capacitors, which then discharge through the spark gap and the primary winding (a detailed description of the operation cycle in the Tesla coil article also applies to the Oudin coil). This cycle is repeated many times per second. During each spark, the charge moves rapidly back and forth between the capacitor plates through the primary coil, creating a damped RF oscillating current in the primary tuned circuit which induced the high voltage in the secondary. The secondary winding (L2) is open-circuited, and connected to the output electrode of the device. In the Oudin coil, one side of the primary winding (L1) is grounded and the other side is connected to the secondary, so the primary and secondary are in series. There were two versions of the Oudin coil:
In earlier Oudin circuits the two coils were separate, not magnetically coupled, with a small horizontal primary "D'Arsonval" coil (L1) of 20-40 turns with a tap connected to a large vertical secondary "Oudin resonator" (L2) with many turns of fine wire (400 - 600 in large coils, 100 - 300 in small ones), connected to the high voltage terminal on top. In this circuit the high voltage was generated entirely by self-resonance in the high Q secondary coil. The addition of the "resonator" coil to the "D'Arsonval" coil was Oudin's contribution; the rest of the circuit was invented by Jacques D'Arsonval.
In later Oudin circuits the coils were magnetically coupled, forming an autotransformer, so the primary induces an EMF in the secondary by electromagnetic induction. Both coils were usually wound on the same coil form, the primary consisting of relatively few turns of heavy wire at the bottom with an adjustable tap, connected to the secondary winding, made of many turns of fine wire. Oudin found this circuit produced higher voltages due to the large turns ratio of the transformer.
Although it doesn't include a capacitor, the secondary winding is also a resonant circuit (electrical resonator); the parasitic capacitance between the ends of the secondary coil resonates with the large inductance of the secondary at a particular resonant frequency. When it is excited at this frequency by the primary, large oscillating voltages are induced in the secondary. The number of turns in the primary winding, and thus the resonant frequency of the primary, could be adjusted with a tap on the coil. When the two tuned circuits are adjusted to resonate at the same frequency, the large turns ratio of the coil, aided by the high Q of the tuned circuits, steps up the primary voltage to hundreds of thousands to millions of volts at the secondary. The secondary is directly connected to the primary circuit, which carries lethal low frequency 50/60 Hz currents at thousands of volts from the power transformer. Since the Oudin coil was a medical device, with the secondary current applied directly to a person's body, for safety the Oudin circuit has two capacitors (C), one in each leg of the primary, to completely isolate the coil and output electrode from the supply transformer at the mains frequency. Because two identical capacitors in series have half the capacitance of a single capacitor, the resonant frequency of the Oudin circuit is
f = 1 2 π L 1 C / 2 {\displaystyle f={1 \over 2\pi {\sqrt {L_{1}C/2}}}\;}
Use
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