Cheap Chinese Induction Coils

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The induction coil, once an essential tool of modern physics that lay behind the discovery of the electron, x-rays and radio, has been relegated to the toybin.  Most of the modern ones are made in China like the rest of our toys.  While the coils of yore were mahogany-cased artisanal masterpieces destined for the most prestigious laboratories, today’s product is pitched at the Wal-Mart Shopper: sloppy mass production and everyday low prices.  I recently bought one such coil for use in an outreach program at Albuquerque’s Explora museum.  Billed as a 60-kV Ruhmkorff coil, the same product seems to have several outlets in the US.  This is a quick review.  Click below for more…

Where to buy:

• www.sci-supply.com is where I bought mine; has new lowest price 4/2/09
• www.unitednuclear.com
• www.enasco.com

The electrical circuit is illustrated above in a scan from the manual.  None of the components are exotic or expensive.  The IC is a regular 555 timer, which when triggered by 60-Hz line-voltage ripple delivers a ~5 ms pulse to the gate of a BT151 SCR.  The SCR dumps a capacitance (chosen from one of six panel-switch-selectable values, 2.2 μF through 13.2 μF) through the primary of the coil.  The manual’s diagram neglects the polarity switch that swaps the direction of current flow through the primary.  The primary capacitors are charged right off the bridge-rectified 120-VAC power line through a beefy current-limiting resistance.  The coil itself is comprised of a 720-μH primary and a two-bobbin, 105-H secondary magnetically coupled on an open ferromagnetic core and packaged in a cylindrical plastic tank (see x-ray above).  The coupling constant is ~0.95.  From radiographs it is evident that the coil is insulated / cooled by a liquid–and that the manufacturer even took care to leave a small compliance bubble of air so the tank doesn’t burst when the coil warms up!  Electrical wiring is surprisingly well-done; the rotary switches are of decent quality, most resistors are 2%, the capacitors are high-Q, 400-V film caps.  All diodes are 1N4007s.

Performance is reliable, with noisy sparks exceeding 4″ and no mechanical interrupter to fiddle with.  I should note that in consideration of the secondary design, in which the inside leads of the bobbins are in close proximity to the core and primary and the output voltage is balanced with respect to these components, one should not attempt to ground either end of the secondary.  This is asking for breakdown that will likely destroy the primary semiconductors and possibly damage the secondary coils.  All loads must be isolated.  This is not a suitable supply for Farnsworth fusors for this reason.  On the other hand, this induction coil is perfect for operating Crookes, Geissler, or x-ray tubes.  The top-right photo above shows a cathode ray tube operating and generating low levels of x-radiation (150 mR / hr. on the ion chamber, easily stopped by a 1/8″ lead sheet).  The lower-left photo shows excitation of a canal-ray (ion) tube belonging to Explora.

Documentation supplied with the coil is useless.  In fact, the English is so poor it’s almost endearing.  Some examples:

• We use them do experiments,such as the performance of electric-flash,,electric-bomber  between liquid solid medium,O 3-produce in laboratory.
• Adhisement: There’s a pair of electric-delive-needle in the package
• Intervolmeter   of Electric-Delive::50mm~60mm(More than two chips of electric-flashes)
  
• Surely put your hands away from the electric-delive-pole.

Safety should be a minor issue with these coils.  While the discharge feels mildly unpleasant and would likely elicit tears and screams from children, this is not Abu Ghraib material.  A more serious potential issue is the power cord; it is possible to yank it out of the chassis due to inadequate strain-relief.  I made a knot in the cord on the inside of the chassis to prevent this occurence.  Would make a great gift for kids of any age who have a budding interest in electromagnetism or x-radiation.

Aesthetics are pretty sorry.  The galvanized sheet-steel chassis has an uneven paint job and an ill-fitting lid on the bottom that fastens in with four sheet metal screws.  If you want a beautiful induction coil, break open your piggy-bank and visit the artisans at PV Scientific Instruments.

Some Induction Coil Links:

• A 1900-vintage text on induction coil construction and repair
• Kurt Schraner’s site, featuring his large homebrew induction coil, experiments, and theory