Solid state Tesla coils on the Tesla list, 1995 to 1996

The list was talking about transistor coils well before the message this card is built on. On 16 Jan 1995 Mark Conway (New Zealand) describes the solid-state coil he built from Duane Bylund's design in the 1993 Electronic Experimenters handbook. He found 120 kHz by twiddling a pot. It is the earliest such post in the archive. On 6 Feb 1996 Mark Barton sums up his own coil again, having posted on it several times before (the card below). Alan Sharp's solid-state posts start on 8 Mar 1996, aiming at 150 V and 500 W into a tightly coupled primary or a bottom-feeding transformer. In June 2004 Dan McCauley of Eastern Voltage Research posted a history of these firsts to the list and asked for corrections. It lists Bylund's bipolar driver, Sharp's MOSFETs feeding the base through ferrite and Barton's full bridge on the primary. None of the replies disputed them.

Sharp's is the fullest early description in the archive. On 24 Sep 1996 Alan Sharp (UK) describes a push-pull driver with three IRF740s a side on a UC3825 and TC4422 gate buffers, feeding a 15:1 ferrite transformer into the base of a magnifier coil. A 555 gates it on and off at about 60 Hz. Current limiting comes off a 0.05 ohm resistor that gives him “weird feedbacks”. He gets a 14 in white arc at 140 V and 420 W from a secondary of about a thousand turns of 0.4 mm wire on a 13 in home-brew bucket, and plans six FETs a side. In passing he mentions Mark Barton's 10 kW full-bridge “Big Fet” and its 3 ft sheet of flame (“of course it blew up”), which Barton had described on the list himself that February. The next day Harri Suomalainen replies from Helsinki University of Technology, the same hut.fi that hosted Jan Wagner's pages a few years later. He is planning his “first self-tuning resonant driver for coiling” around the same chip, sensing coil current so it tracks like a zero-current-switching supply. So interruption, current limiting and self-tuning were all being discussed within one week of 1996. How much of the later work ran in parallel on this list is not known. Anders Mikkelsen, who pointed out these two messages, was not active there, so the archive is the only record.

The printed starting point is Duane Bylund's article in Radio-Electronics of September 1991, and his book. Sharp's page and Rob's circuit both name it. Rob, N3FT, an American ham, drew a “MOSFET current mode resonant coil driver”, a 375 V half bridge on a UC3825 built into a gutted 500 W switching supply. Sharp hosted the hand-drawn schematic for him. Sharp's page also quotes Mark Barton's “Sheets of Flame” from Bill Beatty's site: a FET H bridge at 80 kHz, 10 kW continuous and about 40 kW in 100 Hz bursts, making a 4 ft sheet of electrical flame. Sharp's own CompuServe page, archived in October 1999, has the coil from the 1996 message in its later form, on the card below. Steve Conner's site thanks Alan Sharp by name, and the two later built a coil together.

Schematics and photos

schematic: Rob N3FT's resonant coil driver, hand drawn

Sources

This is the card from the driver tree, written from what the builder published. Send corrections through the forum.