Joan (of arc)

Joan is a portable DRSSTC similar to Sprite but with many improvements.
These include:-
Microprocessor controlled.
Full Bridge Driver
Primary Current feedback
Computer Control via RS232.

Primary: 4T 5mm copper pipe.
Secondary: 200mm tall, 1500T enameled wire former 55mm diameter. wound with 0.125mm Wire
Topload: 40mm x 400mm Toroid.

Joan runs from a pack of 10 NMH AA cells giving a 12VDC supply. This is fed into a small inverter to give 350V DC bus voltage to the Bridge.

The Coil is controlled by a PIC 16F690 micro controller. This controls:- Burst Length, Burst Repetition rate, Number of Bursts
This Controller also allows the Coil to be fully controllable via RS232 from a PC (see opto-isolator below).

The controller also has the ability to implement Over Current Detect (OCD) by using the micro's onboard comparator, but at the moment I am having severe noise issues, so this feature is disabled.

There are two push buttons for controlling the coil, one is a "mode" button and allows the selection of a set of preprogrammed burst lengths, burst rates and burst repetition lengths.

The operation of these buttons can be programmed and stored from the RS232 interface.

Microprocessor

The coil is controlled by a PIC 16F690

This allows the usual DRSSTC functions (including the interrupter) to be achieved in only 4 Chips.

The two chips in the top right hand corner were a pair of microchip drivers that would not supply enough current and are now replaced by a pair of IXYDD414 drivers on the small board bottom right.

The board takes its input from two push buttons for control and from a CT to give OCD.

The variable is to set the max OCD current level via the onboard comparator. At present this is disabled due to noise issues.

There is a 6 way header at the bottom of the board to allow for in circuit programming of the microprocessor.

The Main Circuit Diagram

Inverter

12V is not enough for a decent spark !

So the 12v from the battery pack is inverted to 350VDC by a SG3525 PWM chip driving a pair of IRF460 MOSFETS in push pull.
The output from these goes to a single 25mm Toroidal ferrite.
This Ferrite ring has 3t-0-3t for the primaries and 125turns on a secondary. The output is then rectified and stored in a 400uF capacitor under the bridge. The whole inverter runs at 50Khz and is voltage controlled to save battery life. When the capacitor is charging the inverter provides around 80-90W of power.

Inverter Circuit Diag.

Bridge and GDT

The bridge is made up of 4 x STGW30NC120HD 1200V 30A (135A pulsed) IGBT's. I am of course pushing these up to 200A with very short burst lengths.

You can also see on of the pair of CT's to the left, One gives a phase signal to the driver and the other one is for over current detect.

The main reservoir capacitor is under the bridge and is bypassed with a 0.1uF @1Kv.

The drive is via a gate drive transformer which has one 12T primary and four 18T secondary's this ensures that I get a solid +/-15v gate drive which is clamped by 15V zenners on each IGBT gate.

Primary Cap, Ct's and Feed

The Primary is fed via a 0.4uF capacitor made from an assortment of smaller caps, this allow me to tune the coil by adding or removing capacitance.

The feed to the primary is via a set of double sided PCB "feeders" which reduces the out of circuit impedance.

A close up of the primary feed can be seen here

Complete Unit Inside

This is how the whole lot fits together. into a single case 300mm long, 200mm wide and 55mm deep.

There is now the addition of a charging socket on one side so I can charge the coil without taking the batteries out.

Opto-isolator Tesla Coil end

This box is powered from Joan itself and is a plug in extra.

It allows a degree of isolation (200mm worth) from the tesla coil itself so I can safely attach a laptop to program or control Joan

On the Transmit side there is a single transistor that takes the PIC's output and drives a high speed transmitter LED.

The receive side takes the signal from the opto-diode and amplifies it with a single HEF4049. This is then fed directly into the PIC micro controller.

Opto-isolator PC end

This is the PC-Side of the opto-isolator. As the (cheap) RS232-USB dongle I use only supplies 5V, I need to have a small battery to give the current for the transmit LED. this is achieved with a single transistor.

On the receive side, to ensure the isolator is compatible with "real" RS232 the signal from the opto-diode goes via a HEF4049 amplifier and a MAX232 chip to give +/-12V to the PC.

Both units together will give a throughput of 19.2Kbd.

Sparks

Joan will produce 12"+ arcs to air using a high repetition rate (1Khz) over 10 bursts. Unfortunately at this rate the capacitor needs to be re-charged completely which can take 2-3 seconds.

Using variations of different burst lengths, repetition rates and number of repetitions smaller discharges can be maintained with faster recharge rates.