Joan (of Arc) Battery Powered DRSSTC

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.



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


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.


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.


Joan Sparks