Project Documentation: Tesla Coil Soft Start Safety Box

The Tesla coil Soft Start Safety Box project has been finished. The first version of my soft start box for 3 phased 400 VAC loads was just that, a soft start box. I wanted to include safety features like fault …

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Tesla Coil Soft Start Safety Box

Published: July 15th, 2022.

Introduction to the Soft Start Safety Box

Emergency stop is the most important part of any electrical machine or electrical circuit, that can fail in a dangerous state where its possible to harm either humans, animals or materials. The Tesla Coil can be a dangerous electrical circuit, if there is a failure or someone without knowledge crosses a safety barrier.

The soft start to pre-charge the DC bus capacitors is wanted due to the inrush current a empty capacitor bank has. A empty capacitor present itself as a short circuit when charging is first started, this can make many mini circuit breakers trip or even blow fuses. Slowly charging them through power resistors to limit the charge current and get some charge in, before full mains current is available to the load, makes for a very easy startup sequence.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!

Considerations on safety

The first version of my soft start box for 3 phased 400 VAC loads was just that, a soft start box. I wanted to include safety features like fault detection and emergency stop, but I also wanted to remain galvanically isolated from the mains supply, as it is the case with the interrupters and controllers that used a fiber optical link.

First I want to design a fiber optical link with a encoded signal, so that only a specific signal would make the safety circuits resettable. Secondly I also want it to be tolerant to failures, miswiring and if there is a fail, it should fail to a safe state.

Industrial safety relays are widely available and presents a lot of the wanted features, except fiber optical connections, at least at a affordable price. I had a Siemens Sirius 3TK2822/23 safety relay from machine teardowns and that can be used as a emergency stop device according to DIN EN / IEC 60947-5-5. Depending on the external circuitry, a maximum performance level PLe/Cat. 4 in accordance with DIN EN ISO 13849-1 or SILCL 3 in accordance with DIN EN/IEC 62061 can be achieved with this device. More about its detailed functions with the schematic.

Specifications of the Soft Start Safety Box

Power rating3x 400 VAC at up to 32 A
Safety componentsSiemens Sirius 3TK2822
2-channel safety relay
Input connections1x 3ph 400V 16A CEE
1x 3ph 400V 32A CEE
1x 1ph 230V 10A Control power
1x Fiber optical input
Output connections1x 3ph 400V 16A CEE
1x 3ph 400V 32A CEE
3x 1ph 230V 16A IEC
2x 1ph 230V 10A Socket
4x 1ph 230V Control power
Soft Start Safety Box Specifications

Schematic of emergency stop circuits

My largest challenge in this project was to make a fiber optical link that has a high fault tolerance and will fail in a safe state. I quite quickly set on make a fixed oscillator running at 666 Hz, but that could be virtually any signal frequency for the type of band pass filtering I wanted to use on the receiver. The schematic for the 666 Hz oscillator can be found in part 7 of the DRSSTC Show Controller project about the safety features. I tried my luck with very simple band pass filters like RC filters and discrete logic, Butterworth 2 Pole Bandpass Active Filter and a few other op-amp filters found on google. None of these turned out very stable and I could always get them to fail in a undesired state. Band pass filters are not designed with safety circuits in mind 🙂

I found the inspiration for what I needed, in the “Designer’s Encyclopedia of One-Shots” [1] from Fairchild Semi. On page 26, figure 16, a simplified band pass filter circuit is shown with very little details. Two low pass filters are used in a clever way along with proper timing to create the band pass window. I also used the Texas Instruments guide “Designing With the SN74AHC123A” [2]. The band pass filter output is evaluated by two independent LM331 op-amps that each has their own galvanically isolated relay to switch the safety relays monitored channels.

The safety relay have two instantaneous redundant enabling circuits with NO functions. The enabling circuits are for the safety-related deactivation of the mains supply and charging actuators. The safety relay is configured as “Monitored start” mode which is in accordance to DIN EN / IEC 60204-1.
When the emergency stop button is released and the reset button is pressed, the internal circuit of the safety relay and the external contactors are monitored to ensure that they are functioning correctly. With the safety relay, the ON circuit Y33/34 is checked for short-circuit. That means it is identified as a fault if Y33/34 is closed before the emergency stop button is reset. The devices also monitor the sensor circuits for cross-circuits. This is done based on various potentials in the sensor circuits.

Construction of the Soft Start Safety Box

June 12th, 2022

The enclosure parts was cut to fit and all holes for plugs and switches are made. All components and wiring is harvested from the old soft start box and reused as much as possible. All wiring in the main switching loop (CEE 32A) is made out of 6 mm2 machine tool wire and for the CEE 16 A there is used 2.5 mm2. The control input 230 VAC voltage is run through a EMI filter and feeds the 24 VDC power supply as well as the control power output plugs.

The optical safety receiver PCB with its add-on relay output PCB.

June 18th, 2022

Labels have been added to all plugs, switches and indicator lamps. Everything needs to be visually self-explaining to the operator. The soft start safety box is finally tested with the optical safety transmitter located in the 6 channel Tesla coil show controller.


The soft start safety box has been field tested with a total run time of 3 days, during a show. The start up procedure can be done gradually and checks can be made in a way that leads from emergency stop test to full power in a series of steps. The safety features bring in a safer way of working, as it enforces a certain sequence of steps and is constructed to combat wrongful operation.

The emergency stop was tested daily before each show and it was used once during a secondary coil failure. Works 100% as designed.


Design, construction and test video of the Tesla Coil Soft Start Safety Box.


[1] Fairchild Semi: Designer’s Encyclopedia of One-Shots (AN-366: May 2001)

[2] Texas Instruments: Designing With the SN74AHC123A (AN: SCLA014, October 1999)

3 Phase 400 VAC 6 A Variac


I bought this 6 A variac stack as advertised for 6A. But the physical size suggests it could be a 10A model as well. There is no part numbers or data shields to go by. Variacs are usually rated to be able to handle 200% overload for up to 5 minutes and 300% overload up to 1 minute.

As the lids and housing was missing, it needed to get built into a new enclosure.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!


The variacs are only rated for 220 VAC, so star connection of the 3 variacs for 3 phased 400 VAC is needed. If the variacs was rated for 400 VAC each, they could have been connected in triangle.

There is no feedback of the real voltage and current measure, only the values of the dial. Since it is rated for 220 VAC and there is 230 VAC from phase to neutral in Denmark, there will be a difference. Measurement of all 3 phase-to-phase voltages and currents will be installed in the top panel.


Variac3x Transpower Vario Transformer. 0 – 220 VAC at 6 A
ConnectionStar connection for 3×400 VAC input and 0-400 VAC output
MetersAD101-22VAMS combined AC50-500VAC / 0-100A meter


Mains voltage schematic with 6 A variac star connection, voltage measurement and current transformers are shown.

6 A Variac Stack Construction

24th September 2016

I reused a old enclosure from a 20 kVA UPS system that I had from the junk yard. I had to saw it up in smaller pieces, to bring down weight and get a smaller enclosure.

6th May 2022

A good 5½ years have passed where this project was on hold due to moving, family, job and all those kind of things 🙂

In the meantime I had ordered some combined digital voltage and current meters from ebay. It is a AD101-22VAMS front panel meter with a 22.5 mm mounting hole. It is self supplied through the voltage measurement, why it will not work below 50 VAC. It is rated for 50-500 VAC and 0-100A. The wiring is pretty straight forward, as there is only the voltage and current measurements.

The variac was tested with a 3×400 VAC 9 kW space heater. With the variac set for 400 VAC, the current draw was around 12 A. this gives a total 3 phased power consumption around 8.2 kW.

The finished variac has its wire harness, front panel and one side with cable feedthroughs. The remaining four panels will be made once I have sourced some scrap yard materials in the right size.


The enclosure could be a few centimeters smaller, to bring down overall weight. The reused frame is quite heavy as its a pure steel frame made to carry a much heavier load. But it was free and repurposing is much better for the environment than melting and production of new materials.


6th May 2022

Construction, explanation and test video.

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