# Solid State Tesla Coils and How They Work

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## Introduction: Solid State Tesla Coils and How They Work

High voltage electricity can be DANGEROUS, use proper safety precautions at all times when working with Tesla coils or any other high voltage device, so play safe or don’t play.

Tesla coils are a transformer that operates on self resonating oscillator principle, invented by Nicola Tesla a Serbia American scientist. It is mainly used to produce ultra high voltage, but low current, high frequency AC power. The Tesla coil is composed of two groups of resonant circuits coupled, sometimes three groups coupled. Nicola Tesla tried a large number of configurations of various coils. Tesla used these coils to carry out experiments, such as electrical lighting, X ray, electrotherapy and radio energy transmission, transmitting and receiving radio signals.

There really hasn’t been much advancement in Tesla coils since their invention. Other than solid state components Tesla coils haven’t changed much in over 100 years. Mostly relegated to education and the toys of science just about anyone can buy a kit on line and build a Tesla coil.

This Instructable is on building a solid state Tesla coil of your own, how they work, and tips and tricks to trouble shoot any problems along the way.

### Supplies:

12 volt power supply the SMP supply I used was 12 volts 4 amps.

Torus Glue to mount the secondary coil.

Thermal Silicone Grease for mounting the transistor to the heat sink.

Solder

The tools to assemble the kit,

soldering iron and side cutters.

Multimeter

Oscilloscope

### Teacher Notes

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## Step 1: Electromagnet

To understand Tesla coils and transformers you need to understand electromagnets. When a current, (Red Arrow) is applied to a conductor it creates a magnetic field around the conductor. (Blue Arrows) To predict the direction of the of the magnetic fields flow use the right hand rule. Place your hand on the conductor with your thumb pointing in the direction of the current and your fingers will point in the direction of the magnetic fields flow.

When you wrap the conductor around a ferrous metal like steel or iron, the magnetic fields of the coiled conductor merge and align, this is called an electromagnet. The magnetic field travels from the center of the coil passes out one end of the electromagnet around the outside of the coil and in the opposite end back to the center of the coil.

Magnets have a north and a south pole, to predict which end is North or South pole in a coil, again you use the right hand rule. Only this time with your right hand on the coil, point your fingers in the direction of the current flow in the coiled conductor. (Red Arrows) With your right thumb pointing strait along the coil, it should point to the north end of the magnet.

## Step 2: How Transformers Work

How a fluctuating current in a primary coil creates a current in a secondary coil wirelessley is called Lenz’s law.

All coils in a transformer should be wound in the same direction.

A coil will resist a change in a magnetic; field so when AC or a pulsing current is applied to the primary coil, it creates a fluctuating magnetic field in the primary coil.

When the fluctuating magnetic field reaches the secondary coil it creates an opposing magnetic field and an opposing current in the secondary coil.

You can use the right hand rule on the primary coil and the secondary to predict the output of the secondary.

Depending on the number of turns on the primary coil, and the number of turns on the secondary coil, the voltage changes to a higher or lower voltage.

If you find the positive and negative hard to follow on the secondary coil; think of the secondary coil as a power source or a battery where power comes out, and think of the primary as a load where power is consumed.

Tesla coils are air core transformers, the magnetic fields and current works the same way as iron or ferrite core transformers.

## Step 3: Winding’s

Although it isn’t drawn in the schematic; the taller secondary coil of a Tesla coil is inside the shorter primary coil, this setup is called a self resonating oscillator.

Get your winding’s right; both the primary and the secondary winding’s should be wound in the same direction. It doesn’t matter if you wind the coils with a right hand twist or a left hand twist as long as both coils are wound in the same direction.

When winding the secondary make sure your winding's do not overlap or at the overlap can cause a short in the secondary.

Cross winding the coils can cause the feedback from the secondary tied to the transistor’s base or the mosfet’s gate to be the wrong polarity and this can prevent the circuit from oscillating.

The primary coils positive and negative leads are affected by the twist in the winding’s. Use the right hand rule on the primary coil. Make sure the north pole of the primary coil points toward the top of the secondary coil.

Cross wiring the primary coil to can cause the feedback from the secondary tied to the transistor’s base or the mosfet’s gate to be the wrong polarity and this can prevent the circuit from oscillating.

As long as the coils are wound in the same direction; failure to oscillate do to cross wiring the primary coil is an easy fix most of the time, just reverse the leads of the primary coil.

## Step 4: How a Solid State Tesla Coil Works

The basic solid state Tesla Coil can have as little as five parts.

A power source; in this schematic a battery.

A resistor; depending on the transistor a 1/4 watt 10 kΩ and up.

A NPN transistor with a heat sink, the transistor on these circuits tend to get hot.

A primary coil from 2 or more turns wound in the same direction as the secondary coil.

A secondary coil up to 1,000 turns or more 41 AWG wound in the same direction as the primary.

Step 1. When power is first applied to a basic solid state Tesla coil the transistor in in the circuit is open or off. Power goes through the resistor to the transistors base closing the transistor turning it on allowing current to flow through the primary coil. The current change is not instantaneous, it takes a short time for the current to go from zero current to max current, this is called rise time.

Step 2. At the same time the magnetic field in the coil goes from zero to some field strength. While the magnetic field is increasing in the primary coil the secondary coil resists the change buy creating an opposing magnetic field and an opposing current in the secondary coil.

Step 3. The secondary coil is tied into the base of the transistor so the current in the secondary coil, (Feedback) will draw the current away from the transistors base. This will open the transistor turning off the current to the primary coil. Like the rise time the current change is not instantaneous. It takes a short time for the current and the magnetic field to go from max to zero, this is called fall time.

Then back to Step 1.

This type of circuit is called a self regulating oscillating circuit, or a resonant oscillator. This type of oscillator is limited in frequency by the circuit’s and the transistor’s or mosfet’s delay times. (Rise Time Fall Time and Plateau Time)

## Step 5: Efficiency

This circuit isn’t very efficient, producing a square wave, the primary coil only produces a current in the secondary coil during the magnetic fields transitioning from zero field strength to full field strength and back to zero field strength, called the rise time and the fall time. Between the rise time and the fall time there is a plateau with the transistor closed or on and the transistor open or off. When the transistor is off the the plateau isn’t using current, however when the transistor is on the plateau is using and wasting current heating the transistor.

You can use the fastest switching transistor you can get. With higher frequencies the magnetic field can transition more than it is plateaued making the Tesla coil more efficient. However this will not stop the transistor from heating up.

By adding a 3 volt LED to the transistors base it extends the rise and fall times making the transistors action more of a triangle wave than a square wave.

There are two other things you can do to keep the transistor from over heating. You can use a heat sink to dissipate the excess heat. You can use a high wattage transistor so the transistor isn’t overworked.

## Step 6: Mini Tesla Coil

I got this 12 volt Mini Tesla Coil from an online retailer.

The Kit Included:

1 x PVC Board

1 x Monolithic capacitor 1nF

1 x 10 kΩ resistor

1 x 1 kΩ resistor

1 x 12V Power Socket

1 x Heat sink

1 x Transistor BD243C

1 x Secondary coil 333 turns

1 x Fixing screw

2 x Led

1 x Neon Lamp

The Kit does not include:

12 volt power supply the SMP supply I used was 12 volts 4 amps.

Torus

Glue to mount the secondary coil.

Thermal Silicone Grease for mounting the transistor to the heat sink.

Solder

## Step 7: Testing

After assembling the Mini Tesla Coil I tested it on a neon lamp, a CFL (compact florescent light), and a florescent tube. The ark was small and as long as I put it within 1/4 of an inch it lights up everything I tried it on.

The transistor gets very hot so don’t touch the heat sink. A 12 volt Tesla coil shouldn’t make a 65 watt transistor very hot unless you approach the transistors maximum parameters.

## Step 8: Power Usage

The BD243C transistor is a NPN, 65 watt 100 volt 6 amp 3MHz transistor, at 12 volts it shouldn’t draw more than 5.4 amps not to exceed 65 watts.

When I checked the current at start up it was 1 amp, after running for a minute the current dropped to 0.75 amps. At 12 volts that makes the running power 9 to 12 watts, far below the 65 watts the transistor is rated for.

When I checked the transistors rise and fall times I get a triangle wave that is almost always in motion making it a very efficient circuit.

Top loads allow the charge to build up instead of just bleeding into the air giving you a greater power output.

Without a top load the charges gather on the pointy tips of the wire and bleed off into the air.

The best top loads are round like a Torus or a spheres so that there are no points bleeding off the charge into the air.

I made my top load from a ball I salvaged from a mouse and covered it with aluminum foil, it wasn't perfectly smooth but it worked well. Now I can light up a CFL up to an inch away.

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