Create Your Own Spark

Objective

The goal of this project is to investigate induction coils by creating an apparatus that can generate high voltage sparks.

Introduction

STOP! Important Note About this Project – Please Read this Before Attempting this Project.

There is currently an issue with the procedures on this project. The high-current switching relay (e.g., Radio Shack part number 275-217) listed in the materials section does NOT currently work for this project. Science Buddies has searched for a replacement part but we have been unable to find something that would work with the current design. While the design presented here does not work, the concept of this project is still viable, so someone who is extremely knowledgeable and comfortable building a neef vibrator from scratch could come up with a working design.

Danger — This Project Requires Adult Supervision!

STOP! Before you embark on this project, please be aware that there is currently an issue with the procedure! The high-current switching relay (e.g., Radio Shack part number 275-217) listed in the materials section does NOT currently work for this project. Science Buddies is in the process of searching for a replacement part for this project, and will ammend the procedure and materials list ASAP. Also note that this is a VERY advanced project suitable only for students very comfortable working with electronics, or who have a strong mentor to help them with safety issues. Disclaimer: This project involves high voltage electricity and generation of sparks and heat. You should not attempt this project unless you are comfortable with basic concepts of AC and DC electricity, induction, and reading circuit schematics. You and your adult supervisor are responsible for your safety when doing this project! Safety note: adult supervision is required for this project. Read and follow these High-Voltage Safety Precautions: The induction coil generates HIGH VOLTAGE. Always disconnect the battery before making adjustments to the circuit. The circuit is designed with a series of five current-limiting resistors in the secondary coil. Without these resistors, a spark jumping to the body would cause serious injury and could potentially be fatal. Never touch, adjust or approach the spark gap when the circuit is powered. Although the spark current is limited, a spark jumping to the body would still be very unpleasant. The sparks generate intense heat in the air between and around the electrodes. Never operate the spark coil in the presence of flammable substances. The sparks also create ozone, so operate the spark coil with adequate ventilation.

Air normally acts as a good insulator (or dielectric). However, when the voltage across an air gap becomes sufficiently high, electrons are stripped from the air molecules, ionizing the air and allowing current to flow. This is what is happening when lightning strikes during a thunderstorm. The process is called dielectric breakdown, and the voltage at which it occurs is the dielectric breakdown voltage.

The dielectric breakdown voltage for air is approximately 3000 V/mm (= 3 kV/mm), but also depends on other factors such as the geometry of the gap and the air pressure.

You can create your own high-voltage spark generator using a homemade induction coil apparatus with one or two lantern batteries supplying the power. Figure 1, below, shows a diagram of how the circuit works.

Figure 1. Schematic diagram of spark-gap induction coil circuit.

The idea here is to have two coils: a primary coil with few turns of thick wires wrapped around a soft iron core, and a secondary coil with many turns of thin wire wrapped outside the primary coil. The two coils are insulated from one another with PVC tubing (not shown). Current is induced in the secondary coil when current in the primary coil changes. A battery, as you know, supplies direct current (DC). Thus, the secondary coil would only have current induced when the primary coil was first turned on, or just after it turned off. With steady DC current in the primary coil, there is no induced current in the secondary coil.

The "interrupter" (also called a Neef-type vibrator) in the circuit diagram functions as an automatic switch, rapidly turning the primary coil current on and off. Here's how it works:

• when the momentary-contact switch is closed, current from the battery flows through the primary coil;
• this creates a magnetic field around the core, which both induces current in the secondary coil, and attracts the hammer of the vibrator;
• as the hammer moves toward the core, it's electrical contacts are opened, cutting off the current to the primary coil;
• this reverses the induced current in the secondary coil, and allows the hammer to spring back;
• when the electrical contacts on the hammer close again, the cycle repeats.

The two megohm resistor in series with the secondary coil serves to limit the current in the coil. The lantern battery is capable of delivering 3.5 A of current. If a high-voltage spark carrying that much current were to jump to the body, it would cause serious injury, and could potentially be fatal. The resistor limits the current in the coil so that the sparks produced are not dangerous. Still, it would feel very unpleasant to have them jump to your body, so keep a safe distance from the secondary coil electrodes when the coil is powered. Note: you'll actually use five 470 kΩ, 2-Watt resistors in series. For a single 2 megohm resistor, you would need a power rating of 4.5 Watts, which is hard to find.

There is one additional component in the diagram that we have not yet mentioned: the capacitor or "condenser". (Condenser is the old terminology for a capacitor. You may run across it in articles about spark-gap coils.) The capacitor is connected in parallel across the vibrator hammer's contacts. Adding a capacitor has the effect of reducing unwanted discharge at the vibrator points and increasing the intensity of the secondary spark. Essentially, the capacitor blocks DC current but allows AC current (above some frequency) to pass. By providing a path for high-frequency AC current through the primary coil, more energy is transferred to the secondary coil.

In this project there are a number of variables you could choose to investigate, for example:

• the number of turns on the secondary coil,
• the value of the bypass capacitor,
• the width of the gap between the high-voltage spark contacts,
• the shape of the high-voltage spark contacts,
• the voltage on the primary coil (6 vs. 12 volts).

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• induction coil,
• spark gap,
• dielectric,
• dielectric breakdown voltage.

Bibliography

• The following Wikipedia articles are a good place to start:
  • Wikipedia contributors, 2006. "Induction Coil," Wikipedia, The Free Encyclopedia [accessed May 8, 2006] http://en.wikipedia.org/w/index.php?title=Induction_coil&oldid=47897287.
  • Wikipedia contributors, 2006. "Spark Gap," Wikipedia, The Free Encyclopedia [accessed May 8, 2006] http://en.wikipedia.org/w/index.php?title=Spark_gap&oldid=49741671.
• For historical information on induction coils, see:
  • Greenslade, Jr., T.B., date unknown. "Induction Coils," Historical Physics Teaching Apparatus, Kenyon College [accessed May 8, 2006] http://physics.kenyon.edu/EarlyApparatus/Electricity/Induction_Coil/
    Induction_Coil.html.
  • Jenkins, J., date unknown. "Induction Coils," John Jenkins Sparks Museum [accessed May 8, 2006] http://www.sparkmuseum.com/INDUCT.HTM.
• This link contains detailed instructions on building a spark-gap coil from an article by Willard Doan that appeared in "Experimental Electricity for Boys," published in 1959:
  Carusella, B., 2006. "Construction of a Secondary Induction Coil," Bizarre Stuff You Can Make in Your Kitchen [accessed May 8, 2006] http://www.bizarrelabs.com/ind1.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• one 5/8 in diameter iron carriage bolt, length between 4 and 6 in (hardware store);
• 36 in 12-gauge bare copper wire (hardware store);
• 1/4 lb. spool, 26 gauge (or higher) enameled magnet wire (part number GU-QPM26, Action Electronics, http://www.action-electronics.com/magnetwire.htm;
• 4 to 6 in length of PVC tubing (open at both ends) to fit over primary coil and carriage bolt; this serves as an insulator between the primary and secondary coils (hardware store);
• wax paper (for insulation between layers of secondary coil);
• 1.0 μF 250 V metal film capacitor (e.g., Radio Shack part number 272-1055);
• 0.01 μF 50 V capacitor (e.g., Radio Shack part number 272-1065);
• 0.1 μF 250 V ceramic capacitor (e.g., Radio Shack part number 272-1053);
• high-current switching relay (e.g., Radio Shack part number 275-217); Important! This part currently does not work for this project. Science Buddies has been unable to find a replacement part. Please see the project note at the top of the page.
• two 6 V lantern batteries (e.g., Radio Shack part number 23-016);
• momentary switch, normally open, 3 A or higher (e.g. Radio Shack part number 275-618);
• 5 470 kΩ, 2-Watt resistors (e.g., Mouser (http://www.mouser.com) part number 594-5083NW470K0J),
• soldering iron,
• solder,
• wood (for frame for mounting the components).

Experimental Procedure

Construction of the Spark-Gap Coil

1. The carriage bolt is used as the iron core that will be magnetized by current flowing in the primary coil. It is important to use iron (not steel) for the core. To make the primary coil, wrap 30 in of 12-gauge copper wire around the carriage bolt. (Two 3-in pieces of this wire are reserved for the spark gap contacts.)
2. It is important to electrically insulate the primary and secondary coils. The secondary coil is wound around a length of PVC tubing (not shown in the schematic). The inside diameter of the PVC tubing should be chosen so that the tube slides over the primary coil and carriage bolt. The PVC tube electrically insulates the primary and secondary coils. Note: when mounting the secondary coil over the primary coil, remember that the head of the carriage bolt needs to remain exposed so that the vibrator mechanism can contact it (see step 6, below).
3. The increase in voltage from the primary coil to the secondary coil is determined by the ratio of turns between the two coils. The relationship is described by the following equation:
   
   V_s / V_p = N_s / N_p,
   
   where V is the voltage, N is the number of turns, and the subscripts, s and p, indicate the secondary and primary coils, respectively.
4. In order to generate sufficient voltage to create a spark, the secondary coil should have orders of magnitude more turns than the primary coil. A ratio of 750–1000 is a good place to start. In order not to exceed the power rating of the current-limiting resistor, the ratio of turns (secondary:primary) should not exceed 1000:1. You will need to make a winding mechanism to make this part easier. Wrap as neatly as you can. Be careful not to break the wire. It is recommended that you use wax paper as insulation between each layer in the secondary coil.
5. As shown in the schematic, one side of the secondary coil is connected to the spark gap via the 2 megohm resistor. You will use five (5) 470 kΩ, 2-Watt resistors connected in series. The other side of the secondary coil is connected directly to the other side of the spark gap. This is the high voltage output, so you should be especially careful with its construction. For the spark gap contacts, use two pieces of 12 gauge bare copper wire, each about 3 inches long. Do not use insulated wire. The insulation may get hot and melt. Bend each of the pieces into a semi-circle, and place the tips about 1 inch apart. Make sure that base of each spark gap contact is fastened down tightly to an electrically insulated surface (e.g., a piece of wood).
6. Making the Neef-type vibrator will require some ingenuity on your part. To construct a Neef-type interrupter, you will have to break the relay from its plastic box and use a part of it for your experiment. A relay usually has a two contact plates separated by a hammer and an iron core. Depending on the voltage applied, the hammer can be in contact with either of the plates on its sides, thus completing one circuit and breaking other. For our purpose we don't need the core from the relay, (because we will use the core that we have constructed). We will need the two contact plates and the part of the hammer between them. We will use this contraption such that the hammer remains in touch with one of the contact plates until the magnetic field from the activated core pulls it away, breaking the contact and the circuit. (See Figure 1 in the Introduction for more details.)
7. A DC power supply is used to feed the primary coil. This DC source can be a 6V lantern battery or two such batteries in series. These batteries can supply current in the range of 0.5 and 3.5 amps depending on the adjustment of the breaker points on the hammer.
8. We suggest that you design and build a wooden frame or platform to hold the coil components and the spark-gap contacts firmly in place. The momentary contact switch to activate the coil should be located well away from the spark gap.

Testing and Operation

1. Before beginning, review the safety precautions above. Before making any adjustments, always disconnect the battery.
2. Adjusting the vibrator mechanism.
   1. It is the distance between the two contacts and the hammer in the center that determines the amount of switching in the interrupter. You will need to adjust this. If the separation is too large, the hammer will barely switch and you won't notice a spark. Keeping the hammer too close to the contact will barely break the circuit, and again there will be no sparks. At the correct distance, the hammer will toggle between the two contact plates, and you should hear a constant chatter. Remember: disconnect the battery before making any adjustments!
   2. Start with a single 6 V battery. Turning on the power should cause the coil to produce a strong 1 inch spark.
   3. If there is no spark, turn the power off, disconnect the battery, and change the distance between the two contacts and the hammer slightly. Make your adjustments conservative, the operational range is not very large.
3. Once you get a spark, turn off the coil, disconnect the battery, and make the spark gap about 1/4 inch larger.
4. Then reconnect the battery, turn on the power and check for a spark. Repeat the procedure until the gap is too wide for a spark to pass. That is one simple way to determine the maximum output of your coil.
5. If you have an additional battery, raise the input voltage to 12 V. Repeat the procedure until the gap is too wide for a spark to pass.
6. When everything is just right, the system will deliver a thin intermittent lightning-like sparks close to 2 inches long. These long sparks are best observed in a dark room.

Variations

• What happens if you change the value of the capacitor connected across the contacts of the interrupter?
• If you are very curious you can enclose the spark gap in a transparent, sealable container. You can then investigate the dielectric breakdown voltage for different, noncombustible gases. Air, nitrogen, carbon dioxide, and helium are such gases. Do background research to find out the dielectric breakdown voltage of each gas you intend to test. Will the maximal spark gap distance increase or decrease compared to air? Do an experiment to confirm your hypothesis.

• For more project ideas in this area of science, see Electricity & Electronics Project Ideas

Credits

Written by Mayank Gupta

Edited by Andrew Olson, Ph.D., Science Buddies

Last edit date: 2007-12-00 15:20:00