Nuts and Volts - March 2008

BY KARL P. WILLIAMS

change in position of the armature and the current flowing through the coil.

The force applied to the armature will always move it in a direction that increases the coil inductance. These systems are very quiet when projectiles are fired at velocities lower than the speed of sound, are clean, and require little maintenance. More advanced coil launcher designs incorporate a number of accelerator coils switched in sequence as the projectile moves down the barrel.

The multiple coil design is intended to maximize projectile velocity. The major problem with electromagnetic weapons at the moment is the huge amount of energy lost when converting the electrical energy into kinetic energy.

www.thinkbotics.com/military.htm

Circuit Theory

Project Overview

This article will describe the general construction of the electromagnetic coil launcher shown in Figure 1. The EM- 15 coil launcher is a hand-held, battery powered ( 12 VDC) rifle that is capable of launching a . 30 caliber metallic projectile at adjustable velocities. This is a great project to explore a number of analog electronics concepts.

The electronic circuit consists of a voltage step-up transformer converter, a Cockcroft-Walton voltage multiplier cascade, a capacitor energy storage bank, a voltage comparator to set the charge voltage on the capacitor bank, an SCR switching section, and an accelerator coil. Other components of the launcher are the barrel, breech loading mechanism, battery supply, control panel, display, projectile, pistol grip with trigger assembly, and an aluminum stock that contains all of the components.

The design, construction, and operation of the transformer used in this project is explained step-by-step since it is a key component that is often overlooked in articles and books concerning high voltage. When the launcher is completed, it will be calibrated and fired. Be sure to watch the video of it in action at

The EM- 15 coil launcher schematic diagram is shown in Figure 2. The inverter section of the circuit produces a high frequency, high voltage using an oscillator configuration consisting of transformer T1 being switched on and off by transistor Q1. When power is applied to the circuit by switch S1, resistor R2 initiates transistor Q1 to turn on and conduct a current of 12 volts DC through the primary winding ( 10 turns) of the transformer. The current passing through the primary winding induces a magnetic field in the iron core causing it to produce a current in the secondary (500 turns) and feedback (eight turns) windings. The feedback voltage holds transistor Q1 on as the current flows through resistor R1 and capacitor C2. Resistor R1 and capacitor C2 control the base current and operating frequency of the oscillator.

When the core of the transformer saturates, the induced base voltage goes to zero and turns the transistor off. The magnetic field in the ferrite core then collapses and produces 600 VAC in the secondary windings of the transformer. At this point, the transistor turns on again and the cycle repeats.

The high voltage AC output from the secondary winding of the transformer is doubled and rectified to 1,200

VDC by a Cockcroft-Walton voltage multiplier made up of diodes D1, D2, and capacitors C3, C4. The DC output voltage from the voltage multiplier charges the capacitor bank through the accelerator coil L1, to a voltage that is determined by IC1, a 741 operational amplifier configured as a comparator.

The Cockcroft-Walton voltage multiplier is an interesting device that was named after Douglas Cockcroft and Ernest Walton. In 1932, the scientists used this voltage multiplier cascade design to power a particle accelerator and perform the first

artificial nuclear disintegration in history. The two eventually won the 1951 Nobel Prize in physics for “Transmutation of atomic nuclei by artificially accelerated atomic particles.” The voltage multiplier device was actually discovered earlier, in 1919, by a Swiss physicist named Heinrich Greinacher. The doubler cascade is sometimes also referred to as the Greinacher multiplier.

The capacitor storage bank is comprised of 10 1,500 μF, 200V capacitors configured to achieve 600 μF, 1,000V (C8–C17). These capacitors are available at most electronics supply companies. When the capacitor bank is charged to 800 VDC, the amount of energy that will be switched to the accelerator coil is 192 joules. With the capacitor storage bank charged to 1,000 VDC, the amount of energy is 300 joules. The capacitor bank should only be charged to 1,000 volts if you have installed an SCR that can handle it.

The 741 operational amplifier (IC1) is configured as a voltage comparator and is used to set the amount of voltage charge on the capacitor bank. The reference voltage for the comparator is taken directly from the 12 volt DC source through resistor R10. The voltage charge accumulating on the capacitor bank is dropped down to a value of approximately 1: 20 through a voltage divider made up of resistors R3, R4, and 100K potentiometer R11, and is then connected to the comparator. The potentiometer is used to set the exact voltage level on the capacitor