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
■ FIGURE 1. EM- 15
electromagnetic coil
launcher.
March 2008 51