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For our project we chose to build and analyze a coil gun with the schematic shown in
Figure 1-A:
Figure 1-A
This design was chosen because it incorporated the zener voltage regulation and transistor
switches that were discussed in class, while at the same time being simple enough for us to
fully analyze. We also took advantage of Multisim to simulate the circuit.
A coil gun is a device that pushes a high current through a coil for a couple of
milliseconds to induce a magnetic field onto a ferromagnetic projectile. Electromagnetic
theory states that an electric field creates a magnetic field. This circuit takes advantage of this
property by using a strong current to create a powerful magnetic attraction between the field
in the coil and a ferromagnetic materiel. This attraction will create a strong force for an
instant that will accelerate the material out of the firing chamber as a projectile. Tests show
that this device is not very energy efficient, but still works as a proof of concept. The coil's
main components are the 36V power source which powers the entire unit, the 37.6 mF
capacitor bank which holds the charge, and the coil which induces the magnetic field.
The user controls the operation of the device with a DPDT switch, a physical switch
that controls the connections as shown in Figure 1-B:
Figure 1-B (DPDT Switch)
The component divides the operation of the circuit into three stages. The first stage,
called the charging stage, connects the voltage source to two branches to move current into
the capacitor bank and the indicator. The next stage is the holding stage, which disconnects
the capacitor bank from the rest of the circuit and holds the stored bank voltage until the user
is ready to fire. The final stage is the firing stage, which connects 9 volts to the Silicon
Controlled Rectifier (SCR) thyristor, which shorts across the coil to the capacitor bank and
releases the built up charge as current through the coil.
The 36 V power source can be achieved through four 9 V batteries, but testing shows
the batteries will need to be replaced after a few shots. Alternatively, a combination of wall
adapters that achieve a combined 36 volts can be used instead so battery replacement will not
be an issue. Because the capacitors are rated at 35 V, and the voltage supplied by both wall
adapters and batteries tend vary several volts above and below what they are rated, a certain
amount of voltage needs to dropped between the capacitor bank and the voltage source for
safe operation.
A first intuitive response to do this would be to use a resistor to drop the voltage, but
Figures 1-C and 1-D show that as voltage across the capacitors increases, the voltage
difference between the bank and source will decrease over time, causing the current across
the resistors to fall.
Recall that Ohm’s Law states that the voltage dropped across a resistor is dependent
on the current through it. This means that if the current in the resistor is decreasing, the
voltage being dropped over it will also decrease until it becomes so negligible that eventually
the source voltage will equal the voltage in the capacitors. The circuit needs a constant
voltage regulator that is unaffected by the changing current between the source and bank. The
most reasonable solution is to use a zener diode which drops a constant 5 V in series with the
four 9 volt sources giving us a safe range of 31 to 35 volts, perfectly within the operation of
the capacitor, to make the power supply match the worse case for fresh batteries we made our
testing jerry rigged configuration give out 40 volts of charge.
The process of moving current from the source to the capacitors is split into two
braces one through the 100 Ohm resistor and another through the LED (with an estimated
operation current of 5 mA) and a 2.7k Ohm resistor The resistance prevents excessive heat
from a short and controls the current so the LED will not be damaged. The 100 Ohm branch
just charges the capacitor, the one with the 2.7K and LED powers our indicator. The indicator
moves a small amount of current through the LED, indicating that the capacitors are still
charging. As the voltage between the capacitor bank and the voltage source decreases, the
current will be so small that it can no longer turn on the LED, indicating that the capacitors
are fully charged, thus completing the charging stage.
The charging stage uses the capacitors to store electrical energy that will eventually
be transferred into the projectile as kinetic energy. The charge is stored within eight 4700 mF
dielectric (must be used in accordance to polarity) capacitors for a total capacitance of 37.6
mF. At 31 volts these capacitors deliver 1.1656 coulombs of charge and 18 joules of stored
energy. Not all this is turned into kinetic energy as a projectile. According to the Multisim
simulations as shown in Figure 1-E , the total time of discharge would be 5 milliseconds with
a peak amperage of about 510 A. All this will pass through the SCR and coil.
We use a Silicon Controlled Rectifier to direct when current will flow through the
coil. An SCR thyristor is a 4 layer semiconductor, and can be logically represented as two
BJT transistors, with their base and emitters hooked together. Note how in Figure 1-F, the top
transistor is a pnp and the bottom transistor is a npn, though outside of design under
specifically frequencies it can be the other way around without affecting its function.
(c)
The SCR thyristor works as voltage controlled current gate, which allows current to
flow between the anode and cathode when a voltage is applied to the gate and cathode. When
the voltage is applied, the diodes turn on and current is allowed. Even if this trigger voltage is
removed from the gate and cathode, the current is still allowed through until the current goes
back to zero ampere. We use an SCR instead of a physical switch or BJT because of it’s
ability to handle the tremendous current, which could cause a regular transistor or switch to
be severely damaged. The SCR of our design calls for it to be specifically rated to be able to
handle the voltages and current within our design as you can see in Figure 1-G its datasheet
lists its peak surge current for a 10 milisecond pulse, comparing this to our multisim
simulation seen in figure we can see that it is within the specifications the design.
Figure 1-G
The SCR has three relevant pins for operation, an anode, a gate, and a cathode. The
SCR thyristor is rated to turn on when gate-cathode voltage is 9V, and must be switched on to
allowed the stored charge in the capacitor bank to flow through the solenoid. The power
source is divided so that nine of the 36V source can be applied to the SCR, throwing the
circuit into the firing position. This discharges all 31V in our capacitor bank over 5
milliseconds through our thyristor and into the coil. The SCR will stay shorted until current
no longer flows through the anode and cathode, so even if the trigger voltage is applied for
less than 5 milliseconds, the SCR will still say on until the capacitors are fully discharged.
This is useful for creating an energy efficient firing stage. Once set in the firing stage the
SCR will allow current to flow through our coil.
The coil is uses the resulting current to induce a strong magnetic field for our
purposes. The coil is made of four meters of 22 AWG copper wire with the estimated
wrapped turns being 181 and the length of the coil being two cm. The coil is effectively an
inductor with an inductance of 79 µH. This coil converts the electrical energy from the
capacitor bank into kinetic energy. This whole process however is not 100% efficient. If we
take the amount of energy in the capacitor bank (18 Joules) and use it to solve backwards
1
with the kinetic energy equation (18 J = 2mv2) with the estimated mass of the nail being one
gram we would find that nail would travel at 189 meters per second , which goes far beyond
what testing the circuit indicates. Possible ways we can improve upon the current coil design
would be to use a smaller diameter barrel and a shorter coil with a thinner wire with more
turns in order to increase the density of the field generated.
In conclusion the coil we analyzed was a simple yet effective design that utilized the
zener diode and semiconductor switches that we learned about in class. Though we did not
necessarily cover thyristor in class we were easily able to use concepts taught in class to
analyze its function. With this we can prove our skill at apply concepts outside of the
classroom which will carry with us through our careers.