thyristor fired coilgun is a common variant, due mainly to their
high voltage and pulsed current capabilities. A diagram of the system
that was designed for these experiments is given in fig 1.
1. Thyristor fired coilgun circuit
thyristor is an International Rectifier device, part number 50RIA100
(50 A / 1000 V). The pulsed current capability is 1200 A for a half-sine
pulse of 10 ms.
series of experiments investigates the effect of charging voltage
and launch delta on the projectile velocity and coilgun efficiency.
The value of capacitance that can be effectively employed in this
type of coilgun is limited due to the fact that the thyristor cannot
be turned off once it has begun to conduct current. This requires
that the current pulse be of similar duration to the projectile
transit time, otherwise suckback will vastly reduce the muzzle velocity.
In view of this only the 13,600 uF and 33,000 uF capacitors are
current sensor was placed in the capacitor ground return path to
monitor the capacitor discharge, or in the coil current path to
record the full coil current pulse. Placing the sensor in the coil
current path means that the measuring equipment (PC 'scope and laptop)
floats up to some arbitrary voltage since no part of the system
is grounded to the earth potential. The charging voltage was limited
to 80 V for these measurements.
switches were considered for commutating the trigger pulse; a mercury-wetted
relay and a microswitch. Unlike most mechanical contact switches,
the mercury-wetted relay doesn't exhibit any contact bounce, so
the gate current will be maintained without interruption during
the critical turn on phase. Figure 2 shows the rising edge of the
trigger pulse. Fig 3 shows the initial part of the trigger pulse
voltage that is applied to the gate resistor, and fig 4 shows the
2. The mercury relay pulse has a 10 % - 90 % rise time of around
3. The microswitch's first contact period is about 10 us.
4. The microswitch exhibits plenty of contact bounce.
intermittent trigger pulse produced by the contact bounce is probably
not a problem when the A-K current pulses have low di/dt, such as
those typical of low power coilguns. If, however, the thyristor
is going to be operated close to its peak rated di/dt then the nature
of the gate pulse needs to be examined in more detail, otherwise
the device might fail thermally due to the excessive current density
brought about by insufficient gate charge diffusion.