Plain silicon rectifier
diodes provide a simple means of clamping the inductive voltage which appears
across the terminals of the switching device as it turns off. However, the use
of a single commutating diode leads to an exponential type RL current decay which
has been shown to interact with the projectile as it exits the coil - the current
is 'boosted' at the expense of projectile kinetic energy. This interaction retards
the projectile so the question arises, how can this effect be reduced?
current more quickly should lessen it's effect on the projectile. The time constant
of the current decay can be reduced by increasing the resistance of the commutating
circuit. Another possibility is to connect several diodes in series thereby increasing
the conduction voltage and the subsequent decay rate. Using a series arrangement
of several diodes results in a pseudo-linear current decay. This is due to the
threshold voltage of the diodes - consider the following equation:
each diode demands an induced voltage of e in order to conduct then as
long as energy is available in the field the current will decay at a rate of:
n is the number of series diodes. The actual decay isn't purely linear due to
the exponential behaviour of the diode forward conduction. A more thorough analysis
will be added to the theory section as time permits.
This series of
experiments explores the effect of multiple series commutating diodes. In order
to quickly connect several different numbers of diodes together I constructed
a simple diode array which can be configured to give 1-10 diodes in series. The
diodes are 1N5401 silicon rectifier diodes with a pulsed current rating of 200A
(8ms duration). Fig 1 shows the array.
1. 10 segment diode array.