Diode Commutation
Results
Coil A
The
graph of fig1 shows the difference in the commutating current decay for 1 diode
and 10 diodes.
Fig
1. Difference in commutating current decay.
The
decay time for 10 diodes is around half that of 1 diode. This should have a pronounced
effect on the resulting muzzle speed.
The
following graphs show the effect of the commutating diode arrangement on the muzzle
speed and energy. Five speed measurements were made for each diode arrangement.
The data points on the graphs represent the average of the five readings.
Fig
2. Muzzle speed gain using multiple series diodes
Fig
3. Muzzle speed gain using multiple series diodes
Fig
4. Muzzle speed gain using multiple series diodes
Table
1 shows the gain in velocity and energy using 10 diodes instead of 1. Note that
the values presented in these tables are the end result of calculations rounded
to 3 significant figures.
Projectile 
Speed (m/s) 1 Diode 
Speed (m/s) 10 Diodes 
Gain (%) 
Energy (J) 1 Diode 
Energy (J) 10 Diodes 
Gain (%) 
Solid 
6.73E+0 
7.44E+0 
1.06E+1 
2.88E1 
3.51E1 
2.22E+1 
4mm Core 
7.11E+0 
7.89E+0 
1.09E+1 
2.73E1 
3.36E1 
2.30E+1 
6mm Core 
7.63E+0 
8.45E+0 
1.06E+1 
2.45E1 
3.00E1 
2.24E+1 
Table 1. Gains in speed and energy
using multiple series diodes.
The
limiting factor for the number of diodes is the maximum rated terminal voltage
of the switching device. When the device switches off it sees the supply voltage
plus the induced 'commutation voltage' so the maximum number of diodes is determined
by the difference in the rated voltage and supply voltage divided by the conduction
voltage of the diode. For 'large' current pulses through silicon diodes the conduction
voltage can be taken as about 1.5V. A more accurate value can be gleaned from
the diode's IV characteristic.
