Barry's Coilguns

Low Voltage Power Supply

A low-voltage power source is required for SCR gate control. How was this designed for the Mark IV coilgun?

Low Voltage Power Supply

Schematic diagram of low-voltage power supply The actual SCR gate control voltage is not critical, as long as it does not exceed the SCR's gate rating, typically 20v or less. Current must be sufficient to quickly turn on the gate.

An inexpensive and convenient transformer is the standard 12.6vac “filament” transformer. A center-tap gives the choice of 6.3 or 12.6vac. We chose the 12.6v because it would reduce the current demand.

The rectified DC output peak voltage is:

Equation to find DC output peak voltage

A filter capacitor provides the charge needed to turn on the SCR. The capacitor value is not critical; I used 15 uF but anything from 5 - 50 uF or more will work fine.

A series resistor R1 limits the SCR gate current.

Calculation of resistor R1

The minimum power rating for R1 is:

Calculation power dissipation in resistor R1

The Mark 4 coilgun used a 200-ohm 2W resistor for R1.

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