Barry's Coilguns

Damping Resistor Construction

When two parallel wires carry a large current, their magnetic fields interact. Of course, this is widely known, but when does it become a problem? How much current and how much distance between the wires is needed to notice the effect?

Damping Resistor Construction

We observed a construction problem. The damping resistor is a length of wire looped onto brass brads. We were a bit surprised to see the wires jump around and rearrange themselves when subjected to these large surge currents. It was fun to watch, but the overlapping wires increases the inductance and the chance of short circuits.

The brass brads in the first damping resistor were 1/8 inch (3 mm) apart The second damping resistor (shown below) used 1/4 inch (6 mm) spacing.

Damping resistor showing straight parallel wires

The ¼ inch spacing (shown below) was also found to be insufficient. The wires cross over each other in pairs after each shot, as shown below.

Damping resistor showing wires crossing over each other

The third damping resistor used ½ inch spacing, and the wires remained in their correct positions. There is observed, however, some twitchy motion as 150-joule shots are fired.

Damping resistor showing wider spacing to avoid crossover wires

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