A Faraday cage is required to protect electronic devices operating
in environments where extreme, time variant, electric and/or magnetic fields
are present, such as near operating pulse power equipment, lightning strikes
or nuclear detonations. Recently, I have been experimenting with
pulse power driven wire explosions,
which appears to produce moderate local electromagnetic pulse effects,
particularly in the HP M305 digital camera I am using to record the experiments.
Conceptually, a Faraday cage is very simple, just a container made of an
electrically conductive material which completely surrounds the equipment
to be protected. For a camera, this is a bit difficult, since openings
are required to allow light in and for actuation of the recording functions.
The size of the openings which can be tolerated, depends on the wavelength
of the radio frequency energy emitted by the pulse. For nuclear EMP,
most of the emitted energy is in the wavelength range from ~100 meters
to 1 centimeter. For my wire explosion application, the fundamental
frequency can be calculated by treating the system as an LC oscillator:
nu = 1/(2 pi sqrt (LC)) where the greek letter nu represents the frequency
in Hz, the greek letter pi represents the ratio of a circle's circumference
to its diameter, L is the inductance of the circuit in Henrys and C is
the capacitance of the circuit in Farads. The measured capacitance
of my pulse capacitor bank is 1.488 microFarads and the loop made by the
wire and ground return has a measured inductance of 0.3 microHenrys.
The math tells me that the fundamental frequency should be 240,000Hz, corresponding
to a wavelength of 1200 meters (which really should not effect the camera...).
This drawing shows the plan for my Faraday cage. The main body
is made from a 4.5" long, 3", schedule 40, steel pipe nipple (actual dimensions
are OD 3.500", ID 3.068") . The ends are closed by 3", malleable
iron, pipe caps. The aperture for the camera lens was made by first
drilling a 1/2" hole and then filing a 7/8" X 1" flat on one side
of the nipple, drilling and tapping four 6-32 threaded holes around the
1/2" hole and covering the hole with a piece of 160 X 160 mesh, 304 stainless
steel, wire cloth held in place by a matching 1/32" thick, spring tempered,
phosphor-bronze plate and four round head, 18-8 stainless steel , 6-32
screws. The camera is bolted to a 1 7/8" wide, 5" long piece of 3/8"
thick ultra high molecular weight polyethylene with a 5/8" long, round
head, 18-8 SS, 1/4-20 screw and does not touch the shell of the cage anywhere.
The video mode button is actuated via a mechanism made from a 1/2" wide,
2 1/2" long strip of 1/32" thick, spring tempered, phosphor-bronze drilled
to accept a 6-32, nylon, round head screw and bolt on one end and a 1/4"
long, 1/4-20, 18-8 SS, round head screw on the other.
This is a close up of the flat and holes machined into the pipe nipple
for the camera aperture.
This shot shows the assembled Faraday cage (except for the camera,
And here is a close-up of the assembled aperture.
And the shutter release mechanism.
Unfortunately, the use of this Faraday cage did not seem to mitigate
the camera problems I was experiencing. This makes me think that
the camera problems do not arise from EMP effects, but the rapidly varying
light levels which occur during a wire explosion. Oh well...another
paper-weight for my collection.