chokes101

External chokes intended to be placed over a cable can be a solid
toroid or doughnut, or they can be in two pieces that can be placed
over a cable and then somehow secured to each other, say with a piece
of black tape. The latter is easier to place on a cable with big
things at each end, like a mouse cable. Performance is similar.

Chokes are made of a ferromagnetic material, usually a fine iron
powder mixed with other materials (boron, neodymium) and baked under
pressure. The recipe (bake temperature, bake time, pressure, specific
ingredients) determines the final characteristics. One may be
intended to filter 100 kHz while another is for 100 MHz.

The normal case for a cable is to have two wires, one to carry
current to the device (e.g. mouse) and the other to carry current
from the device back to the source. The magnetic fields from the two
equal and opposite currents tend to cancel out. An external choke
intercepts the external magnetic field. If this field is negligible,
then the choke has no effect on the normal and desired function of
the cable.

There are other possibilites for the cable where current flows the
same direction in both wires and the external magnetic field is not
negligible. Suppose the cable is 2.5 meters in length. It can act as
a quarter-wave antenna, either receive or transmit. If a quarter wave
is 2.5 meters long, then a full wave is 10 meters, with a frequency
of 30 MHz. Frequency and wavelength are inversely proportional. A 1
meter wave has a frequency of 300 MHz.

A choke basically acts as a resistor in series with the unwanted
current. It dissipates power according to the formula I*I*R where I
is the current in amps and R is the resistance in ohms. If we had a 5
ohm choke at 30 MHz and a current of 0.1 amp, the power loss would be
0.05 Watts. This loss appears as heat in the choke material and
finally as an increase in room air temperature. Two chokes in series
would have a total resistance of 10 ohms, and a power loss of 0.1 W
if the current stayed the same at 0.1 amp.

The choke does not stop or block the unwanted current. It just
increases the resistance of the circuit, thereby reducing the
unwanted voltage, current, and electric and magnetic fields.

My guess is that these chokes would probably not help the ES
symptoms. However, the cost is relatively small, and they are very
simple to install, and they will certainly not hurt anything, so why
not try some?

Gary