Biomedical Engineering Reference
In-Depth Information
V
COIL_dldt
d/dt
+ -
33.3u
R_DUMMY2
1T
0
R_CHARGE
100k
+
IC =1600V
V
TX1
I
CAPACITOR
30m
R_COIL
R_BODY
100k
R_CAP
10m
7uHCoil
+
-
V1
1600V
C1
100uF
R_DUMMY
1T
SCR
C228A
V
TRIGGER
Dbreak
D_BACK_EMF
+
-
V_TRIGGER
0
0
0
0
0
0
Figure 7.27
A magnetic stimulator that uses nonpolar capacitors is capable of recovering the energy stored in the magnetic field.
hiPot testing (see Chapter 3) for reinforced insulation. You may remember that parts
carrying voltages U between 1 and 10 kV require their reinforced insulation to be
tested at a voltage of 2( U
2500) for 60 s.
2. The intense pulsed currents driven for stimulation cause very strong expanding and
compressing forces in the coil. This is because when magnetic
flux lines are packed
together, they create magnetic pressure. The force of the expanding coil is propor-
tional to the energy delivered to the coil and related inversely to the coil size. Typical
pressures for circular coils are in the range 500 to 1500 lb/in 2 , making it necessary
to enclose the coil in a potting material (e.g., epoxy) to prevent the coil windings
from
fl
flying apart when the stimulation pulse is delivered. A detailed analysis of
magnetic stimulator coil forces was published by Mouchawar et al. [1991].
3. Joulean heating of the coil must also be considered, since according to the medical
device standards, the surface temperature of the coil must not exceed 41°C. The
main contributor to Joulean heating is the coil conductor's resistance, which is deter-
mined by the wire material, gauge, and coil geometry. Copper is used almost uni-
versally because of its relatively high conductivity and good mechanical properties.
When the wire cross section exceeds 1 mm 2 , the skin and proximity e
fl
ff
ects change
the current distribution in the wire, increasing the dc resistance signi
cantly. Striped,
foil, or isolated multistrand (litz) wire is often used to reduce the skin and proximity
e
fi
ff
ects.
The term litz wire is derived from the German word litzendraht meaning woven wire.
This wire is constructed of individual
film-insulated wires bunched or braided together in
a uniform pattern. The multistrand con
fi
fi
guration minimizes the power losses otherwise
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