Biomedical Engineering Reference
In-Depth Information
Figure 7.28 When the SCR of Figure 7.27 is triggered, the capacitor discharges through the coil. As the capacitor voltage reaches zero,
the coil current and magnetic field are at their maximum. When the magnetic field collapses, the coil current continues to flow in the same
direction until it charges the capacitor to the opposite polarity. A diode placed antiparallel to the SCR allows the charge to flow back
through the coil in the opposite direction, charging the capacitor to its initial polarity. At this point, the current stops because the SCR has
turned off.
encountered in a solid conductor due to the skin e
ect or tendency of radio-frequency
current to be concentrated at the surface of a conductor. A good supplier of litz wire is
WireTronic. You can download a free Windows utility from their Web site (WTSETUP.
EXE is also provided for your convenience in the topic's ftp site) that will give you dimen-
sional data for a wide range of wire sizes, insulation speci
ff
cations, current-handling
capacities, resistance calculation, break strength calculation, and ordering information.
The coil for our experimental magnetic stimulator was constructed with WireTronic litz
wire type 85/26HPN-DN in a
fi
guration. The inductance of an air-core
pancake coil can be calculated using Wheeler's formula:
fl
at pancake con
fi
r 2 n
1
2
L (
ยต
H)
8 r
1 w
where r is the radius to the center of the winding (in inches), n the number of turns, and w
the width of the winding (in inches). Our prototype coil was wound on the inside of a PVC
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