DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

SUSCEPTIBILITY 187mode after a prolonged power outage. A simple way of simulating these conditions duringthe design phase is to use a variac. Although timing an exact 10 ms for a dip is not an easymanual task, good approximations for these dips and interruptions can be achieved, especiallywhen the variac’s output is monitored with an oscilloscope. Remember, however,that power line voltages are present, and an oscilloscope with grounded input channelsshould not be used without appropriate isolation. As shown in Figure 4.31, a small filamenttransformer (e.g., 110 V ac/6.3 V ac at 100 mA) provides appropriate isolation for monitoringthe variac’s output with a grounded oscilloscope.Susceptibility to Magnetic FieldsIEC-61000-4-8 deals with interference that may be caused on a device by low-frequencymagnetic fields, such as those generated by the power lines. These magnetic fields can producejitter on CRT displays, distortion in amplified signals equipment, or false readings inequipment magnetic or electromagnetic field sensors. As shown in Figure 4.32, the deviceunder test is placed in the approximate center of a referenced ground plane at a height of10 cm and is powered and operated in a normal configuration. The magnetic field isincreased to 10 A/m (approximately 125 mG) and is applied to three axes of the deviceunder test. The field is maintained for a period of approximately 5 minutes for each ofthree axes, while the device under test is monitored for any indication of erratic operation.As shown in Figure 4.33, a test system for design-time evaluation is easy to build. Usefour pieces of 92-cm-long 3 -in. PVC pipe, four 4 3 -in. PVC pipe elbows, and one 4 3 -in. PVC4pipe tee to construct the 1 m 1 m frame for the current loop. Thread No. 18 insulated copperthrough the loop to form two complete turns. Solder a flexible twisted-pair cable to theloop wires where they exit the pipe. R1 will be used to monitor the RMS current flowingthrough the coil. The coil is powered by a transformer rated at 24 V at 10 A, which is inturn powered from the power line through a variac.J1T1VariacJ2To DeviceUnder TestAC Power Plug MaleAC Power Plug Female110VT21 56.3V4 8J3BNC1ToOscilloscope2Figure 4.31 A simple way of simulating power line voltage dips and interruptions is to use a variac. Although timing an exact 10 ms for adip is not an easy manual task, good approximations for these dips and interruptions can be achieved when the variac’s output is monitoredwith an oscilloscope. A small filament transformer provides appropriate isolation for connection with a grounded oscilloscope.