ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICES - Design and Development of Medical Electronic Instrumentation - page 187

mode after a prolonged power outage. A simple way of simulating these conditions during
the design phase is to use a variac. Although timing an exact 10 ms for a dip is not an easy
manual task, good approximations for these dips and interruptions can be achieved, espe-
cially when the variac's output is monitored with an oscilloscope. Remember, however,
that power line voltages are present, and an oscilloscope with grounded input channels
should not be used without appropriate isolation. As shown in Figure 4.31, a small filament

transformer (e.g., 110 V ac/6.3 V ac at 100 mA) provides appropriate isolation for moni-
toring the variac's output with a grounded oscilloscope.

fi

Susceptibility to Magnetic Fields

IEC-61000-4-8 deals with interference that may be caused on a device by low-frequency
magnetic

fi fields, such as those generated by the power lines. These magnetic fi fields can pro-

duce jitter on CRT displays, distortion in ampli fi ed signals equipment, or false readings in

equipment magnetic or electromagnetic field sensors. As shown in Figure 4.32, the device

under test is placed in the approximate center of a referenced ground plane at a height of
10 cm and is powered and operated in a normal con

fi

field is
increased to 10 A/m (approximately 125 mG) and is applied to three axes of the device
under test. The

fi guration. The magnetic fi

field is maintained for a period of approximately 5 minutes for each of
three 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. Use
four pieces of 92-cm-long

fi

4 -in. PVC pipe, four

3

4 -in. PVC pipe elbows, and one

3 3

4 -in. PVC

pipe tee to construct the 1 m 1 m frame for the current loop. Thread No. 18 insulated cop-

per through the loop to form two complete turns. Solder a flexible twisted-pair cable to the

loop wires where they exit the pipe. R1 will be used to monitor the RMS current

fl

flowing
through the coil. The coil is powered by a transformer rated at 24 V at 10 A, which is in
turn powered from the power line through a variac.

fl

T1
Variac

J1 J2

To Device
Under Test

AC Power Plug Male

AC Power Plug Female

J3
BN C
1

T 2

1 5

To
Oscilloscope

110V 6.3V

2

4 8

Figure 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 a
dip is not an easy manual task, good approximations for these dips and interruptions can be achieved when the variac's output is monitored
with an oscilloscope. A small fi filament transformer provides appropriate isolation for connection with a grounded oscilloscope.