8
polarity switch is in the Reverse position (
REV
indicator is lit), the high voltage will appear on
the
OUTPUT
as a negative pulse relative to the
RETURN
jack. The polarity switch only
operates when the
CHARGE
LED is lit, i.e. the output is not charged.
Note that the voltage meter may indicate that some residual voltage is present on the
main storage capacitor, even when the tester is first turned ON. This is due to inherent charging
of the internal capacitors. Pressing the
TRIGGER
switch will discharge the capacitors (be sure
not to touch the output and return leads when pressing the trigger switch).
Note that the peak amplitude of the measured output waveform is the voltage that is read
on the front panel of the MegaPulse prior to triggering the output. This is the peak open-circuit
output into any capacitive load from 0 to 180nF. If the load is outside this capacitance range, or
is a significant resistive or inductive load, the peak output voltage will be lower than shown on
the front panel meter. The output impedance of the MegaPulse tester is low (approximately 2.5
Ohms) and the MegaPulse is capable of delivering thousands of Amps of current into a short
circuit. While the MegaPulse can withstand a short-circuit at the output terminals, it should
NOT be used in this way. It is important to measure the peak amplitude of the output waveform,
and adjust the output of the MegaPulse to achieve the desired peak output voltage at the
Equipment Under Test.
Determination of Passing and Failing results can prove difficult. To obtain the most
accurate results, it is generally necessary to perform multiple impulse tests on a few different test
samples (that have adequate insulation to pass the impulse test). Take note of the impulse
waveshape, amplitude, and duration. Also note how much variance there is in the waveshape
from test to test. Also (if possible), perform impulse testing on some test samples that are known
to have inadequate (or damaged) insulation. Take note of the impulse waveshape, amplitude, and
duration, when an insulation breakdown occurs. A passing result will generally allow the
MegaPulse to generate the full 1.2 x 50uS waveform as shown in figure 1. A failing result will
typically show an abrupt reduction in the output voltage, either before the expected peak, or soon
after (Figure 2).
Figure 1: Full 1.2 x 50uS waveform (typical)
