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4.5
The Standard Exponential Decay Curve
The System Test Module, STM-1 will follow the standard exponential decay
curve, which is the solid line in Figure 4.5-1. The decay curve is a function of the
STM-1 capacitor plus the capacitance of the electrode configuration and the 400
megOhm resistance in the STM-1.
When testing a sample the charge on the sample bleeds through the electrodes
to ground via the ground relay. The STM-2 simulates an actual sample using a
30 gigOhm resistance and the system intrinsic capacitance. Since the electrode
resistance is much lower than the sample or STM-2 resistance, the decay time
curve is actually a composite of at least two different exponential decays as
shown by the dash line. The field on the electrodes collapses first then the
charge on the sample begins to bleed off to ground as shown in the figure. The
intrinsic capacitance of the Faraday Test Cage, electrodes and discharge circuit
is 50pF. The discharge circuit incorporates a 10 MegOhm resistor that limits the
fastest decay time to 1% to 1.25 milliseconds. The relay switching time is
approximately 3 milliseconds. This time is approximately one order of magnitude
faster than the 10 millisecond resolution of the readout. Samples with non-
uniform antistatic treatment will have multiple exponential decay curves with the
decay characteristics changing from the fastest decay time curve to the slowest
decay time curve.
Figure 4.5-1: Standard and Method 4046 Exponential Decay Curves
4.6
Measuring Laminated Film
The Model 406D is specifically designed to measure the static decay
characteristics of homogeneous static dissipative material having a surface
resistance between 10
than the measurement capability of the instrument (minimum time is 2.3msec).
This is due to the time required for the internal relays to switch. Above 10
Ohms, the time to charge the material ranges from several seconds to not able
to conduct the applied 5kV charge.
Time Constant - τ
8
12
and 10
Ohms. Below 10
26
8
Ohms the decay time is less
12
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