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MIL-HDBK-419A
(b)  Metal oxide varistor (MOV) RV1, RV2. As shown in Figure 1-43, MOVs can be used in
various configurations to provide effective transient suppression. Turn-on time for the MOV is less than 50
nanoseconds, and turn-on voltage ranges from 22 to 1800 volts. Clamp voltage is not as low as for SAS devices
and turn-on time is not as fast. The turn-on time for SAS devices is typically less than 10 nanoseconds, and less
than 1 nanosecond in some configurations. The configuration shown in Figure 1-43c is especially effective for
protecting highly susceptible equipment.  The configurations shown by Figures 1-43a and 1-43b provide
adequate protection when the protected equipment can safely withstand the rated clamping voltage for the
MOV at the equipment entrance.  An MOV with a 20 mm element diameter will normally provide required
protection at the facility entrance, and a 10 mm element diameter MOV will normally provide required
protection at the equipment entrance.  To enable desirable functioning, the turn-on voltage of the MOV
suppressor at the facility entrance should exceed that of the MOV at the equipment entrance by approximately
10%. This is desirable to permit the MOV at the equipment entrance to turn on and dissipate low-amplitude
transients while reflecting a low clamp voltage to protected equipment.  When a high-amplitude transient
occurs, the voltage increase across R1 will cause the MOV at the facility entrance to turn on.  When the MOV
at the facility entrance turns on, it dissipates most of the remaining transient energy, thereby eliminating or
greatly reducing the energy to the MOV at the equipment entrance. Thus, the MOV at the equipment entrance
will conduct only a small amount of current and maintain a low clamp voltage that will appear across the
protected equipment. The MOV operating characteristics are similar to those for a pair of back-to-back zener
diodes. Therefore, the device responds the same to a negative or positive transient voltage.
(c)  Silicon avalanche diode suppressor (SAS) TS1. The SAS device has the fastest turn-on
time of any of the three suppressor devices shown in Figure 1-43.  Turn-on time is typically less than
10 nanoseconds and can be less than 1 nanosecond in some configurations depending on lead length and the path
to ground for the device. Turn-on voltage ranges from 6.8 volts to 200 volts. Devices may be connected in
series to obtain higher turn-on voltages and to improve power handling capability. For example, two devices
connected in series can dissipate approximately 1.8 times the power dissipated by a single device. The clamping
voltage for the device is also lower than for MOV devices. The maximum clamping voltage for the SAS devices
is approximately 1.6 times the turn-on voltage at peak pulse current.  Peak pulse current ranges from 139
amperes for a 6.8-volt device to 5.5 amperes for a 200-volt device over a period of 1 millisecond. Devices
recommended for use at the equipment entrance have a peak pulse power dissipation rating of 1500 watts over a
period of 1 millisecond. Devices are available in both unipolar and bipolar configurations. Operation of a
unipolar device is very similar to that of a zener diode, and operation of a bipolar device is very similar to that
of a pair of back-to-back zener diodes. For the most effective protection, unipolar devices should be used on
lines that carry unipolar voltage provided the ac noise level on the applicable line is less than 0.5 volt.  Use
bipolar devices on lines that carry bipolar (ac) voltage and on lines with an ac noise level greater than 0.5 volt.
Select SAS devices based on the reverse standoff voltage rating. The reverse standoff voltage must be greater
than maximum line operating voltage, and should exceed normal line voltage by 20% when possible.
(d) Resistor R1.  The function of resistor R1 is to provide current limiting for the
suppression device at the equipment entrance and to provide a turn-on voltage for the suppressor at the facility
entrance.  Empirical evidence has shown that the power rating for the resistor should be 5 watts. The
resistance value should be as high as equipment operation will permit. Typical values are 10 to 50 ohms. Values
as low as 2 ohms have been successfully used. However, when the value is less than 10 ohms, the suppressor at
the facility entrance must be an MOV or equivalent type suppressor.
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