18.104.22.168.7 Power Supply Transient Suppression. Power supplies (5 to 48 V dc) that operate from commercial ac
power inputs and furnish operating voltage to solid-state equipment must have a transient suppressor installed
between the rectifier output and case ground. This protection (in addition to the service disconnect arrester
and powerline suppression at equipment entrances) is required because of the adverse electromagnetic
environmental operating conditions for much military equipment. A silicon avalanche diode suppressor will
provide the best protection for this particular application. The silicon avalanche diode suppressor is
recommended because of the very fast response time of the device, since the primary purpose is to clamp very
fast risetime and very short duration transients. In addition, the silicon avalanche diode suppressor provides the
lowest clamping voltage available. Thus, when this device is used, the clamped output of the transient
suppression at the ac input-equipment interface will be clamped to a lower level by the avalanche diode at the
rectifier. This, in turn, provides optimum protection for solid-state voltage regulators and other solid-state
components receiving operating voltage from the power supply. Operating characteristics for the suppressor
installed at the rectifier output must be as follows if the suppressor is to provide the desired function:
Operating (reverse standoff) voltage. Reverse standoff voltage must be 5 percent above maximum
rectifier output voltage.
Leakage current to ground should not exceed 100 microamperes at standoff
Turn-on voltage. Turn-on voltage must be as near standoff voltage as possible using state-of-the-
art suppressors, and shall not exceed 125 percent of reverse standoff voltage.
Discharge (clamp) voltage. Clamp voltage must be the lowest possible value that can be obtained
using state-of-the-art suppressors not to exceed 160 percent of turn-on voltage.
Overshoot voltage. Overshoot voltage must be sufficiently low to preclude equipment damage or
operational upset. Time duration of overshoot voltage shall be limited to the shortest possible time not
exceeding 2 nanoseconds.
Self-restoring capability. Transient suppressors installed in power supplies must automatically
restore to an off state when line transient falls below rated turn-on voltage for the suppressor.
Operating lifetime. The transient suppressors must safely dissipate 1000 surges with an amplitude
of 200 volts above rectifier output voltage and a waveform of 8-by-40 microseconds. Eight microseconds
defines the time from the start of the transient to peak voltage, and 40 microseconds is the time from the start
of the transient until the transient exponentially decays to 50 percent of peak value.
22.214.171.124.8 Landline Transient Suppression. When the equipment withstand level is below the transient energy
level projected to occur at direct la&line-equipment interfaces, transient suppression must be provided by
equipment design. Generally, all direct landline-equipment interfaces will require transient suppression.
However, when the landlines are totally enclosed end-to-end in ferrous metal conduit, a much lesser degree of
suppression is required than when the landlines are direct earth-buried or overhead cable runs. At the time of
new equipment design, when provisions for transient protection must be included, the manufacturer may not
know whether externally exposed landlines will be totally enclosed in ferrous metal conduit. When the