The Uni-Imp family of high voltage two-electrode spark gaps are designed to protect:

• The Uni-Imp family of high voltage two- electrode spark gaps are designed to protect:
• Aircraft from lightning transients induced into sensitive equipment inside the plane
• Critical defense systems, including missile launch control networks, from the effects of nuclear Electromagnetic Pulse (EMP)
• Power tubes, traveling wave tubes (TWT) and magnetrons from damaging effects of internal vacuum arcs
• Electronic circuits of military weapons from lightning, electromagnetic transients and electrostatic discharge

The Uni-Imps, consisting of the UBD, UBT and UGT Series, are miniature sized two-electrode spark gaps that provide an ultra-fast response to mitigate the hazards of high voltage transients originating from lightning, switched reactive loads or nuclear electromagnetic pulse. In conventional spark gaps, the pulse breakdown voltage is much higher than the DC breakdown for fast rising transients. These results are due to a finite amount of time required for the sequence of events that must precede large currents across the gap. These events, referred to as the statistical time lag and avalanche formative time, which occur in conventional spark gaps are not limiting features in the performance of the Uni-Imp products. A chart of comparative values of the impulse ratio of Legacy spark gaps vs. Uni-imps is shown in Figure 1. The ultra-fast speed of the Uni-Imps is based on the addition of an insulating element interposed between the two main electrodes to provide an automatic self energized trigger pulse. The trigger pulse emits electrons into the region of the main gap, which serves to reduce the formative time of the avalanche and the resulting breakdown.

Spark gaps that have been used for high voltage transient protection in legacy applications have been in the microsecond response time duration. It is known that modern integrated circuits being used for military, commercial and industrial equipment applications are more vulnerable to damage than prior generation legacy circuits. To protect these integrated circuits, Uni-Imp spark gaps should be used that have breakdown characteristics equal to the requirements of each system.

Impulse Ratio

One of the most important considerations in surge arrestor applications is the impulse ratio which may be expressed as the ratio between the voltage at which a gap fires in the presence of a steep wavefront and the voltage at which the gap fires on DC. A unity ratio (1:1) means that the device will trip at its rated breakdown voltage regardless of the rate of rise of the wavefront of the transient. Impulse ratios greater than 1:1 define the amount of overshoot the device will permit before tripping when the wavefront is very steep.

In conventional gas-filled spark gap surge arrestors, this impulse ratio can be as high as 10:1 as indicated in Figure 2. Often this poor impulse ratio causes destruction of the component or circuit due to the fact that harmful spikes bypass the gap before it can operate. To avoid such failures it is necessary to specify the trip voltage in relation to the rate of rise of voltage.

Figure 1

EMP and Lightning Characteristics

EMP
The breakdown voltage of the Uni-Imp spark gaps are independent of the rate at which the voltage is applied and with their ultra-fast speed of response, they are needed to provide protection for critical military systems. The damaging effects of a nuclear detonation within its burst area of exposure will cause component failures and complete failure of unprotected electrical and electronic systems. EMP hardening methods have been ongoing for years with gas discharge spark gaps proving to be best able to withstand exposure to bursts of neutron particles and pulsed gamma radiation that are released by the blast. The Uni-Imp will be the leading component to be used for hardening programs of the future. The Uni-Imp has energy, peak current and di/dt capabilities exceeding those of solid state protectors. They will routinely pass peak pulses of thousands of amps at 100 joules energy and current rise rates of 5000A/µs. Infinite resistance is preserved up to the breakdown voltage and the capacitance is in the pico farad range.

Lightning
Among the many lightning protection applications of the Uni-Imp spark gaps are those needed for avionics protection. Modern passenger jets having dozens of computers, instruments that control the engines, windshields, lighting systems and even passenger headsets require the ultra-fast response of the Uni-Imp spark gap. These computers, like all computers, are susceptible to transient voltage damage. Therefore, in addition to achieving a proper lightning protection design of the outer surface of the airplane, the engineer must also assure that no damaging transients will be induced into the sensitive equipment inside the plane.

Applications of the Uni-Imp Family
The top priority of the military for battlefield operations is reliable communications and reliable operation of offensive and defensive weapons. The modern electronic systems consisting of micro-sized integrated transistor circuits are much more vulnerable to fast rising voltage transients. Spark gaps are presently being used for EMP protection in communication networks, launch sites, as well as aircraft and missiles.

The Uni-Imp spark gaps have been designed into a vast number of different circuits. The military use has been extensive due to the need for enhanced reliability of total system performance. The number of failures of components and circuits has been greatly reduced by eliminating the destructive transients.
The critical component in any electric circuit having a maximum voltage rating may be protected by a Uni-Imp surge protector. For example capacitors, inductors, resistors, power tubes and solid state rectifier’s s may all be protected by connecting a Uni-Imp in parallel with the component. An overview of the Uni-Imp Series is shown in Table 2.

Table 2

Common Applications: 

• Aircraft Circuits: Protection against lightning or static discharge
• Antenna Input Circuits: Protection against lightning or excessive electromagnetic induced voltages
• Electromagnetic Interference (EMI): Protection to limit the amount of electromagnetic interference from arcs internal to motors and generators
• Electromagnetic Pulse: EMP protection in aircraft, missiles, and critical circuits such as land based communication and command systems and launch centers
• Pulse Modulator: The secondary of the pulse transformer in a pulse modulator circuit is normally protected against a “miss fire” (missing pulse) of the magnetron.
• Traveling Wave Tube Protection: Protection of solid state components in TWT grid-cathode circuits where high voltage arcs can wipe out entire circuits of rectifiers and Zeners

Note 1: Uni-Imps protect sensitive components against over voltages without regard to rate of voltage rise.
Note 2: Specifications listed for Impulse Breakdown and Capacitance are maximum values while IR specifications are nominal values and Surge Life specifications are minimum values.
Note 3: The range of values corresponds to the low and high member of the Series.
Note 4: Life ratings on select members of a Series are determined by laboratory tests and are dependent on the cumulative charge, in coulombs (Q) that is passed during the tests. By similarity, the Life Rating of the gaps of a Series, tested with different waveforms, can be approximated by dividing the Cumulative Charge Rating by the charge content in the given waveform that is passed without changing its DC Breakdown Voltage by more than 20%. The coulomb content of any surge current can be approximated by determining the area under the current waveform.

Conclusion

The Uni-Imp design has reestablished the spark gap as a commonly used type of high voltage transient protector. These products have a proven track record in military and commercial applications due to their unique design and construction features. These gaps are offered in a wide breakdown range and are currently available with a lead time of 4 – 6 weeks ARO.