Gas Discharge Tubes (GDT)
Training Material

Construction and Characteristics of GDTs

Performance Ratings

Selection of GDT for Application

Applications when NOT to use GDTs

Comparison of Protective Devices
Page 1
Gas Discharge Tubes
Why are Gas
Discharge Tubes
needed?
A bolt of lightning flashes through the sky and hits the ground somewhere around the
world about 100 times every second. That’s 8 million lightning strikes in a single day!
Scientific America- Nov 2014 – Global Warming: New findings suggest lightning strikes may
increase by 12% for every degree (°C) of warming … That comes to a 50% increase (in
number of lighting strikes) by the end of the century
Gas discharge tubes (GDTs) are ideal for lightning surge
protection of electronic equipment, as GDTs can dissipate
large amounts of energy in small size components
Page 2
Gas Discharge Tubes
Construction of GDTs
Electrode
Solder
Solder
Carbon line
Gas
Ceramic Tube
Electrode
Ceramic Tube
Electron emission material
Gas discharge tubes by nature of their construction can handle very large amounts of current, are bidirectional.
They have very high impedance and low capacitance, resulting in very little current leakage or signal loss. So the
GDT is ‘virtually invisible’ to the protect circuit. GDTs are normally used as primary protection devices, in
conjunction with other kinds of protect devices (with faster response speed) as secondary protection. When a
surge voltage reaches the GDT spark-over voltage, the GDT will switch into virtual short, divert the surge current
through the GDT to ground and removing the voltage surge from damaging the equipment. Therefore, GDTs
provide excellent protection during the time period that they are active.
Page 3
Gas Discharge Tubes
Characteristics of GDTs
Visualization of a Townsend Avalanche*
The Townsend discharge is a gas ionization process where free electrons, accelerated by a sufficiently strong
electric field, give rise to electrical conduction through a gas by avalanche multiplication, called an Townsend
Avalanche. An electron avalanche is a process in which a number of free electrons in a transmission medium (gas)
are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium,
thereby ionizing them (impact ionization). This releases additional electrons which accelerate and collide with
further atoms, releasing more electrons—a chain reaction. In a gas, this causes the affected region to become an
electrically conductive plasma. The avalanche effect was discovered by John Sealy Townsend in his work between
1897 and 1901, and is also known as the Townsend discharge.
Page 4
Gas Discharge Tubes
Voltage
Current
Gas
Discharge
Tube
Standoff Resistance
Page 5
Gas Discharge Tubes
Typical GDT specification sheet
The following is a review of the GDT specifications 
Page 6
Gas Discharge Tubes
DC Breakdown &
Spark-over Voltage
The curves (left) show the typical DC breakdown
voltage (100V/S) and Impulse Spark-over Voltage
(1000V/uS) of NGTA series GDT
The value of Impulse Spark-over
Voltage reflects the Gas Discharge
Tube response speed
Selected based upon operating voltage level (VDC)
 SELECTION GUIDE:
Part Number
DC
Breakdown
Voltage
(100V/s)
Breakdown
Voltage
Tolerance
(V)
Impulse
Spark-Over
Voltage
(1KV/μS)
NGTA1812N401TR1F
400V
340~550
≤750V
Better protection; lower impulse voltage = faster response
NGTC1812N401TR1F
400V
360~560
≤950V
Slower response
With the same DC Breakdown Voltage, the lower the
Impulse Spark-Over Voltage, the faster the response
speed and the better protection
Page 7
Gas Discharge Tubes – Arc Voltage
The arc voltage is developed across the GDT during its “virtual short circuit” condition.
This parameter defines the power dissipation of the GDT during its protection mode.
Arc voltage under AC voltage condition
Voltage
 GUIDE:
A low arc voltage is desirable to keep
power dissipation at a minimum,
which in turn increases the life
expectancy of the gas discharge tube.
Higher power dissipation (higher
operation temperature) will act to
reduce the lifetime of the gas
discharge tube.
Low Arc Voltage of GDT construction
makes the GDT ideal protection against
lightning (high-energy) and AC line
overvoltage conditions
The voltage developed
across the GDT component
when it is operating is called
the “Arc Voltage“
( Va in above example )
Current
Page 8
Gas Discharge Tubes – Arc Voltage
While low Arc Voltage is desired to minimize power dissipation, the Arc voltage of the GDT in DC circuits should be
above the operating voltage level to assure turn off of the GDT. When GDT is used to protect 48VDC power port, the
arc voltage of the gas discharge tube should be selected to be higher than the operating voltage, as shown in
Solution ‘A’ below, or used in series with a MOV (such as NIC NVR series) as shown in Solution ‘B’ below
48VDC solution
Solution A
Solution B
Examples:
Two solutions (A & B) for 48VDC power port, solution A (above left) uses a special 5-element, 6-terminal GDT. The Arc
voltage is >60V@1A, higher than 48VDC, so it is safe choice. Solution B (above right) use a lower voltage single GDT (PN:
NGTF2016M091TR5F) in series with a MOV, because the arc voltage of NGTF2016M091TR5F is about 8V@1A, which is
lower than the 48VDC system voltage, so it needs to be used in series with a MOV which will claim above 48VDC
Page 9
Gas Discharge Tubes – Arc Voltage
NIC has developed Higher Arc Voltage GDTs, which can be applied in higher voltage DC/ACArc
applications
Voltage
at 1A
Format
Part Number
NGTA1812N301TR1F
NGTA1812N401TR1F
Arc Voltage
15V
Application
DC12V
AC24V
NGTF2016M351TR5F
NGTF2016M801TR3F
NGTF2016M102TR3F
NGTF2016M122TR3F
15V
DC12V
AC24V
NGTM2332M351TR20F
NGTM2332M471TR20F
NGTM2332M601TR20F
NGTM2332M801TR20F
NGTM2332M152TR10F
NGTM2332M362TR10F
15V
DC12V
AC24V
NGTD3020M231TR5F
NGTD3020M102TR5F
15V
DC12V
AC24V
Page 10
Gas Discharge Tubes - Surge Current
High Surge Current rating is one of the main characteristic of GDTs.
NIC can provide surge current rating: 1KA ~ 20KA @ 8/20uS in single and multiple element construction
1KA / 2KA
20KA
5KA
Single
Element
Construction
NGTA / NGTC
5KA
(4.5x3.2x2.7)
NGTF
(4.2x5x5)
NGTM
(6x8.3x8.3)
20KA
20KA
Multiple
Element
Construction
(2, 4, 5)
NGTD
(7.6x5x5)
Custom
(24.1x8.3x9.2)
Custom
(16.5x8.3x9.5 & 19.9x8.3x9.3)
GDT suggested as the primary circuit protection, due to its high surge current rating performance.
Page 11
Gas Discharge Tubes
Another characteristic of GDT is capacitance value, which is very important for data signal
transfer, especially for high speed - frequency (lower capacitance = less signal loss).
NIC provides GDTs with very low capacitance as shown below.
Typical Capacitance value @1MHz
Series
Typical Capacitance Value
NGTA
0.26pF
NGTC
0.26pF
NGTD
0.57pF
NGTF
0.48pF
NGTM
0.98pF
 SELECTION GUIDE:
To protect high speed
applications, the protection
device must have low
capacitance values
Silicon Avalanche Suppressors, MOVs &
Transzorbs can have >100pF capacitance value
Page 12
Gas Discharge Tubes
Parameters - Characteristics of GDTs
Higher is better
Lower is better
Selection based upon operating voltage
Guidance based
upon application
VAC or VDC
Page 13
Gas Discharge Tubes
GDT Selection
DC Breakdown
Voltage
Must be higher than the
peak work voltage of the
protective circuit
Impulse Spark-over
Voltage
Must be lower than the highest
voltage the protected circuit can
allow
GDT selection
Arc Voltage
When used to protect power port,
Arc Voltage must be higher than the
peak value of the supply voltage
Surge Current Rating
It must be higher than the
highest surge current of the
protected circuit
Page 14
Gas Discharge Tubes
GDTs are suggested for use in primary protection, but not suggested for use in secondary or ESD protection
High surge current rating
Good for large surge current protection
Use in primary protection
Low response speed
High residual voltage
Not suggested for fine protection
Not used in secondary or ESD protection
Example: Tip & Ring xDSL protection solution
Secondary or ESD Protection
TVS
Primary Protection:
GDT
Page 15
Gas Discharge Tubes
When GDT is used in power port, need to be cautious in selection of GDT.
Must ensure the GDT can shut off after surge wave has passed. The arc voltage of the GDT should be
selected higher than the circuit voltage, or use the GDT in series with a MOV.
L
Secondary Protection
TVS
+12V
Protected
Circuit
GDT
Arc voltage 15V@1A
12VDC solution
0V
Page 16
Comparison of Over-Voltage Protective Devices
GDT – Gas
Discharge Tube
TSS - Thyristor
Surge Suppressors
TVS - Transient
Voltage Suppressor
MOV - Metal
Oxide Varistor
Operating
Principle
Gas discharge
Avalanche effect
of PN junction
Avalanche effect
of PN junction
Nonlinear voltage
characteristic
Protection
Mode
Switch Mode
Switch Mode
Clamp Mode
Clamp Mode
Response speed
Slow
Fast
Very fast
Medium
Ability of
withstand high
voltage
High
Medium
Low
Medium--High
Leakage
Very low
<5uA
<5-10uA
<20-30uA
Capacitance
Very low, normally
lower than 1pF
Mid (be related to
lightning level)
Mid (be related to
lightning level)
Large (be relate to
the size and
lighting level)
Failure Mode
Open
Short
Short
Short
Item
Device
Page 17
Comparison of Over-Voltage Protective Devices
GDT – Gas
Discharge
Tube
TVS Transient
Voltage
Suppressor
Switch Mode
TSS Thyristor
Surge
Suppressor
MOV - Metal
Oxide Varistor
Clamp Mode
Page 18
Comparison of Over-Voltage Protective Devices
GDT – Gas Discharge
Tube
TSS - Thyristor Surge
Suppressors
TVS - Transient
Voltage Suppressor
MOV - Metal Oxide
Varistor
High Surge Current
Rating,
Low Capacitance,
Low Leakage
Low Residual Voltage,
Fast Response Speed,
High Accuracy of
Voltage
Low Residual Voltage,
Fast Response Speed,
High Accuracy of
Voltage
High Accuracy of
Voltage,
Low cost
Disadvantage
High Residual Voltage,
Slower Response Speed
Limited
Voltage Range,
Can Not Be Used
In Power supply Port
Low Surge
Current Rating,
Unit cost increases
with Surge Current
Rating Increase
High Capacitance,
Easy Degenerate,
Large Leakage
Typical Usage
Applications
Mainly used to primary
protection of signal ports
with high flow capability
or low voltage power
supply ports
Mainly used primary or
secondary protection of
signal ports, Normally not
used in power supply ports
Mainly used for fine
protection, such as
secondary, third level
protection and ESD
protection
Advantage
Mainly used in power
supply ports, normally
not used in signal ports
due to its’ high
capacitance.
Page 19
NIC SMT Gas Discharge Tubes
SMT Format: Single Element Construction & 2-Element Construction
NGTA / NGTC
QVGQ2.E467518 / E467518
Isolated Loop Circuit Protectors –
Component, Transient-voltage
surge suppressors, gas tube
(4.5x3.2x2.7)
NGTF
(4.2x5x5)
NGTM
(6x8.3x8.3)
NGTD
(7.6x5x5)
Featuring: Ultra High Current, Low Capacitance & Low Insertion Loss
Impulse
Breakdown
Series
Size
Current
Spark-over
Voltage
Voltage
NGTA
1812
150V  400V
1KA
650V  750V
NGTC
1812
200V  600V
2KA
700V  1200V
NGTD
3020
90V  1000V
2KA & 5KA
700V  1800V
NGTF
2016
90V  1200V
3KA & 5KA
650V  2000V
NGTM
2332
90V  3600V
5KA  20KA
600V  5000V
Page 20
NIC SMT Gas Discharge Tubes
Gas Discharge Tubes Applications
 Power Supply Voltage Protection
 ADSL, xDSL Applications
 General Telecommunication Equipment
 CATV & Satellite Applications
 Communications Equipment
 Smart Metering, Green Power
 Data Transmission Line - Port Protection:
 RS485
 RS232
 xDSL
 Ethernet
Page 21
NIC Circuit Protection Products
NVR Series
NFVC Series
Chip Fuse
Metal Oxide Varistor (MOV)
Over-voltage protection
Over-current Protection
NIC Circuit Protection Products
→ NGT_ - SMT Gas Discharge Tubes
→ NVR – MOV High Voltage Varistors
→ NPX– X2 Safety Capacitors
→ NFVC – 125V / 250VAC Chip Fuses
NPX Series
X2 Capacitor
Interference Suppression
NGT_ Series
Gas Discharge Tubes
Over-voltage Protection
Page 22
Additional Information Needed?
Need Samples?
European Engineering Support
Technical Support: tpmg@niccomp.com
Sales Support: sales@niccomp.com
North America Engineering Support
SE Asia Engineering Support
NIC Components offers unique performance passive components that provide advantages to design engineers to
create high performance end products in smaller and lower total cost formats
• Surface Mount SMT formats (high speed auto placement)
• Pb-Free Reflow Compatible (high temperature reflow)
• Performance advantages over competing technologies
Page 23