GDT’s are special type of gas filled
tubes used for wide range of electronic/electrical circuits for
providing protection against lightning and other power surges. These
tubes basically has two electrodes that are kept inside a gas filled
closed envelope. In case of electronic applications, the container is
mostly ceramic in nature. For high grade electrical applications
military tubes are used. The electrical characteristics of this tubes
depends on the pressure and composition of gas, and the distance between
the two electrodes contained inside. The most commonly used gases in
GDT’s are given below.
1) Hydrogen gases
2) Deuterium gases
3) Noble gases
4) Elemental vapors (metals and nonmetals)
5) Other gases
6) Insulating gases
An image of a ceramic discharge tube is shown below. Take a look.
There
will be conduction inside the GDT’s due to ionization of gas
molecules. Each GDT have a specific voltage and current rating. A
simple lightning protector circuit is given below.
Lighting/Surge Protector Circuit
In
power lines, usually large amount of voltage is induced (typically
very short time with high amplitude) due to lightning (direct or
indirect strike) or Transients*
*
(Transients caused by other equipments are usually caused by the
discharge of stored energy in inductive and capacitive components.
Electric motors, such as those used in elevators, heating, air
conditioning, refrigeration or other inductive loads, can create a
continuous stream of 250V to 1000V transients. DC motor drives,
variable speed AC motor drives, DC power supply switching, and portable
power tools are other sources of transients.)
Lightning protection circuit:
The basic surge suppression circuit
shown below consists of a VDR** (Voltage Dependent Resistor) and gas
surge suppressor (GDT) connected in series. The protection circuit is
connected between live and mains lead. Normally no current flows through
GDT and VDR1. When the voltage between the terminals is higher than
the sum of voltage ratings of GDT and VDR1 (here both GDT UZ470B and
VDR S20K250 has 250v 16A rating), current starts to flow through those
components.
If more the voltage rises then
more current starts to flow through GDT and VDR1.When the current is
normal, the circuit is reset and resumes it’s functioning. Thus the
current cannot be raised much over that predetermined value. When the
voltage again goes back to normal values G1 and VDR1, the conducting
stops and the circuit remains normal. If the flowing current is more
than the specified value of main self-resettable fuse, the fuse will
break and the circuit will be protected. After the current is normal,
the fuses resets and continue its functioning (protection against short
circuit and overload).
The circuit is designed to
protect sensitive electronic devices against overvoltage transients in
normal mains voltage and overload/ short circuit. Two neon pilot lamps
are also provided with the circuit diagram to show the status of input
and load supply.
(**A VDR (Voltage Dependent
Resistor) is an electronic component with a “diode-like” nonlinear
current–voltage characteristic. The name is a portmanteau of variable
resistor. VDRs are often used to protect circuits against excessive
transient voltages by incorporating them into the circuit in such a way
that, when triggered, they will shunt the current created by the high
voltage away from the sensitive components. A VDR is also known as
Voltage Dependent Resistor or VDR. A VDR’s function is to conduct
significantly increased current when voltage is excessive.)
- Voltage Dependent Resistor
Advantages:
1) Normal working voltage = 230v AC/DC
2) Maximum current rating=16A
3) Cut-off current =16A
4) Cut-off voltage= >300v R.M.S
5) Protection against overloads
6) Protection for short circuit
Applications:
1) Protection for sensitive components
2) Protection for motor devices
3) Telephone line protection
4) SMPS protection
