Gas Discharge Tubes (GDT) – Introduction
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.)
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.)
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
1) Protection for sensitive components
2) Protection for motor devices
3) Telephone line protection
4) SMPS protection