The
Gas discharge surge protector consists of a
glass
or
ceramic tube with an
electrode in each end.
The
tube contains a low
pressure gas mix. When
the
voltage over the
tube rises over a
certain level, the
gas starts to
ionize. When the
voltage rises
further(to the
so called
arc voltage),
it causes an
avalance effect. This means that fast
moving
electrons from the
ionized gas collides with other the
other gas
atoms and causes further
ionization. The ionized
gas is a
good conductor, and the resistance can fall as low
as 5
mili ohms. The
switching speed of the gas
discharge
tube can be down to 1
micro second. By using
different gas
mixes the arc
voltage can be
changed.
This kind of tube is used to
protect against
surges
caused by
lightning,
switching of
inductive load ,
EMPs and
electro static discharges. To protect against
pulses
with
nano second
rise times, it is used in
parallel with
a
voltage dependant resistor. This
circuit has the
advantages of the
VDR's fast
switching time(<25nS) and the
discharge tube's
abillity to
conduct high
currents(>10KA).
An example of a circuit with discharge tubes and VDRs
(For 230V AC)
~ --------*---------*---------------.
| | | |
/V\ | |/ VDR 1 |
SP1 | | |/| 150V |
\^/ /| | |
| | | |
GND --------*---------*------------| | | LOAD
| | | | |
/V\ | |/ VDR 2 |
SP2 | | |/| 150V |
\^/ /| | |
| | |
~ --------*----------*--------------'