3
STRUCTURE AND BASIC OPERATING PRINCIPLES
The UVTRON is a bipolar tube with a structure similar to that of a phototube. Just as with phototubes, the UVTRON
utilizes the photoelectric emission effect, but the inside of the UVTRON tube is filled with special a gas
rather than be-
ing a vacuum, so it operates as a discharge tube. Figure 2 shows its structure and a schematic diagram of operation. A
voltage is applied across the anode and photocathode (cathode) which is only sensitive to ultraviolet light. When UV
light passing through the UV glass (UVTRON bulb) strikes the cathode, photoelectrons (electrons) are emitted from the
cathode surface due to the photoelectric emission effect. These photoelectrons are then drawn to the anode by the elec-
trical field created by the supply voltage. If the supply voltage is low, the operation is the same as for a phototube and
the current, i, is extremely weak. When the voltage is increased to strengthen the electrical field, the photoelectrons are
accelerated so they collide with the gas molecules within the tube and ionize them. The electrons produced by ionization
continue to collide with other gas molecules while causing ionization until they finally reach the anode. Meanwhile, the
positive ions are accelerated towards the cathode and the resulting collisions with the cathode generate a great number
of secondary electrons. As this cycle is repeated, a large current suddenly flows between the anode and cathode, creating
an electrical discharge. This phenomenon is called gas multiplication and the voltage at which this discharge starts is
called the discharge starting voltage V
l
of the UVTRON.
Once the electrical discharge has begun, the tube is filled with electrons and ions and the voltage that maintains the
discharge drops to a low value. This value is called the discharge sustaining voltage Vs. Figure 3 shows this state. The
UVTRON primarily operates in the glow discharge region, but in this region, since the discharge sustaining voltage Vs
is lower than the discharge starting voltage
V
l
, the discharge will continue unless some means is employed to control
the supply voltage.
TPT C0010EA
TPT C0010EA
+
–
i
R
HIGH VOLTAGE Ebb
DISCHARGE
CURRENT
TUBE VOLTAGE V
CURRENT LIMITING
RESISTANCE
PHOTO-
ELECTRONS
ELECTRONS
FROM IONIZATION
CATHOD
(PHOTOCATHODE)
UV LIGHT
FILLED GAS
UV GLASS
ANODE
Schematic Diagram of UVtron Operation
Figure 2: Schematic of UVTRON Operation
TPT B0029EA
TPT B0029EA
10
-6
10
-4
10
-2
1
10
2
Ebb
V
L
V
S
DISCHARGE CURRENT i (A)
GLOW DISCHARGE REGION
OPERATION
POINT
Rs LOAD LINE
TUBE VOLTAGE (V)
UVtron V-I Characteristics
Figure 3: UVTRON Voltage-Current Characteristics
