UV-4 Flame Detector
372000-28 Rev J
1
Section 1 General Description
The Forney UV-4 flame detector contains an ultraviolet (UV) sensor tube and signal-processing electronics
inside a protective case. These components receive operating power and control inputs from an external
amplifier assembly. During normal operation, light passes through a sight tube or a fiber-optic light guide
and strikes the UV sensor tube. The signal-processing electronics inside the head convert pulses form the
tube into a digital output signal whose frequency is directly proportional to the intensity of UV light striking
the sensor tube. The output signal then is routed to the external amplifier assembly, which converts the
signal from the head into digital flame on/off, analog intensity, and alarm output signals.
The UV-4 flame detector includes two major subassemblies: a head assembly and a base assembly. The
head assembly (figure 1) consists of a machined housing that contains the internal electronics of the flame
detector:
Figure 1 UV-4 Head Configuration
•
The sensor tube contains an ultraviolet-sensitive gas at reduced pressure. When UV light enters the tube,
the gas ionizes, enabling current to flow between two electrodes. Signal conversion electronics on an
internal printed circuit board (PCB) transforms the resulting voltage surges into digital pulses.
•
The PCB contains a voltage level converter, optical isolator, and a single shot. These components
supply the bias voltage required by the sensor tube and transform voltage transients into a well-defined
digital pulse train.
•
A rotary solenoid is located next to the UV sensor tube. When the external amplifier assembly energizes
the solenoid, it moves a mechanical shutter into a position in front of the sensor. This action can be used
to blind the flame detector for a self-test cycle or to disable the system when its operation is not
required.
