Datasheet

PHASE ANGLE
Graphs show load voltage
against time on 1/50 second
repeating time base at 30%
and 60% throughput.
Output is a chopped sine
wave allowing more power
through as conduction angle
is increased.
GENERATION OF RFI
The step function of current
creates a wide range of radio
frequencies and is the main
source of RFI.
BURST FIRE
Graphs show load voltage
against time on 1 second
repeating pattern time base at
30% and 60% throughput.
Output is block bursts of
complete sine waves,
switched on and off at zero
voltage mains crossover.
More power is allowed
through as ON to OFF ratio is
increased.
INHIBITION OF RFI
No step function as current is
only switched on at zero
voltage, therefore the RFI
problem is eliminated.
UNITED AUTOMATION LIMITED
1 Southport Business Park Tel [phone]
Southport, PR8 4HQ, U.K. Fax [phone]
Page No 2 of 2 Issue 1 15/07/97
PHASE ANGLE FIRING FEATURES BURST FIRE FIRING FEATURES
In each mains half cycle the duration of thyristor conduction
is determined by the firing instant, relative to mains polarity
changeover. Once switched on, the driven thyristors conduct
power to the load until the end of each applied half cycle,
resulting in a chopped sine wave output. A ramp and
pedestal input circuit, allowing variation of firing phase
angle by DC signal, enables more power through to the load
with increasing conduction angle. Advantages of phase
angle firing include: Operation with all types of loads
including inductive, soft start, current limit facility and
stepless quick response.
Using Zero Volts Switching (ZVS) burst firing, the
alternative form of triggering, gives interference free
AC power control. This circuit inhibits RFI by
switching ‘on’ and ‘off’ at zero volts mains crossover, in
repeating time periods (typically one second). The
number of complete mains sine waves are varied in its
ON/OFF ratio, or duty cycle, linearly by the control
signal level. The burst firing circuit provides trigger
pulses coincident with mains zero polarity change-over,
ensuring only complete half cycles are passed through to
the resistive loads. This prevents step changes in load
current, and thus virtually no RFI is produced. UAL
Burst Firing Circuits are available and suitable for 2,3,4
or 6 wire load connections (see ‘Stacks’ Product
Technical Datasheets). Stability against temperature
and supply voltage variations is exceptionally good.
The performance of any thyristor system and its suitability for a particular application depends on the
type of control circuit used. The main types, characteristics and features of both Phase Angle and Burst
Firing circuits, showing the waveforms and probable sources of RFI, are detailed below:-
PHASE ANGLE TYPES
(e.g. CSR, QVR and PSR family
ZVS, BURST FIRING TYPES
(e.g. BVR, ZVT family)
These are universally applicable and are often more
economical but are susceptible to RFI caused
interaction between systems.
These are virtually free from RFI problems but
are only suitable for resistive loads.