Advanced Welding Processes: Technologies and Process Control

Welding power source technology 33
system has been used for DC MMA, GMAW, square-wave AC GTAW, [28,
29] GMAW and SAW power sources.
3.4.2 Transistor series regulator
The output of a transistor may be controlled by adjusting the small current
flowing through its ‘base’ connection. The series regulator consists of a
transistor in series with the DC welding supply, the output power being
continuously regulated by means of the base current. It is usual to incorporate
a feed-back control system to ensure output stabilization and an amplifier to
supply the drive signal to the transistor (Fig. 3.8).
Until fairly recently, the capacity of individual transistors was limited and
large banks of devices (connected in parallel) were necessary to handle
typical welding currents. The recent availability of higher-power devices
has, however, reduced this problem. The important characteristics of the
transistor series regulator are its rapid response rate (the response time of
transistors is measured in microseconds) and its ‘ripple’-free output. [30]
The main disadvantages of the system are the poor efficiency and high cost.
The poor efficiency results from the dissipation of surplus power as heat
from the devices, necessitating the use of water cooling for most applications.
The equipment cost is a function of the number of devices used and the need
to balance each transistor to ensure current sharing. The high response rate,
accuracy and low ripple of this type of power supply make it suitable for
small high-precision supplies and, particularly, for process research work.
GTAW, GMAW and SAW units are available.
Transformer
rectifier
Drive
transistor
Main power
transistors
3.8 Principle of the transistor series regulator.
Electronic
control