Precision Control Systems 1029C - Manual (Page 23)

MODEL 1029C 19 10/19/2007
WARNING: the Control Concepts, Inc, model 1029C power controller uses power thyristors to switch
voltage to the connected load. Line voltage must be assumed at the output terminals at all times, even
when the control signal has been removed and the load voltage appears to be off.
It has been mandated by the National Electrical Code and the Occupational Safety and Health Act of 1970
that a physical disconnect be opened ahead of all remotely actuated controls before
performing any maintenance work on the controller or its connected load.
THEORY OF OPERATION
THE SCR
The heart of the Control Concepts, Inc. power controller is
the SCR (silicon controlled rectifier, sometimes referred to
as a thyristor).
The SCR has two states, ON and OFF, and allows current
to flow in only one direction when turned on. SCRs can
remain in the off state even though the applied potential
may be up to 1600 volts.
In the on state, they can pass several thousand amperes.
When a small signal is applied between the gate and
cathode terminals ( Figure 20 ), the SCR will turn on within
10-100 microseconds.
Once turned on, it will remain on until the current through it
is reduced below a very low value, referred to as the
holding current.
CURRENT FLOW
CATHODE
GATE
ANODE
Figure 20. SCR symbol
Because the SCR allows current to flow in only one
direction, two SCR’s are connected in an inverse parallel
(back to back) configuration to control AC current.
Figure 21. “AC Switch”
PHASE-ANGLE OPERATION
In phase-angle control, each SCR of the back-to-back pair
is turned on for a variable portion of the half-cycle that it
conducts. ( Figure 22 ).
Power is regulated by advancing or delaying the point at
which the SCR is turned ON within each half cycle. Light
dimmers are an example of phase-angle control.
Phase-angle control provides a very fine resolution of
power and is used to control fast responding loads such as
tungsten-filament lamps or loads in which the resistance
changes as a function of temperature.
Phase-angle control is required if the load is transformer-
coupled, capacitive, inductive or a variable resistance load
requiring current limiting.
Point at which
SCR begins to
conduct
Figure 22. SCR “ON” time, shown by shaded area, is
varied to apply the desired load voltage