Precision Control Systems 3629C - Manual (Page 28)

MODEL 3629C 24 04/10/2007
THEORY OF OPERATION
THE SCR
The heart of the SCR 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 several thousand volts. In the on state, they can
pass several thousand amperes. When a small signal is
applied between the gate and cathode terminals, 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 31. 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 32. “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 34 ). 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 or inductive.
Figure 33. SCR “ON” time, shown by shaded area, is
varied to apply the desired load voltage
Figure 34. Power is regulated by advancing or delaying the
point at which the SCR's are turned on.
What is the difference between Current Limiting and Over Current trip?
Current Limit is a circuit that causes the controller to stop increasing the output voltage when a predetermined current
limit setting has been reached. The controller will continue to operate normally up to the current limit setting.
Over Current Trip is a circuit that causes the controller to shut down when a predetermined setting has been reached. No
output will be available until the reset has been operated or the power is interrupted.