3.2
Triac control
A Triac or AC switch can be controlled through a DC or pulse gate command. If you apply a gate current until the
Triac current exceeds I
L
(latching current), the Triac remains ON even if the gate current is removed. The Triac
turns OFF when the Triac current reaches zero, if the gate control is released.
Figure 19.
Triac behavior according to gate control and latching current
Gate control
V
M AINS
I
T
I
L
I
L
Triac curr ent doe sn’t reach I
L
before end of gate pulse
Triac doe sn’t remain ON
Triac curr ent reaches I
L
before
end of gat e pulse
Triac rem ai ns ON
Triac t urns-off when the Triac
current reaches zero and if no
gate pulse is app l ied
You can control a Triac gate in one of the following ways:
1.
DC gate control: a continuous gate current is applied. This ensures the TRIAC turns ON, but it is energy-
consuming.
2.
Pulse gate control: a pulse gate current is applied. The gate power consumption is lower but you need to
verify that I
L
is reached before releasing the gate current.
3.3
Operating mode
The following operating modes are available to use the loads by controlling the AC switches:
1.
ON/OFF basic mode: the Triac is ON during the whole period. So the load current is a full-wave sinusoidal
current. This control mode is usually used to control fans, pumps or resistive loads. The gate signal can be
DC or pulsed.
2.
ON/OFF timer mode: the Triac is ON for a defined period, so the load current is a full-wave sinusoidal
current for a chosen duration. This control mode is usually used to control fans, pumps or resistive loads,
and also for specific applications like lighting and other home control applications with timer functions.
3.
Phase control mode: the Triac turns ON with a delay with respect to the ZVS. Current conduction starts at
the end of the half-cycle. The current conduction per half-cycle can then be set between 0 to half-a-cycle in
order to control the load power. This control mode is usually used to control fans, universal motor speeds or
lamp brightness. Only the pulse gate command is available.
The above loads (and corresponding AC switches) can be controlled at the same time or individually. To control
several AC switches at the same time, the control mode and parameters are common to all the AC switch
controls.
3.3.1
Mode selection
The “ON/OFF basic”, “ON/OFF timer” and “Phase control” operating modes are selectable through the three-
position “MODE” switch (SW201), see
UM2304
Triac control
UM2304
-
Rev 3
page 14/46
