The integrated circuit, U217B, is a triac controller for the
zero crossing mode. It is meant to control power in
switching resistive loads of mains supply.
Information regarding supply sync. is provided at Pin 8
via resistor RSync.
To avoid dc load on the mains, full wave logic guarantees
that complete mains cycles are used for load switching.
A fire pulse is released when the inverted input of the
comparator is negative (Pin 4) with respect to the non–
inverted input (Pin 3) and internal reference voltage.
A ramp generator with free selectable duration is possible
with capacitor C2 at Pin 2 which provides not only
symmetrical pulse burst control (figure 3), but also
control with superimposed proportional band (figure 10).
Ramp voltage available at capacitor C2 is decoupled
across emitter follower at Pin l. To maintain the lamp
flicker specification, ramp duration is adjusted according
to the controlling load. In practice, interference should be
avoided (temperature control). Therefore in such cases a
two point control is preferred to proportional control. One
can use internal reference voltage for simple applications.
In that case Pin 3 is inactive and connected to Pin 7
(GND), figure 9.
Firing Pulse Width tp, (Figure 4)
This depends on the latching current of the triac and its
load current. The firing pulse width is determined by the
zero crossing identification which can be influenced with
the help of sync. resistance, Rsync, (figure 6).
w Ǹtp =
Latching current of the triac
Mains supply, effective
Power load (user’s power)
Total current consumption is influenced by the firing
pulse width, which can be calculated as follows:
+ Ǹ w WRsync
VM 2 sin (
Vmains = 230 V∼
IL ( mA)
–VS 2 C2
Figure 2. Pin 1 internal network
300 600 900
tp ( ms )
Rev. A1, 24-May-96