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TRIAC analog control circuits for inductive loads March 2008
Rev 3
1/15
AN308
Application note
TRIAC analog control circuits for inductive loads
Introduction
The TRIACs of today are well suited to the requirements of switching inductive loads.
TRIAC control circuits must be particularly well tuned to be both economical and applicable
to inductive loads.
The purpose of this document is to present different methods of TRIAC control with their
applications and to analyze their relative advantages and disadvantages.
A simple circuit offering all the guarantees of reliability is proposed for inductive loads.
www.st.com Triggering methods
AN308
2/15

1 Triggering
methods
1.1
Triggering with synchronization on the TRIAC voltage
The triggering circuit with synchronization across the TRIAC (See
Figure 1
and
Figure 2
)
turns on the component at an angle
after the current drops to zero, such that
= · Tr.
Time Tr is defined by the time constant (P + Rt)C.
= 2 · · f with f = mains frequency.
Figure 1.
Typical Circuit: synchronization across the TRIAC
Figure 2.
Synchronization across the TRIAC - waveforms (general case)
This is the simplest possible circuit but in certain cases it can have an important drawback.
For example, consider a highly inductive load (L
/ R > 4) where the TRIAC is turned on
with a considerable delay
, perhaps 100° after the mains voltage zero as in
Figure 3
.
If the TRIAC is turned on at point A, the conduction (
) lasts up to about 150°. The TRIAC
turns off at point B at
+ = 250° after the zero voltage point. At that instant a negative
voltage is applied to the triggering circuit which turns on the TRIAC at point C after an angle
of 100°, that is, 350° from the starting point.
AC Mains
C
P
Rt
Z
L
T
D
Diac
1
2
Mains voltage
T
T
Gate pulse
T
: Current lag (full angle)
: Blocking of the component
: Conduction angle
full angle TRIAC voltage
TRIAC current AN308
Triggering methods
3/15
The second turn-on occurs at a very low voltage and the angle
is much smaller than .
The following period begins under similar conditions and the unbalance persists. This type
of asymmetrical operation is not only unacceptable but can be dangerous (high current due
to load magnetic saturation due to the dc content of the waveform).
The unbalance is illustrated for a particular case, starting from zero of the mains voltage.
Other causes also produce this behavior variation of the load impedance, for example, with motors, due to torque variation modification of the control turn-on angle
This phenomenon is due to the fact that the circuit does not take its time reference from the
mains zero voltage. Rather, the synchronization is taken from the voltage across the TRIAC,
which is dependent on the load current, that is, on the load phase shift.
Figure 3.
Synchronization across the TRIAC: waveforms (delayed turn on)
To sum up, this first very simple triggering circuit, synchronized by the voltage across the
TRIAC, has the following characteristics: Advantages: Simple design and low cost Connection by two wires, without polarity issue Absence of a separate power supply Little power dissipated in P and Rt A serious disadvantage: Because of its principle, this circuit cannot be used for highly inductive loads with a
narrow conduction angle because it can result in unacceptable asymmetrical
operation.
This very simple triggering circuit should be reserved for low-cost applications with the
following characteristics: Resistive or slightly inductive loads No stringent requirements concerning the accuracy of regulation Highly inductive loads where the power varies between 85 and 100% of the maximum
power full angle Mains voltage
Gate pulse B A
'
C
T
T
T
TRIAC voltage
TRIAC current Triggering methods
AN308
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1.2
Triggering with synchronization by the mains voltage
This triggering circuit of
Figure 4
is synchronized by the mains voltage. The pulses are
always shifted by 180° with respect to each other, whatever the type of load.
Figure 4.
Typical circuit: synchronization by mains voltage
Figure 5.
Synchronization by the mains voltage: waveforms
Angle
is the delay between the mains zero voltage and the triggering pulse. It can be
adjusted by means of potentiometer, P, from 0 to 180° to vary the load voltage. The current
in an inductive load (L.R) lags the voltage by an angle
: (tan = · L/ R).
For triggering angles,
, higher than , operation is perfectly symmetrical and stable.
This simple circuit can still present the risk of a fault in the case where
is smaller than , as
shown in
Figure 6
.
As an example, take the case of a highly inductive load and an angle
= 60°. The TRIAC is
turned on at point A (60°). It conducts over more than 180°, up around 230°. It is blocked at
point B: (290°).
AC Mains
C
Z
L
T
D
P
Rt
1
2
Mains voltage
T
Gate pulse
: Current lag (full angle)
: Blocking of the component
: Angle of conduction
T T full angle A
A
: Triggering delay angle
TRIAC voltage
TRIAC current AN308
Triggering methods
5/15
The second triggering pulse occurs at point C when
+ = 240°. It has no action on the
TRIAC which is still conducting. The TRIAC is not turned on for the next half-wave. As in the
previous case, the operation is totally asymmetrical, and thus unacceptable.
Figure 6.
Synchronization by the mains voltage - asymmetrical operation
<
To prevent this fault, it is necessary to limit the turn-on angle to maintain
> . This is
possible for loads whose L and R parameters remain strictly constant.
Experience shows that for the majority of inductive loads used in industrial applications (like
motor controls and transformers) the values of L and R are not constant and vary a great
deal during operation. For these types of applications it is not possible to limit the turn-on
angle without considerably reducing the voltage excursion.
To sum up, this simple triggering circuit, synchronized by the mains voltage, is more
developed than the previous one. It has the following charactersitics: Advantages: Simple design More accurate control than the previous circuit No auxiliary power supply or transformer required Disadvantages: Connection of the circuit by 3 marked wires, instead of 2 without polarity in the
previous circuits Higher power dissipated in passive components P and Rt (since the mains voltage
is continuously applied across them) Operation becomes completely asymmetrical if the control angle
is less than .
This triggering circuit can be used only for applications in which the phase shift of the load
remains constant (air inductor) or if operation is restricted to values of
much higher than ,
that is, for low load voltage operations.
full angle
Mains voltage
Gate pulse
B T
T
T
C TRIAC voltage
TRIAC current Triggering methods
AN308
6/15

1.3
Triggering synchronized by the mains voltage and suitable
for industrial applications
This new circuit is derived from the previous one by improving the triggering pulse
generator. The improvement consists of maintaining the triggering signal during each half
wave between values
and 180°. This is done simply by sending a pulse train after the
initial pulse so as to maintain the triggering order, as shown in
Figure 7
.
Figure 7.
Triggering by pulse train synchronization by the mains voltage
For example, suppose that angle
is equal to 85° and is equal to 60°. At the first pulse,
the TRIAC is turned on at point A (60°). It conducts for angle
1 greater than 180° and close
to 240°. It is blocked at point B, but is immediately triggered at point B by the next gate
pulse. During the first half-waves, operation is slightly asymmetrical but gradually the
durations of conduction become balanced (refer to the dotted line curve in
Figure 7
).
Figure 8
gives the circuit diagram. A small sensitive auxiliary TRIAC, Ts, is used to produce
the required pulse train. The delay time constant, defined by capacitor C, compensating
resistor Rt and potentiometer P, sets the angle
. The capacitor charges from 0 V. DIAC D
triggers TRIAC T as soon as the capacitor voltage reaches the DIAC breakover voltage
(Vbo). This time is the same for both half-waves, it just depends on Vbo symmetry.
A first pulse is applied to the gate of the main TRIAC, T. A voltage pulse occurs across Rd
and triggers sensitive TRIAC Ts. Once turned on, this Ts bypasses potentiometer P. Thus
the remaining charging cycles of the capacitor have a much shorter time constant Rt · C.
Mains voltage
Gate pulse
: Current lag full angl