ST AN392 Application note

AN392

Application note

Microcontrollers and TRIAC-based dimmers

Introduction

Today, electronics is used in home appliances for applications as widely varying as motor
regulation in a washing machine, control of a vacuum cleaner, dimming of a lamp or heating
in a coffee machine. This evolution has increased pace rapidly because appliances require
enhanced features that are easy to build and modify while electronics-based solutions
become cheaper and more sophisticated.

Within this evolution, microcontrollers (MCU) progressively replace analog controllers and
discrete solutions even in low cost applications. MCUs are more flexible, often need less
components and provide shorter time to market. With an analog IC, the designer is limited to
a fixed function frozen inside the device. With a DIAC control, features like sensor feedback
or enhanced motor drive cannot be easily implemented. With an MCU the designer can
include his own ideas and test them directly using EPROM or one time programmable (OTP)
versions.

The TRIAC is the least expensive power switch to operate directly on the 110/240 V mains.
Thus it is the optimal switch for most of the low-cost power applications operating online.
The logic level or snubberless TRIACs can operate with low gate current and can be directly
triggered by the MCU.

This application note describes two different MCU based applications: a universal motor
drive, and a light dimmer. They all operate with the same user interfaces and almost the
same software and hardware.

April 2009 Doc ID 1863 Rev 2 1/11

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Universal motor drive AN392

1 Universal motor drive

1.1 Power control

The power device is a TRIAC because it is the most economical online switch. In a TRIAC­based controller the output power, and, for example, the motor speed, are controlled by the
phase delay of the TRIAC drive. This delay is referred to the zero crossing of the line voltage
which is detected by means of a connection to the mains neutral (Figure 1). Changing
operation from 60 Hz to 50 Hz can be achieved by making simple modifications to the MCU
EPROM/ROM table defining the TRIAC conduction angle versus power level. Automatic
selection of the 50 Hz/ 60 Hz tables can be implemented.

The TRIAC can be directly driven by the MCU. A very short gate current pulse (~ 100 µs)
could be enough to trigger the TRIAC for rms load currents above 2 A. Such pulse control
allows the low voltage MCU power supply consumption to be reduced. The snubberless
TRIAC is driven in quadrants QII and QIII with a 60 mA gate current provided by three I/O
bits of the ST6210 in parallel. This pulse is sufficiently long to ensure the TRIAC is latched at
the end of the pulse. Pulse length can be modified if another TRIAC or motor is used.

Figure 1. Mains synchronization

1.2 User interfaces

Different user interfaces can be implemented - a touch control, a push button or a

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potentiometer. The circuit diagram in Figure 2 show that the three modes are implemented
on the board to let the system designer choose the preferred user interface.

Control action is obtained when the sensor or the button is touched for more than 330 ms. If
the touch duration is between 50 ms and 330 ms, the circuit is switched on or off. A contact
of less than 50 ms causes no action.

AN392 Universal motor drive

Figure 2. Motor drive circuit diagram

Optional

GND

user interface

4.7 M

4.7 M

Push button

GND

+5 V

Potentiometer

Touch sensor

4.7 M

Line

Load

Neutral

Fuse

Fuse

47

47

BTA16-600CW

-

1M

1M

5.6 V

5.6 V

820-1/2W

820-1/2W

220 nF/400 V

GND

Version

1N4148

1N4148

19

19
18

18
17

17
16

16

13

13

12

12

11

11

+5 V

100 uF/10 V

1

1

VDD

VDD

PA 0

PA 0
PA 1

PA 1
PA 2

PA 2
PA 3

PA 3

PB2

PB2

PB3

PB3

PB4

PB4

OSCIN

OSCIN

3

3

8 MHz

8 MHz

22 pF 22 pF

+5 V

ST6210

ST6210

OSCOUT

OSCOUT

4

4

1 2

1 2

GNDGNDGND GNDGND

RESET

RESET

PB0

PB0

PB1

PB1

NMI

NMI
VPP

VPP
VSS

VSS

7

7

10 nF

15

15

14

14

5

5
6

6
20

20

1.3 Circuit components

The MCU chosen (ST6210) includes an 8 bit accumulator, 2 k ROM, 64 bytes RAM, an 8 bit
A/D converter that can be connected to 8 different inputs, 4 I/O lines with 40 mA sink current
capability and a timer. Hysteresis protection is included in series with each I/O pin. The
ST6210 is packaged in DIL or SMD packages. The ports, the timer and interrupt
configurations can be chosen by software, providing high flexibility. The ST6210 has been
designed to operate in very disturbed environments. Each I/O line contains internal diodes
which clamp the input voltage between V
continuous current of 1 mA (typ.).

The snubberless TRIAC (BTA 16-600CW) has been specially designed to drive loads which
generate very strong dynamic constraints such as a vacuum cleaner motor. This TRIAC can
be triggered in quadrants QI, QII or QIII with gate and latching current of 35 mA and 80 mA
respectively. In this application it is driven by three I/O lines of the ST6210 in parallel. This
TRIAC has high current switching capability ((dI/dt)c > 8.5 A/ ms and 5.5 A/ms for
BTA10600CW), and high static dv/dt ((dV/dt) > 500 V/ms). So, in this circuit, it can operate
without any snubber.

Total consumption of the board is 3 mA with an 8 MHz oscillator. The board supply comes
from the mains through a simple RCD circuit. The +5 V is referred to anode 1 of the TRIAC
in order to provide the negative gate current necessary to drive the TRIAC in quadrants QII
and QIII. The 5 V supply capacitance is connected as near as possible to the MCU with very
short interconnecting traces to maximize RFI immunity.

The touch sensor is a voltage divider between line and neutral. It works only if the +5 V
supply input of the circuit is connected to the line. This connection to the mains must be
ensured according to local electrical safety rules.

and Vss. These diodes are sized to withstand a

dd

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Universal motor drive AN392

1.4 Software

All operating features are contained in a 700 byte program. So more than 1 byte of ROM is
available for additional features.

A look-up table relating delay time to the power requirement contains 64 different levels. The
conduction time of the TRIAC can vary from 1.7 ms to 6.7 ms for a 60 Hz application and
from 2 ms to 8 ms to a 50 Hz application. The user can easily adjust the minimum and
maximum power levels because the corresponding delay times are slowly changing at the
top and bottom of the table.

It is recommended that all MCU inputs be filtered so that an input is validated only if it
remains constant for 15 s or more so that passive filter components can be saved. The
mains supply carries disturbances (glitches, telecommand signals, ...) which could disturb
the TRIAC drive. For this reason, a mains voltage zero crossing is only validated if it occurs
during a window of time (1.7 ms each 16.6 ms for 60 Hz operation and 2 ms each 200 ms for
50 Hz operation) selected by the internal timer of the MCU. This block acts as a filter and
again eliminates external components (Figure 3).

This circuit can be used in the following applications:

● Vacuum regulation in a vacuum cleaner

● Speed control in a food processor

● Speed regulation with torque limiting in a drill

● Unbalance detection in a washing machine

● Washing machine door opener with remote control

Figure 3. Major steps of the software

RESET

Initialization

Read version

Line synchronization

Sensor acquisition

Power level requirement

Delay time td1 in timer

Calculation next delay

TRIAC firing

Delay time td2 in timer

Calculation next delay

TRIAC firing

Window for zero

crossing mains

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