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ST6 - Microcontroller (MCU) and TRIACs on the mains (110/220V)

Application Note

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Last Updated: 27/04/2009

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16 A Snubberless", logic level and standard Triacs

8A TRIACS

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Source file for microcontroller and TRIACS on the 110/240V mains

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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.

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Universal motor drive

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1
1.1

Universal motor drive
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 s) 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 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.

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AN392 Figure 2. Motor drive circuit diagram

Universal motor drive

Fuse Line
19 18 17 16 1 1

+5 V

Optional user interface

RESET 7

4.7 M 10 nF
GND

Touch sensor 4.7 M

V DD

47 BTA16 -600CW
GND

PA0 PA1 PA2 PA3 PB2 PB3 PB4

PB0 15 PB1 14

4.7 M

Version

Push button

13 12 11

ST6210 T6210
5 NMI VPP 6 VSS 20 OSCOUT OSCOUT 4 4 GND

1M
+5 V OSCI N 3 3

+5 V

Load 5.6 V Neutral 820-1/2W 1N4148
GND

100 uF/10 V

1

2

Potentiometer 22 pF

8 MHz 22 pF

220 nF/400 V
GND GND GND GND

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 Vdd and Vss. These diodes are sized to withstand a 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.

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Universal motor drive

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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 Major steps of the software

Figure 3.

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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Light dimmer

2

Light dimmer
For a light dimmer application, the board can be plugged in series with the line wire like a mechanical switch. The line synchronization and the auxiliary supply are obtained from the voltage across the TRIAC (Figure 4). Figure 4 shows a schematic which can operate either on 110 V or 240 V mains. It uses an MCU ST6210 and a logic level TRIAC. This circuit drives halogen or incandescent lamps supplied directly from the mains or through a low voltage transformer. It includes soft start and protection against transformer saturation and against open load. The user interfaces are the same as previously presented. Figure 4. Light dimmer circuit diagram
Optional user interface 4.7 M Touch sensor 10 nF GND 4.7 M 4.7 M

Fuse Line 100 BTA08 -600SW
+5 V 19 18 17 16 1

+5 V

RESET V DD PA0 PA1 PA2 PA3 PB2 PB3 PB4 PB5 OSCOUT 4 OSCI N 3 PB0 PB1 13

7 15 14

Version
12 GND 11

Push button

100k 12 V
T?

ST6210 T6210
5 NMI 6 VPP 20 VSS GND

200k
Trans

100k

+5 V

200k
+5 V

1

2

Potentiometer 22 pF

22k 5.6 V
GND

8 MHz 22 pF
GND

GND

GND

GND

Neutral

820-1/2W 220 nF/400 V

1N4148 100 uF/10 V

GND

2.1

Power control
Power is controlled by the phase delay (td) of the TRIAC drive. In the previous design, td is referred to the zero crossing of the line voltage. To avoid a connection to the mains neutral as the circuit is in series with the load, the trigger delay is referred to the zero crossing of the current (see Figure 1). When the TRIAC anode current reaches zero, the mains voltage is reapplied across the TRIAC. Synchronization is achieved by measuring this voltage. This voltage is monitored over each half cycle with a network of resistances connected to two I/O lines of the ST6210. This allows detection of spurious open load and the retriggering of the TRIAC with multipulse operation if it is not latched after the first gate current pulse.

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Light dimmer

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2.2

Operation with a transformer
Low power halogen spots use low voltage lamps (12 V typ.) usually supplied through a low voltage transformer. For good application performance, the MCU program should ensure the following:

At start-up, the delay time between the first gate pulse and the synchronization instant is greater than 5 ms. This limits transformer coil induction and the risk of saturation with associated high peak current. The circuit starts on a positive line half cycle and stops on a negative one. Thus it starts with positive induction and stops after negative induction has been applied. This helps to minimize the size of the magnetic core material, and the current rating of the TRIAC. The timer is precisely tuned in order to obtain 8.3 ms (for 60 Hz) or 10 ms (for 50 Hz) delay between two gate pulses. As a result, the TRIAC is driven symmetrically in both half cycles so that DC voltage content is avoided across the transformer terminals. Saturation risk is then also reduced here. Otherwise, the voltage across the TRIAC is monitored to detect a spurious open load condition at the secondary of the transformer. The inrush current at lamp switch-on (halogen or incandescent) is also reduced due to the soft start feature of the circuit (Figure 6).

2.3

TRIAC drive
The TRIAC is directly driven by the MCU. The pulse driving the TRIAC lasts 50 s. The logic level TRIAC is driven in quadrants QII and QIII with a gate current of 20 mA provided by two I/O lines of the ST6210 in parallel. The logic level TRIAC has a maximum specified gate triggering current of 10 mA at 25 C. The TRIAC is multi-pulse driven. Therefore, inductive loads can be driven without the use of long pulse drives. As a result, the consumption on the +5 V supply can be reduced and the supply circuit components are downsized. Before supplying the first drive pulse, the TRIAC voltage is tested. If no voltage is detected, a spurious open load or a supply disconnection is assumed to have occurred and the circuit is stopped. After the first driving pulse, the TRIAC voltage is monitored. If the TRIAC is not on, another pulse is sent. The same process can be repeated up to four times. Then, if the TRIAC is still not on, the circuit is switched off.

2.4

Circuit components
The light dimmer board (Figure 4) is almost the same as the motor drive board (Figure 2). The major differences concern the point where the voltage is measured and the TRIAC choice. When the board is dimming a resistive load, an RFI filter should be added to limit the conducted noise. In a dimmer, because of the resistive load, dynamic constraints are lower than in a motor control, so a logic level TRIAC (BTA08-600SW) can be used. It is a sensitive TRIAC (IGT < 10 mA) which can be triggered in quadrants I, II and III. This TRIAC has high switching capabilities ((dI/dt)c > 2.98 A/ms, (dV/dt)c > 10 V/ms). Thus it can also operate without any snubber across it. The MCU board in Figure 4 is supplied only when the TRIAC is off. A minimum off-time of the TRIAC (1.7 ms/60 Hz and 2 ms/50 Hz) is necessary to ensure a good VDD level. The RCD circuit is the same as the one used for the board in Figure 2.

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Practical results

2.5

Software
The software for a light dimmer can be the same as for motor drive. The major difference concerns the mains disturbances rejection in order to prevent lamp flickering. The timing is carried out internally by the MCU timer. The mains period can be calculated internally by the MCU to detect the mains frequency. But this mains frequency must not be disturbed by noise coming from the line, as the mains synchronization signal is received every cycle.

3

Practical results
Figure 5 presents the current and voltage in a TRIAC driving a universal motor. Figure 5. Universal motor drive: TRIAC current and voltage

TRIAC current IT : 5 A/div

IT

TRIAC voltage VT: 250 V/div

VT

2 ms/div

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Practical results

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Figure 6 presents the soft start operation for an incandescent lamp rated at 150 W, 230 V. Thanks to the embedded soft start, the peak in-rush current is about 3 times the nominal current compared with 8 to 10 times otherwise. Therefore, the lamp life time is improved. Figure 6. Soft start for 150 W, 230 V incandescent lamp

TRIAC current IT: 1.0 A/dic T

100 ms/div

Voltage across TRIAC VT: 100 V/div

100 ms/div

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Conclusion

4

Conclusion
Microcontroller units (MCU) are in common use in most areas of home appliances. The applications described in this Application note show that enhanced appliance circuits can be designed with ST6210 MCU and a snubberless or logic level TRIAC. The presented circuits are a universal motor drive, and a light dimmer operating from the 110/240 V mains. The motor drive can be adapted, for instance, to vacuum cleaners, food processors, drills or washing machines. The light dimmer drives incandescent and halogen lamps supplied either directly from the mains or through a low voltage transformer. Those circuits include soft start and protection features. Different user interfaces can be chosen: touch sensor, push button or potentiometer. Such features are obtained with only few components: an ST6210 MCU in 20 pin DIL/SMD package with a logic level or snubberless TRIAC in TO-220 package and some passive components. Additional features such as motor speed regulation, torque limitation, vacuum or unbalance control, IR presence detection, remote control, alarm, homebus interface can also be implemented.

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Revision history

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Revision history
Table 1.
Date Jan-1998 24-Apr-2009

Document revision history
Revision 1 2 Initial release. Reformatted to current standards. Updated for current products. Changes

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Document Number: 1863



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