AN392 STMICROELECTRONICS | Alldatasheet
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April 2009 Doc ID 1863 Rev 2 1/11 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.
1 Universal motor drive
1.1 Power control
selection of the 50 Hz/ 60 Hz tables can be implemented. 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
on the board to let the system designer choose the preferred user interface. of less than 50 ms causes no action.
Figure 2. Motor drive circuit diagram
1.3 Circuit components
continuous current of 1 mA (typ.). short interconnecting traces to maximize RFI immunity. ensured according to local electrical safety rules.
8 MHz VDD 1
1.4 Software
available for additional features. top and bottom of the table. again eliminates external components (Figure 3).
- 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
2 Light dimmer
voltage across the TRIAC (Figure 4). are the same as previously presented. Figure 4. Light dimmer circuit diagram
2.1 Power control
TRIAC with multipulse operation if it is not latched after the first gate current pulse.
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 µs. 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 GT < 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.
2.5 Software
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
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, I R presence detection, remote control, alarm, homebus interface can also be implemented.
5 Revision history
Table 1. Document revision history 24-Apr-2009 2 Reformatted to current st andards. Updated for current products.