AN2839 Freescale Semiconductor / Motorola, AN2839 Datasheet - Page 5

• Manufacturer Part Number: AN2839
• Description: Implementing a Lamp Dimmer with an HC908Q Family MCU
• Manufacturer: Freescale Semiconductor / Motorola

Application Description
Figure 2. Dimmer with the HC908Q Family
The triac must be chosen according to the required load current. For a 100-W lamp, the load current at
115 V is 0.87 A and at 240 V, it is 0.42 A. Therefore, for the triac MCR22-8, 600 V isolation is a
reasonable choice and can be used for both 115 V and 240 V AC. The resistor R3 limits the triac gate
current.
All discrete devices in the circuit are not critical and similar devices can be substituted. The user must be
careful about reverse voltage and direct current on diodes and zener, isolation voltage of capacitors, and
maximum current in the triac.
Two push buttons and a switch are used to set the lamp brightness and turn the circuit on/off. When the
up/down control is pressed, the MCU receives a low level and varies the conduction angle by shifting the
short pulses that trigger the triac. The down button allows the MCU to apply pulses reducing α and the
lamp brightness is reduced continuously. Conversely, the up button is used to increase α, which increases
lamp brightness.
Because there is no transformer for power-line isolation, the user must be very careful during assembly
and testing to provide the appropriate isolation from the AC power line.
Figure
3,
Figure
4, and
Figure 5
show the power line signal (a), zero-crossing reference signal (b), MCU
short pulses triggering the triac (c), and the waveform of the voltage applied to the load at different
average powers (d). Notice that only a portion of the sine wave is applied to the load. The average power
is proportional to absolute average voltage.
Implementing a Lamp Dimmer with an HC908Q Family MCU, Rev. 0
Freescale Semiconductor
5