AN2839 Freescale Semiconductor / Motorola, AN2839 Datasheet - Page 2

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

Dimmer Features
Dimmer Features
Control Method
Many homes have lamps that can be made brighter or dimmer by rotating or sliding a control on the on/off
switch. Years ago, this was done using a device called a rheostat which consists of a large variable
resistor. To control the amount of energy going to the light, the rheostat had to dissipate the excess
energy as heat. For example, at half brightness for a 100-watt bulb, approximately 20 W would be
converted to heat in the rheostat.
Modern dimmers work in an entirely different way. They use transistor-like devices called triacs to switch
on the current to a lamp part way into each half-cycle. Unlike the silicon-controlled rectifier (SCR), the triac
can conduct current in both half-cycles when turned on. As soon as it is triggered, the triac will allow the
current to flow through the bulb until that current gets to zero, which happens whenever the voltage
crosses zero.
The amount of energy that reaches the bulb during each half-cycle depends on how long the control waits
before triggering the triac. The longer it waits, the less energy reaches the bulb and it will glow.
Triacs
To successfully apply triacs for power control, an understanding of the triac’s characteristics, ratings, and
limitations is imperative.
Figure 1
that are triggered into conduction when a low-energy signal is applied to their gate, allowing a full wave
AC control. In
used for triggering the device. To avoid confusion, it has become standard practice to specify all currents
and voltages using MT1 as reference.
Triggering a triac requires meeting its gate energy specification. Therefore, the gate should be driven hard
and fast to ensure complete gate turn-on, which helps to prevent false triggering. Usually that means a
gate current of at least three times the gate turn-on current with a pulse train. It is also important to keep
up the input trigger pulse synchronized with the AC power line in order to have a constant conduction
angle.
The dashed region in
angle, measured in electrical degrees, during which the device is blocking the line voltage. The period
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110 V or 220 V, 60 Hz or 50 Hz supply voltage
Up to 100 W lamp dimming
Full wave AC phase control
No transformer for AC power isolation
Up/down touch control option
Customized programmable timer
Low-cost 8-pin MCU implementation
shows the triac power control principle. Triacs are three-terminal AC semiconductor switches
Figure
1a, terminals MT1 and MT2 are the current-carrying terminals; G is the gate terminal
Figure 1b
Implementing a Lamp Dimmer with an HC908Q Family MCU, Rev. 0
corresponds to the voltage applied to the load. The delay angle is the
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