Halogen lamp dimmers
4 April 2012
Opto-Electronics
LSI/CSI has developed two ranges of lamp dimmers for halogen lamps used in different applications.
The LS7232NT is designed for brightness control of incandescent and transformer-coupled, low-voltage halogen lamps. Brightness is controlled by adjusting the firing angle of a triac in series with the lamp and triggered by the IC. A phase-locked loop keeps the device’s phase pointer locked in phase with the line voltage.
An optical user interface provides for a smooth control of the lamp intensity without the need to touch any sensor plate. A mechanical switch interface for dimming control is also provided.
The IC includes features to address problems associated with inductive loads such as transformers in low-voltage halogen lamp applications. The voltage to current phase lag resulting from the inductance may prevent the triac from shutting off at AC zero crossover and/or prevent the triac from maintaining conduction following removal of the gate trigger.
The LS7232NT includes an extension input for remote control, safety shutdown for transformer malfunction, and pin-selectable operating modes.
For pushbutton or touch control of high-voltage lamps and all electronic transformer-coupled, low-voltage halogen lamps, the LS7636/LS7637 are available, as well as their ‘FO’ versions which power up fully-on after application of AC.
Since the operating current of some electronic transformers are below the holding current of most triacs, a typical triac-based (leading-edge) dimmer may not operate properly when driving an electronic transformer-coupled, low-voltage halogen lamp. The filtering components in the electronic transformer which prevent its operating frequency from generating line interference can also inhibit triac triggering.
The LS7636/LS7637 family devices drive FETs or IGBTs and slowly turn on current beginning at the AC input zero-crossing. This slow current turn-on is ideal for interfacing with the primary of any electronic transformer and also eliminates the RFI generation caused by the sudden turn-on of current when using a triac-based dimmer.
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