I am building a street lamp controller that should be able to set a street lamp to on, off, or dim state. I have successfully built an incandescent lamp dimmer (phase dimming - using zero cross detector, AVR microcontroller, and controllable TRIAC driver). Now I am interested in building a similar thing for street lamps.
From my reading most street lamps still use HID lamps (metal halide, mercury, high pressure sodium) and they are not purely resistive so that some people in other forums state that they cannot be dimmed using phase dimming method. For example see http://www.edaboard.com/thread115980.html
My question is how should these kind of lamps (metal halide, high pressure sodium) be dimmed. This seems possible since I have seen such dimmer products in the Alibaba store but the manufacturer cannot tell me their method. What waveform should be expect?
- Based on NEMA standard, HID lamps can be dimmed: step level or continuous (phase control, high frequency shifting). The limitation is: power level and RMS voltage: cannot below specified value (determined by ANSI or lamp manufacturer)
- Phase control is somehow unsuitable (leading phase control may introduce flickering while lagging phase is dangerous for inductive load such as HID ballasts)
- High frequency shifting is a method that is so far, according to NEMA, seems safe enough for dimming.
- So far, the recommendation is to use step level switching (be aware of minimum power requirement) and, most importantly, consult lamp manufacturer
My question is, according to Russell's link here, HID lamps is operated at low frequency to avoid acoustic resonance (may ended in damage or lamp explosion). So, if I apply high frequency shifting on the ballast input, this will not affect the low frequency output of the ballast. Is this true?