I've been interested in dimmer technologies these days, and spent some time in reading about this topic on the web.
According to what I understand, the problem with usual dimmers is that with inductive or capacitive loads, the current is not in phase with the voltage, which prevents the dimmer from working. So, they invented various technologies like phase adaptive dimmers, or pulse width modulation etc.
I just want to understand: if a dimmer were able to deliver a pure sine voltage of settable amplitude (and of the same frequency as the main), without regard to the current flowing through the device, would the problem be solved ? would led lamps, transformers etc. work well under this "dimming", or is there something else?
Electrical – dimmers – is the problem solved with pure sine wave
acdimmerpower supply
Related Solutions
You say you're not familiar with digital oscillators yet, but I think it's worth looking into. Using a microcontroller with PWM output you can input the signal directly in the amplifier's format. So you won't need an A/D conversion at the amplifier's input stage.
The code for the microcontroller is very simple. Basically it's like this: you need a sine lookup table, and a timer interrupt generating your sample clock. This can be 5kHz, for instance, then you get 100 samples per cycle, which is enough for an inverter application; the transformer will work as a low-pass filter. Every 200\$\mu\$s you take the next value from the table and set the PWM register with this value. The output will be pulse-width modulated following the sine pattern.
This means that your class-D amplifier doesn't need the input stage with the triangle wave generator + comparator (this would otherwise be you A/D converter), so it only consists of the driver stage for driving the output MOSFETs.
You get a crystal precision frequency virtually for free.
PS: a center-aligned PWM signal will give you a cleaner sine approximation, but many microcontrollers only can generate edge-aligned PWM. This Freescale controller is available in an SOIC-8 package and is an example of a microcontroller which can do center-aligned PWM.
image from this document
Wikipedia has a nice article on phase angle control which is used for most house hold light bulb dimmers. They usually don't work for halogen, LED, TL. This is a typical case where a single image says more than a thousand words:
It works by chopping a (varying) part from the sine wave using a triac. The image shows a rectified wave, but the mechanism works equally well with an unrectified one, the latter being most common in household light bulb dimmers. The dimmer is connected in series with the lamp and therefore needs to see a minimum load (in the order of 10W) to work.
This is the classical dimmer. Not sure how halogen / LED / CFL differs from this, but I know from experience (with CFL) that an unmatched dimmer can make the lamp flash at a really annoying frequency.
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Best Answer
Incandescent filament lamps would dim well on the waveform you propose, but then they work well with conventional TRIAC phase shift dimmers.
As flourescent lights need a minimum voltage, and both compact flourescents and LEDs use electronics which rectifies the peak voltage provided, a variable amplitude sine wave would fail to dim all of these types, but in a slightly different way that the phase shift dimmer fails to dim them.
Ballast flourescent lights would dim as the voltage reduced a little, then probably flicker and fail to light at all as the tube temperature dropped at further reduced voltage. For high frequency flourescent, CCFL and LED, it all depends on how the control circuitry has been designed. 'Passive' LED with the cheap'n'cheerful capacitive input would dim quite a long way, as the current dropped with voltage. LEDs with a constant current source would keep going at constant brightness, drawing more current from the supply as the voltage dropped, until they went out at a low enough voltage.
Note that dimmable LEDs are available. They rectify the peak voltage, take their control input as the width of the pulse from the TRIAC dimmer, and program their current output accordingly. Some clever types even change their colour temperature according to the dimmer setting, hard white when full on, and warm white when dimmed.
Dimmers. The problem is solved when the type of dimmer and the type of lamp are matched.