Light Dimmers have gone through several design generations. Originally, dimmers were contructed out of resistive bridges or autotransformers. These were big and lossy (hot!). In the 60s, SCRs where invented. These were followed by Triacs. These solid state devices were much more compact and efficient (cooler). Recently, IGBTs have been used to make even smaller and cooler devices.

However, SRCs, Triacs and IGBT designs have a problem that until recently was not an issue. That is, they generate a lot of EMI. That is because they switch at very low frequencies (multiples of 60Hz). By 'clipping' the 60Hz signal at various places in the cycle, you generate a lot of harmonics that get reflected into the power source or into the load. To mitigate this situation, designers have resorted to putting large output filters on the devices to contain the harmonics. However, recent/pending policy decisions might require that harmonics be reduced farther than what is presently considered acceptable. It this becomes the case, the required filters will become too large for many applications.

One advantage the old resistive/autotransformer designs had was that they generated no harmonics. Sine wave in. Sine wave out. A Power Factor of one. They could drive resistive, capacitive and inductive loads. Phase dimmers (Triacs and IGBTs) can't do that.

There is a new generation of dimmers coming onto the market that have the advantages of both the old designs and the new designs. They are called 'Sine Wave Dimmers' or 'PWM Dimmers'. They are compact and efficient. They transform a sine wave from one power level to another. I couldn't find many designs floating around on the web. However, there are patents.

Sine Wave Dimmers operate by pumping the current through the dimmer at very high frequency. In doing so, they are free to vary the duty cycle of the pump to control the power delivered to the load. If the switching frequency is high enough, then the effects of the switching can be filtered away by a somewhat small filter -- leaving only the low frequency (60Hz) power.

One approach that seems to be prevalent is to treat the dimmer as a switch mode power supply and treat the light as the load. The idea is to filter the power supply lines (but pass the 60Hz) and then use matched switches to pump the positive and negative cycles through an output filter (an LC filter) which has the load in parallel. Active clamps are used to reduce the ringing in the circuit. By varying the duty cycle of the switches, the amount of current that passes through the load can be controlled (thus controlling the power). I have tried a simplified/modified version of this approach below.

One requirement Jon and I have is that the dimmer be able to replace existing dimmers. It is common practice for electricians to bus the neutrals together in multi-dimmer panels. They do this because they think of the neutral as ground and the phase as providing the power to the load. They don't expect the dimming device (and most commercial dimmers don't) to insert any electronics between the load and ground. Thus, very often, it is not easy to spot the neutral wire running to a specific load in a multi-dimmer box.

Another requirement we have is that the dimmer present AC power to the load. One approach to solving this problem is to simply make a DC to DC buck converter and be done with it (in fact, this is how LED dimmers are done). However, DC currents tend to reduce the light-span of incandecent bulbs. They are also inapropriate for other types of loads (fans, motors, ..etc).

Here is the schematic for my experiment. In this design, I am trying to pump the current through the load and filter away the high frequency switching effects. To do this, I use the inductor (L1) and output capacitor (C1) as a high pass filter. The idea is to let these components pass the high frequency stuff and leave the 60Hz signal for the load. The LC filter formed by L1 and C1 has a cutoff frequency of about 33KHz. However, the load resistance can be as high as 1500 ohms (for a 10W load). This makes the filter resonant at 33KHz so it is important to stay well away from that frequency.

I decided to pump the circuit at 100KHz and live with some bigger components. This is just a demonstration at this point so I decided to keep the frequency well within the capabilities of the MOSFET I am modeling (an IRF840).

Here are some results:

Results. Here is the power (purple), voltage (red) and current (green) waveforms across the load. The current is hard to see because the power curve dominates the y-axis. Notice how clean these curves are! This is a 280W load (100 ohms) at 50% duty cycle. Notice that the power is no where near half of 280W. The power control does not vary linearly with the dutycycle in this technique. I haven't looked into the cause yet. However, I have tried dutycycles from 95% to 5% with success. At 5%, this load was less than 10W.

FFT. Here is the FFT of the power, voltage and current. Notice that the harmonics start at 100KHz and go up from there. Also notice that they are small and well defined. This is in contrast to the phase dimmers which have large harmonics starting at 120Hz and assending.