A dim view of dimmers

In the great recessed lighting experiment I tried three different kinds of LED lights. I learned a lot about LEDs. I also learned a lot about dimmer switches, kicking and screaming the whole way.

The first LED strip I used never turned completely off. Ever. Regardless of the dimmer setting, or position of the on/off switch. So I hooked it up to a scope. Ewww:

I confess that up to this point I had never thought much about dimmer switches or how they worked. I somehow preserved a naive view of them as little variable resistors. That doesn’t make any sense, though, because a resistor would need to dissipate power.

Old dimmer switches did in fact dissipate significant power, and occasionally killed people. Head over to Wikipedia and you’ll learn about some crazy dangerous historical designs, such as using salt water or mercury to make contacts.

As soon as I started reading about today’s dimmers I got a bad feeling. Lutron, a giant lighting manufacturer, has a wide variety of dimmers, and a web page that explains in broad categories when they are meant for. There are many, many types, and dubious branding and sales-talk electronics. For example, the phrases “magnetic transformer” and “electronic transformer” are used as if it is self-evident what these are. An “electronic transformer” presumably means things like switching power supplies (which have inside them a “magnetic” transformer – oh well.)

Unlike the exciting historical dimmers, modern dimmers use a cheap semiconductor device called a thyristor or triac. This is an on-off switch, and the dimming is achieved by turning off more or less of a fraction of the AC waveform. It’s a blunt tool to reduce power, polluting the 60Hz AC with high harmonics.

Although cheap, these designs create a long list of problems. They often require a significant load to function, and unless the load looks like a resistor, the lights may not come on, go off when they shouldn’t, or even flash. They make incandescent bulb filaments “sing” by suddenly turning on and off the power (a coiled filament is a small electromagnet, after all, and bucks by the change in magnetic flux when turned on and off).

What’s worse, dimmers interoperate badly with different types of bulbs in ways that are hard to predict. This remarkable pdf document shows a giant test matrix of 15 dimmers against hundreds of different bulbs, and many combinations don’t work well. It makes me sad that this approach – test every possible combination – is the state of the art.

After I educated myself, I bought a triac dimmer specifically for an LED load. It also had a trim pot to control the brightness where it would cut out, since that seemed to be an issue. It worked slightly better – at least turning it off actually cut the power.

At this point I lost faith. The only good dimmers I can find for sale – promising a simple sin wave, like my benchtop variac – are hundreds of dollars.

Fortunately, the final fluorescent form factor LED bulb with integrated electronics has solved the problem for me, by having either sufficient capacitance or inductance to help with the pulses. It does flicker slightly at intermediate dimness, but the highest and lowest settings provide very stable light, and the lowest dim setting now makes a fine night light.

We’ve achieved adequancy, but I’m itching to design a dream dimmer for myself.

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