Hi. I'm Bill Griffith, an Application Engineer with Keysight Technologies. Today, I want
to talk about using dimmer switches with LED light bulbs. I went to the hardware store
and they had a broad selection of switches and they also had an even broader selection
of dimmable light bulbs. I want to take a look at some of those products today and show
you how they work and how they work together. To get started, we will look at how our dimmer
switches work. Right now I have a typical dimmer switch hooked up. It is actually driving
a 60 watt incandescent bulb. We have two waveforms here. We have the waveforms going into the
dimmer switch and the waveforms coming out of the dimmer switch. What I will do is first
I am going to go ahead and just move our waveform down so you can actually see the two waveforms
together. The yellow set of waveform represents our input and the green represents the output
of the dimmer switch. We have our voltage waveform; we have our current waveform; and,
we have our power waveform at the bottom. An incandescent bulb is basically a resistor,
so all the signals look very similar. Now, as we start to dim our light bulb, you can
see what happens. The way the dimmer switch works is it actually keeps the amplitude the
same. The green waveform is our output, so the amplitude – the voltage – stays the
same that is coming out of the dimmer switch. But what it is doing is it is doing phase
control. This is called forward phase control, or leading edge control, where they actually
remove part of the waveform and that reduces the power. We can actually look at that using
our power analyzer. You can see the watts that are going into the dimmer switch is about
32 watts. We can look at the output as well. Now we are looking at the output; it is very
similar. Around 31 watts is what we are outputting. As we continue to dim it, you can see the
watts drop. As the waveform gets smaller the amount of energy that goes to the light is
greatly reduced for about half of the amount of wattage when it is at its full brightness.
That is the way a dimmer switch works. This is called forward phase control, or leading
edge. I wanted to show another dimmer switch. This dimmer switch has a slide control. I
thought it was interesting to look at because it starts with quite a bit of that leading
edge already missing. Just by installing this dimmer switch, it drops down the power to
our light bulb to 35 watts and normally the light bulb at full power is over 40 watts.
Just putting the switch in starts to dim the lights, and then from there it dims down even
more. Again, the same sort of leading edge, or forward phase control, on that switch.
The third – and final – switch I wanted to show you is called a trailing edge switch.
I will go ahead and turn it on. What it does is it actually deletes the trailing edge of
the waveform. You can see the trailing edge is moved as we dim the light, and then we
can put it back. That is known as reverse phase, or trailing edge control. This type
of dimmer switch is usually used with a capacitor device. A lot of low voltage lights use a
capacitive power supply, so this is the ideal switch for that. I just wanted to show you
the difference between the leading edge and the trailing edge control. When we want, I'll
switch light bulbs and we will take a look at our LED light bulbs. We've gone ahead and
installed our inexpensive, dimmable LED bulb. Right away you can see the watts is much different.
Our incandescent bulb used over 42 watts on full brightness whereas the LED we're less
than 10. This is the reason why everyone is switching to LED bulbs, is the energy savings
of greater than 4X is tremendous with these LED bulbs. What we will do with our inexpensive
LED bulb is pay particular attention to our current waveform. I am going to go ahead and
start dimming it. Right away you can see it goes from this really nice current waveform
and it goes into a really complex waveform. As it does that, the LED actually kind of
flashes as we dim it. Then, once we start dimming it and it is in this other mode, then
it dims pretty consistently. But, we do see kind of a spike at the beginning of the LED
when we dim it; when we go from full brightness down to a dim state and a really complex waveform.
At this point we will go ahead and switch over to our more expensive dimmable LED bulb.
Now we've installed our more expensive LED bulb. It works a lot more like the incandescent
bulb. As we dim the LED, we'll watch the current waveform. It dims very, very evenly throughout
the range of the dimmer switch. It is compatible with a lot of different types of dimmer switches,
and the manufacturer on their website actually published a long list of dimmer switches that
their light bulb can be used with. As you can see it is very even from one end of the
scale to the other. In conclusion, we looked today at how dimmer switches actually work
and we also looked at a couple of different types of LED dimmable light bulbs and how
the two work together. I thought you'd find it interesting to actually see the power waveforms
that are generated by the dimmer switches and the LED light bulbs. Thanks for watching.