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How do Dimmer Switches Work?

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We’ve all used dimmers switches before.  Ever wonder how they work?  Or how about why they seem so much more complicated than they used to be?  In this article I will try to shed some light, (pun intended), on these issues.


Really Old School


Back before dimmer switches became common in homes, dimmers were used in commercial settings to control light levels.  Stage and theatrical settings were most common.  Lighting fixtures used bulbs with simple filaments which when encased by a bulb working in a halogen gas, noble gas or a vacuum.  These are called incandescent bulbs.  One property of incandescent bulbs is that they are pure resistors.  As voltage is applied to the bulb, the filament heats up and glows.  More voltage, more light, up to a point of course.  Light bulbs are designed for a rated voltage, and their power usage depends on the resistance of their filament.  If you want to dim an incandescent lamp, the easiest way would be to add a resistor to the circuit.  Add a variable resistor and you in essence have a dimmer switch.  More resistance means less voltage to the lamp, and the dimmer it gets.  There are two problems with such a simple solution though.  First is that there is really no energy savings gained by putting a resistor in the circuit.  The second problem is that resistance being added means that heat must be dissipated at the dimmer switch.  This heat has nowhere to go with a switch encased inside an electrical box, so dimmers of this type just were not practical for use in homes.  Even modern dimmers, which are much more efficient, still have to dissipate some heat.  Let’s look at an admittedly crude attempt on my part to graph this behavior.



Figure (a.) represents normal house current, 120 volts alternating at 60 cycles per second (Hertz).

In figure (b.), the load sees less voltage as a resistor has been added to the circuit.  The difference between (a.) and (b.) must be soaked up by the resistor as heat, so no energy savings occurs.  In fact, the light bulb itself will be less efficient as well, putting out far less light compared to the heat it will generate.


So how did household dimmers work?


Since the mid 1960’s or so dimmers have used circuitry to “clip” the house current signal.  Semiconductors like triacs and thyristors were used to cut off the leading edge of each alternating current cycle.  See figure (c.).  In this figure, the load is operating at roughly half power, while the dimmer circuitry only absorbs a small amount of power in order to operate.  Since incandescent light bulbs act as simple resistors, this clipped power system worked very well to efficiently dim them all the way down to around 1 or 2% of their normal output.  This is the dimmer performance we all grew up with, and the only problem with it was that dimmer switches were physically larger than regular switches.  Sometimes it was hard to fit them inside a standard electrical box when it had more than a few wires in it as well.  Not a big problem really, until light bulb technology brought us compact fluorescent (CFL) and light emitting diode (LED) light bulbs.


Compact fluorescent bulbs are built in a very different way from incandescent bulbs.  Instead of simply sending house current through a glowing filament, CFLs have an electronic ballast built into their base to control the power going into the lamp.  The ballast first takes the 120 volt/60 Hz AC house current and rectifies it to DC.  This DC power then goes to a half bridge that converts it to very high frequency AC power.  The frequency can vary from 20K Hz to over 50K Hz.  An IC then takes this power and controls voltage, current and frequency sweep in order to ignite the fluorescent tube and then maintain light output.  CFLs provide a capacitive load to the dimmer, and are somewhat limited in the range of light output they can handle.  At best, a dimmable CFL will only dim to roughly 30% of its full output.  Even then a special dimmer switch needs to be used to get this performance.


LED light bulbs are much closer to incandescent bulbs in their dimming ability, but also present a complex load on a dimmer switch.  Because of this many LEDs that should be dimmable are not, or end up flashing, flickering or even shutting down when dimming is attempted.  The circuit board inside an LED bulb transforms the 120 volt house power down to the roughly 3 volts DC that each LED element needs.  How a given bulb’s electronics handle the clipping action of a current reducing dimmer will vary.


More recently, bipolar transistors have been used for dimmer circuitry.  These have the advantage of switching off the trailing edge of the voltage sine wave.  See figure (d.).  This tends to reduce any “singing” vibration created in filament bulbs, and also helps with bulb types that are capacitive rather than simply resistive in design.  There are also dimmer circuit designs that can decide which method of wave clipping to use and adapt to whatever load it sees.  These types will perform with a wide variety of light bulbs.




Why has buying a simple dimmer switch become so complicated?  The answer lies in the fact that each manufacturer of light bulbs has their own design for the circuitry that drives them.  That makes dimmer design difficult.  What Lutron has done, for example, is to create dimmers with a C*L rating that are more likely to be able to adapt to modern CFL and LED bulb electronics.  They have also published a list of light bulbs that have been tested and approved to work with their C*L dimmers.  You can find it here:  LINK


So now you have a tool that allows you to pick out the type of dimmer you want to use, and then match the light bulbs you need to that dimmer.  Now they will work as a team.



For more information about dimmers and frequently asked questions, see this link from Lutron:  LINK





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Posted 2015-07-23T18:05:00+0000  by Chris_HD_CHI Chris_HD_CHI