Dumb and dimmer
Dimmers are a fantastic way to improve comfort, enhance the home’s appeal, do your bit for the environment and best of all save a few bucks in the process, writes Harry Simidis.
During a recent site survey, I was told in a “matter of fact” kind of way that dimmers didn’t actually save on electricity consumption.
This was because mains power supplied to the dimmer was constant and therefore being consumed all the time.
Tragically, all of this was said in front of the home owner, who quickly quashed any intention of implementing simple and inexpensive energy saving measures using his existing dimmers. Hence, the topic of this issue’s article will be to explain how energy savings are achieved through the use of dimmers with incandescent or halogen lamps.
Most if not all lighting dimmers manufactured in the last 30 years use a TRIAC. As explained in previous articles, the TRIAC is a semiconductor that literally switches the current through the dimmer, on and off 100 times every second (AC mains supply in Australia is at 50Hz – electricity changes direction 50 times every second = 2 half-cycles x 50 = 100cycles/sec). The length of time this TRIAC remains on within each of these half-cycles, determines the brightness of the lamp.
Figure 1 – Phase Angle Control of Lamp Voltage
In the case of a leading edge phase control dimmer, shown in Figure 1, the mains voltage as applied to the dimmer is in green. The dimmer’s output voltage as applied to the lamp is in red. The TRIAC, along with a bunch of other electronics, is responsible for switching the lamp ‘On’ part way through the mains half-cycle. Tracing the red line we can see that current can only be flowing through the lamp when there’s a voltage applied across it. Even though the dimmer is being supplied with the full mains voltage, the actual portion of this that’s being allowed through to the lamp is small, depending on how dim the lamp is.
The conduction phase angle shown in the above example is sitting at around 70o. This means that for approximately 10o of mains phase supply, the dimmer’s output voltage remains ‘Off’. The actual light output corresponding to this graph would be around 18W if we were using a 100W incandescent globe, as shown in Figure 2. This is typically a slight glow, ideal for watching TV.
|Brightness||Volts RMS||Current RMS||Power||Resistance|
Figure 2 – Measured Voltage & Current for 100W Incandescent Lamp (source: http://sound.westhost.com/articles/incandescent.htm)
It should be noted that the power consumption for the fully ‘On’ state exceeds the lamp’s nominal power rating of 100W due to the filament’s resistance changing with temperature and brightness. The level marked as ‘Half Brightness’ is a visual estimate, but corresponds well with the setting that gives a conduction angle of 45° – power at this conduction angle is just under 60W as shown. This means that when the lamp is dimmed to around half its brightness, there’s a saving of around 40% in electricity usage.
It’s worth noting that using an incandescent lamp at a slightly lower voltage than rated will give significantly increased life. Operation at around 90% of rated voltage will increase life by a factor of three, but light output (i.e. lumens) is reduced to about 70%. The overall efficiency of a filament lamp is reduced even further by using a dimmer. Admittedly this is disconcerting, especially when one considers that 95% of all energy consumed is converted into heat. However there are very few options that provide the versatility offered by the dimmer… and you do still save power.
incandescent bulbs last longer
Figure 3 – Incandescent lamp life
The “dimming the lights” column in Figure 3, refers to a visual approximation of Lumen output, as opposed to rated voltage across the lamp. Another major contributor towards extending lamp life is the dimmer’s ability to “soft start”. Typically, most incandescent globes fail when they’re switched ‘On’, due to what’s known as filament shock. This is caused by the filament’s initial lower resistance and hence larger inrush currents when cool.
Soft start operation can also dramatically improve halogen lamp life when operated at lower than maximum voltages.
With the gradual phasing out of incandescent type lamps, newer more efficient lamps such as compact fluorescent (CFL) and LEDs (Light Emitting Diode) will soon become the typical lighting source found in and around the home. Existing phase angle dimmers were never designed for these types of lamps and shouldn’t be used to dim such unless the lamps are specifically designed to be used in this way. Although a topic in own respect, CFL’s and the dimming of these lamps requires careful planning and consideration as they are a completely different light source with different characteristics to incandescent lamps. To date, I’ve had much success in using On-Off control only for CFL lamps seeing as they are vastly more energy efficient with respect to the amount of light they generate. A comparison is shown in Figure 4.
|Lamp Type||Power||Luminous Efficacy (lm/W)||Efficiency ¹|
|Fluorescent (compact)||5W – 24W||45 to 60||6.6% – 8.8%|
|Fluorescent tube (T8 120cm / 4 ft)||36W||93 (max, typical)||14% (max, typical)|
|Fluorescent tube (T5 115cm / 45 in)||28W||104||15.2%|
|Xenon arc lamp||n/a||30 – 50 (typical)||4.4% – 7.3%|
|High pressure sodium||n/a||150||22%|
|Low pressure sodium||n/a||183 – 200||27% – 29%|
|Ideal white light source||242.5||35.5%|
| ¹ – The term “efficiency” is actually fairly meaningless. This is a measure of the “overall luminous efficiency”, and is included as a comparative figure only, calculated such that the maximum possible efficiency is 100%
Figure 4 – Lamp Efficiency
So I guess I’m writing this article in the hope that my friend, and those of similar persuasion, may get a chance to read this and do some research for themselves.
Dimmers are a fantastic way to improve comfort, enhance the home’s appeal, do your bit for the environment and best of all save a few bucks
in the process.
Yes indeed. Please meet our good friend, the dimmer….