Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
Compact Fluorescent Lamp
A long time friend, K8AQC, asked about the AC current and
of the compact fluorescent lamps (CFL) he uses.
Originally written June 2008
revised 27 Sept 2008
Introduction and Economics
I've made measurements of the AC current and power and
also looked at the broadband and line spectrum noise coupled back into the AC
line by a Sylvania 100 watt (equivalent) 23 watt actual screw-in CFL, model
This lamp appears typical of those sold as of mid 2008. I
purchased this one at a Sears Hardware store for about $7.00. The packaging says
the expected life is 8000 hours with an output of 1600 lumens. The packaging
also says a typical 100 watt incandescent lamp has an output of 1680 lumens. The
electricity and replacement lamp savings are stated as $61. (The price on CFLs
continues to drop and with a bit of shopping around, you can find 60 watt
equivalent lamps for $2 to $3 and 100 watt equivalent lamps for $5.)
Before looking at the electrical and noise characteristics
of this lamp, are the cost savings figures approximately correct?
The industrial supply house I deal with, McMaster-Carr,
sells a 1600 lumen 100 watt incandescent lamp, with an estimated 750 hour life,
at $0.98 for a pack of two lamps. For a reasonable quantity purchase, and
including shipping, these lamps would run about $0.70 each, delivered. In 8000
hours, therefore, our lamp capital costs are:
||Number for 8K Hours
||Cost for 8K Hours
Capital cost is essentially a wash. I'm ignoring the time
cost of money in this simplified analysis, but that won't make too much of an
error. (If you can find a 100 watt equivalent CFL lamps for $5.00 each, the
capital cost shifts by $4.00 or so towards the CFL.)
To look for the main savings, therefore, we must look at
power costs, where the CFL consumes 77 watts less power. Over an 8,000 hour
operating life, therefore, the CFL consumes 616 fewer kilowatt hours.
Northern Virginia Electrical Coop (NOVEC) serves Clifton
and has prices considerably below many other utilities--the consumption-based
portion of our power bill in Clifton is 7.81 cents/kwh according to this month's
bill. Hence, our power savings would be $48.10 over the CFL's lifetime, assuming
no price increases and again neglecting the time value of money.
Electrical power in other areas can be considerably
state-by-state averages (2006 data), and shows Virginia at 8.49 cents/kwh. New
York, in contrast, is nearly 17 cents/kwh. (Good for me as a small shareholder
of ConEd, but not so good for New York residents) California averages 14.3
cents/kwh. The lowest priced state seems to be West Virginia at 6.35 cents/kwh,
and New York is the most expensive. The 2006 national average is 10.4 cents/kwh.
||2006 Price (cents per kwh)
|West Virginia (lowest)
|New York (highest)
The claimed $61 savings are therefore quite believable.
It is worth noting that the packaging notes that the CFL
life may be reduced (and light output is also reduced) when the lamp is used in
base-down position. It also notes that lamp life is reduced when used in
enclosed or recessed fixtures. It is also the case that CFLs experience a marked
reduction in lifetime when cycled with short on times.
In that regard, I've found that CFL flood lamp
replacements used in ceiling fixtures have a very short life. However, the
operating cost savings are so considerable that even if the CFL lifetime is 3000
or 4000 hours, it still is cost effective. I also found McMaster-Carr carries a
line of CFL lamps with 10,000-15,000 hour life ratings, although to get the
price down to $10 each, 12 units or more have to be purchased.
What's Inside a CFL?
A CFL contains a DC power supply and an AC inverter
typically operating in the 40-50 KHz range. The voltage inverter causes a
current-limited current to flow through the gases contained within the CFL
envelope. The ultraviolet light produced by the gases is converted to visible
light by fluorescent phosphors deposited on the glass tube's inside surface. The
inverter is designed to limit the current through the tube to its rated value,
and usually employs a resonant tank-type output.
Historically, conventional fluorescent lamps, powered by
the 120V line, limit lamp current through a series inductor, called a "ballast."
For that reason, the CFL's electronics inverter module is often called an
I had not intended to disassemble the lamp, but that
plan was changed when I dropped it onto the concrete floor in my basement shop.
After cleaning up the remains (and being suitably careful to avoid potential
mercury vapor exposure from the broken lamp), I disassembled the electronics
CFL lamp partially disassembled. The four wires (one is
broken and no longer connected to the printed circuit board) are the
fluorescent tube connections.
Component side of PCB.
Current and Power
As the CFL module has a DC power supply, we may expect that it will behave like
any DC power supply, i.e., it will draw current from the mains power only
when the instantaneous AC voltage exceeds the DC voltage on the filter
capacitor, plus the rectifier diode drop.
To see whether this is the case, I powered the CFL from
the AC mains, and monitored the mains voltage and supplied current. (This
must be done carefully, using an isolation transformer. Don't do this at home
unless you thoroughly understand the safety of life issues and have the
necessary equipment and knowledge to use it correctly.)
I used a Tektronix TDS430 digital oscilloscope, with the
line voltage on Channel 1 and the current, sensed through a TCP202 Hall-effect
current probe on Channel 2. I also used the TDS430's mathematical option package
to compute and display the instantaneous power, i.e., the product of the
voltage (CH 1) and current (CH 2) traces. This trace is displayed as M2.
In addition, I used the display measurement feature to compute the average of
the instantaneous power, shown at the right margin as "M2 Mean." The display
identifies current is VΩ and average power as VV. In the case of CH 2, the scale
should be understood as 1.00 A/div and 200 watts/division for M2.
We can learn quite a bit from studying this oscilloscope capture. Looking at the
current, for example, we see that, as expected, current is drawn over a
relatively small fraction of the total AC cycle. An expanded view of the current
trace is shown below. Although the average power consumed by the CFL is 21.2
watts, the peak current is 1.17 amperes. And, current is drawn only for 2.6
milliseconds for each half-cycle. At 60 Hz, a half-cycle is 8.33 ms, so the
lamp's full required power is consumed over about 31% of the AC cycle.
Looking at the M2, or instantaneous power waveform, in the
oscilloscope trace above we see a similar effect�the instantaneous power peak is
a bit over 200 watts.
The oscilloscope capture below shows a 40 watt incandescent
lamp in the same test setup. It looks quite different than the CFL.
Although the instantaneous power still varies with time�as it should since the
lamp looks more or less like a resistor and the instantaneous current varies
proportionally with the instantaneous voltage�the peak effect is much less
pronounced and the power is more uniformly spread over the full cycle.
So what, you're probably thinking. I have many electronic power supplies running
everything from televisions to computers to amateur radio equipment and
all have this same peak cycle current draw.
true. I can't find a definitive number in a quick Internet check, but the
figures thrown around say that 15-20 percent of electrical power consumption in
the US results from lighting and the great majority of this represents
If we switch to CFL lamps, the total lighting-related
power consumption will drop, a good thing generally. However, having a
significant proportion of the total electrical power load represented by AC
waveform peak devices such as the Sylvania CFL I looked at is definitely not a
good thing. It increases the harmonics in the power line and also presents a
highly non-linear load to the power utilities. These things complicate both
electrical generation and the distribution network, including power
transformers. The European Union, in fact, has adopted a standard, EN61000-3-2,
placing limits on the relative amplitude of the current pulse harmonics, with
the thought of avoiding or at least reducing problems caused to the electrical
power grid by peak-charging loads such as the CFL.
This is sometimes said to be a "power factor" issue.
Normally the power factor relates to the phase between the voltage and current
waveforms. Power factor is the cosine of the phase difference between the
voltage and current, sometimes expressed as a percentage and sometimes as a
ratio over the range -1 to +1. A resistive load has a power factor of 1.0,
as the voltage and current are in phase. Motors are inductive and have a power
factor that varies with motor size and construction, but is usually in the 0.8
to 0.9 range. Overall, a power utility's load is normally inductive due to
customer's motor loads, and the power factor can be brought closer to 1.00 with
capacitors, if necessary. You will sometimes see a capacitor package installed
on pole tops by some utilities for power factor correction. (At the risk of
being inaccurate due to brevity, the power generation and distribution network
must be sized for apparent power, i.e., VARs or the product of voltage
times current, called volt-amperes. The actual load that residential and small
business customers pay for, however, is voltage times current times power factor
or watts. Hence, it's most efficient for the utility if power factor = 1.00.
Large scale industrial and commercial users, however, often are billed for both
VARs and kwh.)
The issue with a CFL is not so much the power factor, as
it is that the entire power is drawn over a relatively small segment of
the 60 Hz waveform. This spike-type current waveform, if analyzed in a Fourier
series, or looked at with a spectrum analyzer, will show that it causes
harmonics in the power system. It also causes a problem similar to the power
factor issue, in that the power network must be designed for the instantaneous
amateurs, we are also concerned with radio noise generated and radiated by
electronic devices such as the CFLs. Measuring emissions requires a calibrated
test range and specialized equipment, such as a line impedance stabilization
network or LISN, which I do not have. However, I did look at a conducted
differential line noise, i.e., radio frequency noise measured as current
over one side of the power line. At the lower radio frequencies, the power
wiring looks like a transmission line and crud induced on it will not
necessarily be completely radiated or coupled into your antenna or receiving
To measure the differential mode induced noise current, I
used a Tektronix P6022 current probe. To increase the probe sensitivity, I
wrapped 10 turns of small diameter wire through the probe jaws and connected the
CFL to the wire.
A more complete investigation would also look at common
mode noise, i.e., noise excited on both the hot and neutral conductors
against ground, as this signal has a greater potential to radiate. However,
these measurements are not going to mean much without a defined environment with
respect to the common mode line impedance.
For the range up to 100 KHz, I connected the P6022 current
probe to an HP3562A dynamic signal analyzer. The capture below shows the noise
for a 40 watt incandescent lamp. The P6022 rolls off and is not accurate below
10 KHz, so you should take the data below 10 KHz with some caution.
Compare the CFL data below with the incandescent data above. We see the
inverter fundamental at around 47-48 KHz and its second harmonic. In addition,
there's a tremendous amount of broadband hash, being 40 to 50 dB above the level
seen from the 40 watt incandescent lamp.
I traced the CFL's input circuit to see what the
designers did for RFI suppression. The circuit fragment below shows that the RFI
filter consists of a single bypass capacitor and an RF choke. Incidentally, the
component ratings seem marginal for reliability. R1 is a resistor, but its
primary purpose seems to be as a fuse. It's a 1/4 watt, 0.47 ohm resistor which
will quickly open if excessive current is drawn. The filter capacitor is rated
at only 200 V, even though it will normally see 180 volts applied. That's a thin
margin indeed. Likewise, C1's 250 volt rating is much lower than would normally
be used for a line bypass.
The RF choke is unmarked,
but looking at the core material, it is likely in the few millihenry range.
Earl, N8ERO, has written to say he has disassembled several
CLF ballast assemblies and found the chokes identical at 1.3 mH, and with a Q of
50 at 60 KHz. He also reports the self-resonant frequency is around 300 KHz.
I also looked at the differential noise over a wider
frequency range with the same test setup, but employing an Advantest R3466
spectrum analyzer. Since the spectrum analyzer has a 50 ohm input impedance, I
added a 10 db gain Z10000-U broadband amplifier. (The P6022 current probe is
designed to operate into a 1 MΩ instrument, such as an oscilloscope or the
As a calibration point, I connected
the output of an HP8657A signal generator at 1 MHz and -40 dBm output. This
results in a current of approximately 89 microamperes through the current
The image below shows the test setup with a CFL installed,
but the power supply switch turned off. One strong local AM broadcast station is
seen around 1440 KHz, and a marine navigation beacon at just over 200 KHz.
Engaging the AC line power starts the CFL and the noise floor
increases substantially, particularly below 1 MHz. With the CFL powered up, the
stray broadcast pickup is greatly diminished, as can be seen in the 1440 KHz AM
signal, which is not visible when the CFL is powered.
The remaining question is how much of the CFL trash induced into the power line
is radiated. I can't provide a good answer to this question at the
moment. Casual listening with my antenna system shows no noise, but my antennas
are 100 feet or more from the CFL test position.