Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

A long time friend, K8AQC, asked about the AC current and power characteristics of the compact fluorescent lamps (CFL) he uses.

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 CF23EL/BL/1.

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.

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:

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.

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 higher. http://www.eia.doe.gov/bookshelf/brochures/rep/index.html provides 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.

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.
 

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 "electronic ballast."

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 module.
 

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.
 

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.
 

Quite 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 incandescent lamps.

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 voltage/current peaks
 

As radio 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 equipment.

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.

The RF choke is unmarked, but looking at the core material, it is likely in the few millihenry range.

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 sensing circuit.