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Douglas C. Smith

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Technical Tidbit - September 2007
Electronic Fluorescent Light Induced EMI

scope probe held near electronic fluorescent bulb

Figure 1.
Electronic Fluorescent Light Near Scope Probe

Abstract: Electronic fluorescent lights are capable of generating significant EMI. The nature of the EMI resulting from internal switching used by such a light is presented. The amplitudes measured are significant and could affect high impedance analog circuits.

Discussion: Figure 1 shows a scope probe with a one meter test lead attached to the probe tip and held near an electronic fluorescent light in an overhead fixture. One might expect the probe to pick up some EMI at such a close distance, and it does, but the setup shown in Figure 2 also picks up significant EMI. In Figure 2, an overhead view is shown of a scope, probe, and one meter test lead attached to the probe tip. The same light fixture from Figure 1 is just visible at the top of Figure 2. The distance from the fixture to the tabletop was just over one meter.

view  from above measurement

Figure 2.
Overhead View of Experiment
(edge of light fixture at top)

The probe is a standard 10X Hi-Z probe with a 10 Megohm input resistance in parallel with 10 picofarads. Figure 3 shows the resulting 60 Hz pickup of the probe with a higher frequency signal riding on it. The 60 Hz peak to peak amplitude is about 600 mV. Near the center of the screen, the amplitude of the high frequency component is about 700 mV. A new plot is shown in Figure 4 where the time scale is expanded from 5 ms/div to 20 us/div.

scope plot showing 60 Hz with noise

Figure 3.
Probe Pickup at about One Meter Showing EMI Riding on 60 Hz Waveform
(Vertical = 200 mV/div, Horizontal = 5 ms/div)

The peak to peak amplitude in Figure 4 is also about 700 mV and the frequency is about 48 kHz. This signal was picked up by the test lead on the table over a meter from the lamp! In order to estimate the source impedance of this voltage, a 100 kOhm resistor was placed across the probe input as shown in Figure 5 to reduce the probe input resistance from 10 MOhms to 100 kOhms.

scope plot showing 48 kHz riding on 60 Hz

Figure 4. Probe Pickup at about One Meter Showing ~48 kHz EMI
(Vertical = 200 mV/div, Horizontal = 20 us/div)

100K resistor across scope probe

Figure 5. 100 kOhm Resistor Across Probe Input

Figure 6 shows the result. The peak to peak amplitude was reduced to about 400 mV indicating that the source impedance was somewhat larger than 100 kOhms, I suspect a capacitive reactance of around 150 kOhms. This would imply a source capacitance (frequency was ~48 kHz) of 21 pF.  The 21 pF equivalent capacitance would include the probe input capacitance and capacitances to nearby objects from the test clip as well. The experiment was performed on a wooden table so that should not affect the calculation much.

48 kHz with 100K resisitor across 10  Meg probe

Figure 6. Probe EMI Pickup at about One Meter Across 100 kOhm Resistor
(Vertical = 200 mV/div, Horizontal = 20 us/div))

The test was also run with a square sheet of aluminum foil about 1 foot, ~30 cm, on a side as shown in Figure 7. The peak to peak amplitude was increased to about one Volt under this condition, not a large change.

overhead view of foil measurement

Figure 7. Overhead View of Foil Attached to Probe Input

When the probe and test lead were held close to the lamp as shown in Figure 1, repeated below for convenience, the recorded amplitude was much larger as shown in Figure 8.  A peak-to-peak reading of ~6.5 Volts was recorded.

repeat of Figure 1

Figure 1. Electronic Fluorescent Light Near Scope Probe
(repeated for convenience)

scope plot of probe and wire held near light as in Figure 1

Figure 8. Probe Pickup at ~5 cm Showing ~48 kHz EMI
(Vertical = 1 V/div, Horizontal = 20 us/div)

This kind of EMI would not likely be a problem for low impedance digital circuits. However, some analog circuits can have a high impedance at 48 kHz and could be affected. In fact, this Technical Tidbit was written as a result of EMI from the same light fixture shown in Figure 1 affecting some of my experiments over the last several years.

The waveforms of Figures 4, 6, and 8 also are rich in harmonics. Given the amplitudes observed, enough high order harmonics to cause RF interference to AM radios and other equipment is a possibility, especially radios with high impedance antennas like AM car radios. A small AM radio does in fact pickup broadband noise modulated by 60 Hz in the vicinity of the lamp.

Summary: An example of EMI at 48 kHz from an electronic fluorescent light was given. It appears that such lamps could pose an interference problem for nearby sensitive anlog circuits and AM radios. 

Additional articles on this website related to this topic are:
  1. April 2001, Measurement Error Caused by Probe Input Impedance
  2. April 2005, Inductive and Capacitive Coupling - Induced Current Characteristics
The scope used in this Technical Tidbit is an Agilent DSO5054A, a great little scope that is easy to use and boots in 9 seconds flat!

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Click here for a description of my latest seminar to be available soon titled:

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).

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Copyright © 2007 Douglas C. Smith