Testing Induction Lamps using Digital Oscilloscopes

Dr. Michael Lauterbach
January 18, 2012
Summary
There are many types oflighting devices including thefamiliar incandescent lamp,
LED (light emitting diode), HID(high intensity discharge),
fluorescent, and induction
lamps. All of them can convert
AC power (and in some cases
DC power) into light. But there
are differences in efficiency,complexity of construction,
lifetime, brightness and in thecolor of emitted light. However
the testing of all of thesedifferent types of devices canbe done using digital
oscilloscopes. This article will
discuss testing of induction
lamps.

TECHNICAL BRIEF

Similarities and Differences between Fluorescent Lights
and Induction Lamps
In some ways, induction lamps are a subset of fluorescent lamps. The
light emitted from the lamp comes by transferring the energy fromultraviolet waves (UV) to a fluorescent coating on the inside of the bulb.
The fluorescent coating converts the UV into visible light and the glass wallof the bulb absorbs any remaining UV. So only the visible light is emitted.
The coloration of the light depends on the types of fluorescents used tocoat the inside of the bulb.
But the method of producing the UV in typical fluorescent lamps is muchdifferent than the process used by induction lamps. UV is produced byestablishing an arc discharge. In the case of conventional fluorescentlamps the arc termination is at tungsten filaments (electrodes) at each endof the tube. An electrical current sustains the arc. The electrons in the arc
through the lamp stimulate mercury vapor in the lamp thereby producing
the UV waves that in turn excite the phosphor. Once the lamp starts up, it
becomes a device with negative resistance- the more current that flows in
the lamp, the lower its resistance becomes. So a ballast is required tomoderate the flow of energy.
By contrast, induction lamps do not have filaments. This is also a huge
difference from conventional fluorescent bulbs where the arc goes fromone electrode to the other. The Induction lamp has one continuous arc,eliminating the need for the wear out electrodes. Induction lamps work on
the same principle as transformers, commonly encountered by electronic
design engineers in power supplies. An alternating current passing
through a conductor (the “primary”) will generate a magnetic field whosestrength varies with the amount of current. The time varying magnetic field
will induce a current in a nearby conductor (the “secondary”).
In the case of induction lamps, the 50-60 cycle AC line current is stepped
up to RF frequencies. The wire carrying the RF signal is wrapped arounda ferrite core that is exterior to a bulb containing mercury gas. The wire
wound core acts as the primary of a transformer and the gas is thesecondary. The energy transferred by this process excites the gas and it
starts to emit UV. An electronic ballast is used to control the start-up andsteady state operation of the bulb. These types of lighting devices can behighly efficient and long lasting. Efficiency is rated in lumens of light
output per watt of power consumed.

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Testing Induction Lamps using Digital Oscilloscopes

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An Example of Testing an Induction Lamp
Both the efficiency and the longevity of an induction lamp depend crucially on the ballast used to control it. In
many cases this is a microprocessor controlled circuit – and considerable design and test expertise is needed tocreate a reliable, low cost yet effective ballast. Typically, a lighting device needs to operate in AC circuits withvoltages ranging from 108 to 270 volts and frequencies of 50-60 Hz. Figure 1 shows an example of testing on aninduction lamp. In this case, the lamp ignition is having a hard time starting. The input voltage is 277 volts. Thisshould be stepped up to about 450 volts and 200 kHz to operate the lamp. The upper half of the oscilloscope
screen shows the capture of four signals over the course of one second during start up. Note the time/div in thelower right corner is 100 ms/div and there are ten horizontal divisions on the screen. The lower portion of thescreen shows a zoomed detail at 20 ms/div of the four upper traces – basically, the zoom shows the portion of thendrd
signal captured in the 2 and 3 horizontal divisions of the upper grid. In the upper gird you can see highlighting
on the early portion of the signal which indicates which piece of the signal is being shown in the zoom.

Figure 1: Failed ignition of an induction lamp operating at 277 volts. The upper grid shows one
second of capture time for four signals. The highlighted portions of those signals are shown aszoom details in the lower grid. Channel 1 (orange) is the rail voltage, Channel 2 (red) is theboost diode current, channel 3 (blue) is the bias supply voltage and channel 4 (green) is theoutput voltage of the lamp

Channel 1 (orange) is the rail voltage which should be boosted to 450 volts. Note in the lower left corner of thescreen the sensitivity of channel 1 is 100 Volts/div. In the highlighted portion of the lamp start up channel 1 only
reaches (just barely) the fourth division, 400 volts. If you examine the upper trace you can see it reaches 450 volts
about half a division later. Channel 2 (red) is the boost diode current (5 amps/div). For proper operation of thelamp, it should be running at a frequency of 100 kHz. In the zoomed detail you can see channel 2 is starting toLeCroy Corporation

Testing Induction Lamps using Digital Oscilloscopes

page | 2 of 5switch current, but at a much slower rate than 100 kHz. Channel 3 (blue) is showing the charging of the internalbias supply to run the electronics (5 volts/div). It needs to be above 10 volts and though it eventually does getthere (as shown in the upper trace), during the start-up portion of the signal shown in the zoom it never reaches
10 volts. Consequently, channel 4 (green) of the oscilloscope ( 1 kV/div) which shows the output voltage of thelamp reveals the lamp tries to start up and then fails.
Perhaps the first lesson concerning testing of induction lamps is that you need several types of probes to acquire
the signals. Channel 3 is “easy”, just 5 volts/division so the typical probe supplied with the scope can handle it.
For channels 1 and 4 the signals have much higher voltages so specialized voltage probes may be required.
Channel 2 is a current signal, so you either need a shunt to convert the current signal to a voltage or you need touse a current probe capable of capturing AC currents at frequencies up to at least 200 kHz.
When you want to use an oscilloscope to examine multiple signals and multiple zooms or multiple math traces it isgood practice to use multiple grids. The ADC (analog to digital converter) on the front end of a typical 8 bit
oscilloscope has 1 part in 256 resolution to measure the amplitude of the signal. If a scope is set up to display
many signals on a single grid the user “squashes” each signal to fit a small vertical portion of the grid. Since theADC is spreading its dynamic range across the full vertical height of the grid, if a scope user places a signal to fit
into one vertical division (in order to see 8 signal shapes on a single grid) the scope winds up using only one
eighth of its codes, 32 counts, to digitize the signal. Essentially, the user has paid for eight bit ADCs but is only
using them as 5 bit ADCs. Note in the upper grid on Figure 1 that each of the four signals is occupying about halfof the vertical range of that grid – which means the four signals are each being digitized using half of the verticalrange of the ADC. In an optimal case, the user would want the signals to cover near the full range of the grid, but
in actual engineering practice, some headroom often needs to be left in case the signal goes higher or lower thanexpected. Also, in this case, no precision measurement is needed. The scope is operating more as a simple
viewing tool. The shapes of the signals are enough to confirm the lamp is not operating properly.
Second Example of Induction Lamp Testing
A second example of testing the same induction lamp is shown in Figure 2. This time the basic operating
conditions are 208 volts and 60 Hz. The ignition sequence is better than in Figure 1 but still not sufficient to get
the lamp started during the one second of time captured by the oscilloscope. The four traces (and the fourzooms) are all the same signals as in Figure 1, the horizontal time base of the acquisition and the zoom are thesame and the vertical sensitivities (volts/div and current/div) are also the same as in Figure 1. The highlightedthth
area of the upper traces is now the 4 and 5 horizontal divisions of the upper grid. You can see the orange trace
on channel 1, the rail voltage, reaches 300 volts near the end of the first horizontal division on the upper grid andeventually gets to the targeted 450 volts near the end of the zoom portion. But during the crucial time where thelamp is starting up (as shown in the green trace on channel 4) the rail voltage oscillates at around 300 volts.
Channel 2, in red, shows the boost diode current attains a much higher frequency than in Figure 1. In the zoom
you can see a comb shape of quickly changing diode current. Channel 3 (blue), attains the 10 volts desired forthe bias supply charging voltage shortly after the third division on the upper grid. But as the light starts to turn on(channel 4, the green trace) the bias voltage is disrupted. You can see bursts of noise on channel 3 which line up
with the bursts of current on channel 2. The lamp fails to stay on and all four signal collapse back to steadystates.

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Testing Induction Lamps using Digital Oscilloscopes

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Figure 2: The same test setup as Figure 1 but this time the input voltage
is 208 volts rather than 277 volts

In addition to the oscilloscope usage lessons discussed above in the first example there are a few new points thatarise. In Figure 1 the activity of the boost diode current could be clearly seen as nine fairly slow peaks of roughly
triangular shape. But in Figure 2 the same signal has much faster activity. This could be viewed more clearly ifchannel 2 was zoomed to a faster time base. Some oscilloscopes only allow one common timebase for all zoomtraces, as is shown in both figures. But in many real world test situations there are both slow signals and fasterones. So it can be desirable to have multiple zoom timebases. Many LeCroy oscilloscopes have this capability.
rdthYou can even have a zoom of a zoom – perhaps turning on a 3 and 4 grid that showed just the green and red
traces on faster timebases than the existing zoom. That is a useful tool for viewing details of multiple signals.
And there is a useful technique for measurements in this situation. Is the boost diode current running at its
desired 200 kHz ? You can’t tell from Figure 2. But if the scope was set up to show the boost diode current on athird grid, at a faster zoom display timebase you could visually ascertain the frequency , you could do an FFT ofthe faster zoom trace or apply a parameter measurement to that trace. Using a zoom to select of portion of a
waveform for measurement is a good technique. If you want to ignore a portion of a signal and just do math orparameter measurements on a piece of the signal, you can activate a zoom to select the interesting portion of thesignal, then apply math or parameters to the zoom trace (only).
A similar sort of capability with respect to measuring parameter values on just a portion of the signal can beaccomplished using a feature called a “measure gate.” Oscilloscopes with this feature (including many LeCroyoscilloscopes) allow the user to turn on two vertical cursors and use them to define the piece of the signal to use
in making parameter measurements. The measure gate can be placed on the original acquisition channels or it
can be placed on a zoom. In both Figure 1 and Figure 2 the user has placed a measure gate on the lower grid.

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In Figure 1 the vertical dashed, black lines can be seen to line up with the start and end of the burst of activity on
the green trace (the lamp output voltage). In Figure 2 the measure gate is noticeably inside the envelope of lamp
activity. The oscilloscope user can even move the measure gate to see if parameter values are different onvarious portions of the signal. Is the diode switching frequency (red trace) the same in the first burst of activity asndrd
2 , 3 , etc?
There is one final point. Suppose you have the lamp shown in Figures 1 and 2 – and when you flip a switch itturns on? Certainly the view shown of the signals during one second of acquisition time does not look good. But
maybe after the circuit has a few aborted tries it starts to work and the ballast regulates the light. What you mightneed is an oscilloscope with a longer acquisition memory in order to record the complete action of the lamp. If
you look at rectangle near the lower right corner of both Figure 1 and 2 you can see the oscilloscope is capturing“10.0 MS” which means ten million samples of data on each of the four input channels. You can also see thesampling rate is “10 MS/s” – 10 million samples per second. This sample rate is clearly a good one for capturing
the sort of details in this application. Let’s suppose you could keep the sampling rate the same, but instead ofcapturing one second of data you would like to capture for three seconds. You would need 30 million sample
points. There are a couple of ways to do this. Some oscilloscopes have very long memory and you can simply
go into the horizontal setup menu and tell the oscilloscope to use more. In other cases, you can sacrifice thenumber of input channels acquiring data in order to have longer memory available for the channels that are inuse. An example is the WaveSurfer MXs-B from LeCroy (though there are also many other types of scopes withthis capability). If you use all four channels of this type of scope, there are 16 Mpts of memory per channel. But
the scope can also be told to operate in 2 channel mode and apply 32 Mpts of memory to each channel. So youcould capture 3.2 seconds of data on 2 channels at 10 MS/s sampling rate. In order to test the operation of yourlamp, maybe you only really need to see channel one and channel two of the two previous examples. Or, if
desired, you could capture two signals, save them to internal memory in the scope, then capture two moresignals. So you could even have all four signals – but use two acquisitions to acquire them.
Summary
There are many different types of interesting and useful lighting devices. To obtain longer lasting and more
efficient lighting, good R&D engineering and careful testing are needed. This necessitates a wide variety ofprobes and can be enhanced by a knowledge of how to use the measurement capabilities of oscilloscopes.

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Testing Induction Lamps using Digital Oscilloscopes

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