Why can't my DVM read the voltage of my electronic transformer?

Why measuring the voltage on an electronic transformer with a Digital Voltmeter (DVM) can be problematic? This post is intended as a demonstration of the two most common misunderstandings, assuming there's nothing actually wrong with the electronic transformer.

"Why can't my DVM read the voltage of my electronic transformer, even though the transformer appears to work fine?"

Repeatedly, I've been asked this question because friends were getting wildly inconsistent results when measuring the voltage output of an electronic transformer, typically used as a halogen light driver.  This has lead to a number of false diagnoses of broken electronic transformers as well as incorrectly assuming that the voltage supplied by the electronic transformer is too low.  This erroneous situation comes about by a basic misunderstanding of both the workings of a typical electronic transformer and the capability of the test/measurement equipment being used. There are two main reasons for this confusion:

1. The power output side of the electronic transformer has automatically switched off, due to lack of sufficient loading, so cannot generate an output voltage to be measured.


2. The digital multimeter being used isn't capable of reading the complex high frequency output of the electronic transformer.
   

Clearly, I have not done the best job of explaining these scenarios. So I think within this post, I need a better way to explain and illustrate the fault by including real world pictures. Hopefully the following demonstration will drive home and cement the two most common reasons for this problem assuming a good working device.

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Index.

Framing the problem.

Describing the expected output of an electronic transformer.

Demonstrating the results.

• The various multimeters used in this post.


• Video with live demonstration of issue


• The results for the reference magnetic transformer.


• The results of measurements for electronic transformers which have a minimum load.


• The results of measurements for an electronic transformer with no minimum load.

The intentions of this post.

The calculation of RMS values in this post.

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Framing the problem.

There are many reasons why you might want to measure the output voltage of an electronic transformer. The most obvious is to check it's working as expected, especially after an event such as the blowing of the fitted bulb. Another is if you're considering updating the technology from halogen to LED and you want to assess the suitability of the transformer.

• If you're considering the update you may be interested in this post I wrote regarding points to consider when assessing an update of a transformer for LEDs. The cost implications can be calculated with the lighting cost calculator created in this previous post.


• Most people can get hold of a DVM quite easily.  Modern DVMs are relatively cheap, accurate and useful bits of test kit. For a small sum of money you can buy some very accurate multimeters but they do tend to be build to a price and construction may not be as robust as the more expensive meters. Also some of the cheaper meters may have a question mark around safety when they are used in high voltage and high current environments. Luckily, these high voltage, high current environments are not normally found in the typical home. Though there is enough voltage and current in your home to cause serious harm to the reckless and slapdash.


• Fortunately, YouTube does contain lots of good reviews on a multitude of DVMs, so I thoroughly recommend using Internet based reviews on YouTube to get a better understanding of what you're going to buy.

All too often there is a confusion when measuring a transformer, electronic transformer or SMPSs. In this post I will be demonstrating the problem relating to the AC output of an SELV electronic transformers and using a SELV 12v magnetic transformer as an initial benchmark. Predominantly, these electronic transformers are used to drive "low voltage" halogen lamps, such as the MR16/MR11 or GU5.3/GU4. I will not be looking at or investigating a dedicated SMPS driver, which are quite different to the more "primitive" variety of electronic transformers. Specialist SMPSs are more often seen when they are used for driving LED GU5.3 equivalents. Also SMPSs are found where it is necessary for current regulation.

• In this post, when I refer to "low voltage" I will be referring to 12v AC. When I refer to "mains" I will be referring to the 240/120v AC domestic mains.


• Note, professional electrical engineers and technicians tend to refer to "low voltage" as the domestic mains voltage of 240v or 120v.  When they refer to 12v electrical systems, these engineers and technicians term it as "Extra" low voltage or ELV. In fact ELV is often considered as any generated voltage under 50/60v.


• The term SELV only differs from ELV by implying protective circuitry built into the devices such as isolation or short circuit/overload protection, etc. The "S" in the acronym SELV stands for Safety.

If you're not sure what I'm writing about when I use acronym terms such as "SMPS" and "electronic transformer" or more specifically what the differences are between them, then see this earlier post I wrote regarding transformers, electronic transformers and SMPSs differences.  Hopefully the earlier post will cleanup any confusion relating to the different types of power delivery options, especially regarding 12v low voltage systems.

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Describing the expected output of an electronic transformer.

Previously I wrote another post on the circuit analysis of an electronic transformer.  In that post I explained in greater detail the circuit design considerations of a typical electronic transformer and how it operates.

Essentially, the electronic transformer first DC rectifies the mains and then generates a high frequency, high voltage switched output. This high frequency, high voltage switched output is then channelled through an output transformer. The output transformer then drops the switched voltage to the desired output voltage. The value of the output voltage is governed by the turns ratio of the output transformer just like a standard passive power transformer.  The output voltage is still AC but at a much high frequency. Thus the output voltage is dependent on the input mains voltage which allows it to dimmed by reducing the input voltage or more commonly by using, "off the shelf," phase dimming techniques.

The advantages of a electronic transformer is that it's small, light and cheap and can deliver substantial amounts of power.

In this post I describe the waveform as a modulated high frequency with a mains envelope, which looks something like this plot below. Note the peak-to-peak value is nearly 34v even though the transformer is considered a low voltage 12v lighting driver.

This plot is just a mathematically generated approximation, as the frequency of most electronic transformers is much higher, often centred around 40 khz. So would look much more like the following graph, which is based on a frequency of 5 khz.  Further along into this post, in the following pictures, you can see the actual output trace on the oscilloscope display.

Sadly this output is very difficult to measure with an ordinary multimeter.  The best way to demonstrate this is by actually showing you pictures with a selection of multimeters

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Demonstrating the results.

The various multimeters used in this post.

In the following pictures I used 5 multimeters, which are listed in the following table:

Equipment used
1)	Maplin Gold	Old 80's accurate home multimeter
2)	Vici 99	Modern budget home multimeter
3)	Fluke 73	Workhorse technician's multimeter
4)	Fluke 87	Workshop engineer's multimeter
5)	HP 34401	Laboratory engineer's multimeter
	Tektronix 2445B oscilloscope	Only included for a visual reference

I also used an oscilloscope as a visual reference.  This allows you to see the waveform in the time domain, which helps to illustrate the actual output waveform being measured by the multimeters. The nice advantage of this particular oscilloscope is that it has display cursors, which allow you to see the values they've been set to.

Note:  For the average person/worker, the first three representative DVMs are the type that would be commonly found around both home and work.  The last two DVMs aren't normally available as they're in a totally different budget class, which is normally outside that of the typical domestic user. They are here as a reference to show what an elite multimeter would measure in the same scenarios.

The results for the reference magnetic transformer.

This is a power transformer commonly known as a magnetic transformer. I will use this transformer as a reference so you can compare and contrast with the measurements of the other electronic transformers further down in the post.  None of the multimeters have a problem measuring the output of this type of transformer. In the tabulated results you can see they're all consistent. Click on the image to see a better quality view of the results.

Tabulate results:

The magnetic transformer's measurements
Device		0w load	20w load	50w load
1)	Maplin Gold	13.50vac	12.92vac	12.07vac
2)	Vici 99	13.45vac	12.86vac	12.02vac
3)	Fluke 73	13.43vac	12.84vac	12.00vac
4)	Fluke 87	13.49vac	12.91vac	12.07vac
5)	HP 34401	13.45vac	12.87vac	12.02vac
	Tektronix 2445B oscilloscope	34.3vpp ~ 12.0vrms	33.3vpp ~ 11.7vrms	31.1vpp ~ 11.0vrms

The results of measurements for electronic transformers which have a minimum load.

The next two are more typical electronic transformers that have a minimum load.  This is something that is frequently forgotten that these transformers don't produce an output voltage until they have sufficient load on their output.  Another problem is that the transformer generates a very difficult voltage waveform for budget multimeter's to measure usually resulting in them failing to give a sensible reading.

• The output of the transformer is usually easiest to get to by removing the bulb, thus allowing access to the terminals in bulb fitting. Unless the electronic transformer supplies multiple fittings then removing the bulb removes the load, so it's actually the wrong way to measure its voltage, as can be seen in the "no load" pictures below.

Note that, the first three multimeters are not reading anything sensible for these electronic transformers, neither loaded nor unloaded. The elite meters are only successfully measuring an output when the electronic transformers are loaded.

• The first electronic transformer has a minimum load of 20w and second has a minimum load of 10w.

Tabulated results:

The (20-60w) electronic transformer 1's measurements
Device		0w load	20w load	50w load
1)	Maplin Gold	2.31vac	5.55vac	9.73vac
2)	Vici 99	0.226vac	0.881vac	1.587vac
3)	Fluke 73	0.082vac	0.350vac	0.804vac
4)	Fluke 87	3.74vac	11.83vac	11.75vac
5)	HP 34401	3.80vac	11.70vac	11.52vac
	Tektronix 2445B oscilloscope	17.5vpp (? vrms)	32.3vpp ~ 11.4vrms	34.1vpp ~ 12.0vrms

The (10-60w) Electronic transformer 2's results
Device		0w load	20w load	50w load
1)	Maplin Gold	0.31vac	9.09vac	4.94vac
2)	Vici 99	0.048vac	0.264vac	0.839vac
3)	Fluke 73	0.02vac	0.063vac	0.306vac
4)	Fluke 87	1.51vac	11.80vac	11.73vac
5)	HP 34401	1.61vac	11.78vac	11.58vac
	Tektronix 2445B oscilloscope	16.9vpp (? vrms)	32.7vpp ~ 11.5vrms	34.0vpp ~ 12.0vrms

The results of measurements for an electronic transformer with no minimum load.

The last Electronic transformer does not need a minimum load and so will produce an output irrespective of whether it's loaded or not.

Note that, the first three multimeters aren't reading anything sensible for this electronic transformer, neither loaded nor unloaded. The elite meters are successfully reading a voltage, in all scenarios.

Tabulated results:

The (0-50w) Electronic transformer 3's results
Device		0w load	20w load	50w load
1)	Maplin Gold	12.33vac	9.97vac	9.95vac
2)	Vici 99	1.959vac	1.605vac	1.469vac
3)	Fluke 73	1.015vac	0.796vac	0.701vac
4)	Fluke 87	13.02vac	12.54vac	12.26vac
5)	HP 34401	12.85vac	12.31vac	12.03vac
	Tektronix 2445B oscilloscope	31.1vpp ~ 11.0vrms	34.6 vpp ~ 12.2 vrms	35.7vpp ~ 12.6vrms

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The intentions of this post.

I have put this post together as a demonstration of an issue I've seen and heard multiple times.  It is not intended as a scientific study as none of the items have been calibrated or checked with, or against, a piece of calibrated test equipment. Equally, this post is not intended as a comment on the quality or manufacture of the test kit shown. It's really to show some of the limitations you may come against when trying to check or fault find an electronic transformer.

"Remember test kit is not a fashion accessory!"

I would happily recommend any of the test equipment shown, as I have used them regularly myself on many occasions.  The cheap multimeters are excellent for most standard domestic use and you should always consider your budget before splashing out ridiculous amounts of money for kit you will rarely use.  Being seduced into buying test equipment, which is over elaborate or expensive for your usage is as much a shame as buying something too simple. Remember test kit is not a fashion accessory!

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The calculation of RMS values in this post.

RMS stands for the term Root Mean Square and it's the way that electrical engineers and technicians refer to the voltage of an AC waveform. Without going into the nitty-gritty about the derivation for a pure sine wave (without a DC component) the calculation is:

RMS (of sine wave) = peak / √2

With regard to the oscilloscope readings I have only noted the "peak-to-peak" value or "V_pp".  With regard to the magnetic transformer, making the relatively safe assumption that there is no DC component and the slightly unsafe assumption that it's a pure sine wave AC, I have used the equation as below:

V_rms = V_pp / (2√2 )

Simplified for speedy calculation on calculator:

V_rms = V_pp * 2 ^-1.5   (Remembering those old maths lessons about indices addition)

The calculation of the RMS for the electronic transformers is much harder and I have taken lazy and incorrect shortcut of using the Vrms calculation for a "pure" sine wave as above.  Fortunately, the output of the electronic transformer is more a high frequency square wave modulated by a mains frequency sine wave.  This is close enough to the calculation for a pure sine wave for the purposes of this post, especially as this post is intended more for an indication rather than as a reference.

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