First off: I live in a part of the world where our grid runs at 230V, 50Hz. In case that matters.

I bought this house with numerous 12V 50W Halogen downlights, each with their own step-down transformer. The transformers are, as far as I can determine, good old heavy iron core transformers that supplies 12VAC on their output.

In order to save costs, I very soon started switching the house to LED lamps. These are sold as 5W drop-in replacements that you simply plug into the same MR16 connector of the old lamp, and they appear to work fine on the 12VAC they are supplied with.

I expected the actual consumption on the AC side to be slightly higher than 5W, because transformers are after all not 100% efficient, but what I am seeing now is a little horrifying. It would appear that I use around 15W per lamp, measured on the 220VAC side, rather than the expected 5W.

A quick current measurement on the 12V side shows that I am running 0.3 amps at 12.5VAC, which seems a bit odd but if I compensate for peak vs RMS it gets me exactly 5Watts peak (clever LED sales people, it is sold as a 5W...).

So my question now, is it possible that the efficiency of the transformer, which is of course designed to drive 5 amps rather than 0.3, could really cause such terribly efficiency?

I read on some internet forums that some transformers really do consume 5 to 10 watts of power on their own. Is this what I'm seeing?

What should I use instead?

  • \$\begingroup\$ How are you measuring voltages and currents, because halogen "electronic transformers" are simple switched mode PSUs which have a large high frequency component which makes them almost impossible to measure with most multimeters, including true RMS ones due to the high peak to rms factor. Even using a scope can be problematic due to the bandwidth involved. The best way to "measure" the rms voltage is to do a thermal comparison with a known sinusoidal voltage into the same load. Otherwise you get misleading results. \$\endgroup\$
    – Martin
    Commented Feb 5, 2014 at 9:18
  • \$\begingroup\$ It's good old iron core transformers. I measured the 230VAC side with a clip-on-current-transformer type meter, and the 12VAC side with a multimeter. The lamp and transformer is installed in the roof so there is really very little I have access to. The exact symptoms is simply that if I turn on the bathroom lights (3 x 5W LED) I see my power consumption jump by around 50W. That seems wrong. \$\endgroup\$
    – izak
    Commented Feb 5, 2014 at 10:27
  • \$\begingroup\$ This is what the lamps look like: landmarkonline.co.za/Home/ProductView?pgrp=83305 \$\endgroup\$
    – izak
    Commented Feb 5, 2014 at 10:30
  • 1
    \$\begingroup\$ How are you measuring power? LEDs are likely to have a low power factor. \$\endgroup\$
    – Phil Frost
    Commented Feb 5, 2014 at 12:30

4 Answers 4



You advise 3W load on a 50 W transformer.
I suggested 85-90% efficiency.
Transformer will draw magnetising current when unloaded. This will incur copper losses but will not be the full loaded current.
10W additional = 80% of rated power which sounds much too high.

You have still declined to tell us HOW you are measuring power - you say it jumps by 50 Watts. Measured how & with what. The only certain measure would be a power factor correct Watt meter. ie the house meter would qualify - a plug in watt meter quite possibly wouldn't.

More information is needed.
You mention "halogen mal transformers" - are they 50 Hz iron core transformers or smps (switch mode power supply) converters?

You imply that the LEDs operate from 12V and
imply that they may usually run on DC.
You suggest that they run OK on the 12 VAC from the "transformers" which may be
50 Hz AC or
10's to 100's of KHz AC?

You say "it would appear that you are using 15W/LED lap but knowing how you measure this in as much detail as possible could be useful. Or not :-).
You imply that it is not just the VAC x IAC product as yoyt say that in the output VAC x IAC = 15W but compensating for RMS gives 5W. This raises the question as to what it is you measure and how to get the 15W and how you RMS convert to get 5W. (The factor of 3 does not untuitively drop out of any conversion that comes to mind).

SO if you can fill in all the bgaps, and add a diagram if your word picrure would benefit from it, it may help.

Iron core power supplies may be in the 85% - 90% efficiency range but better is conceivable, and any fool can get less than that, and some do. (A 60Hz designed transformer run at 50 Hz can get nice and toasty. Ask me how I know :-).)

An electronic "transformer" can get just about any efficiency at all. 95%+ is doable and 65% would not be a surprise in some few cases. 80%-90% should be hoped for if not always expected.

Running LED lamps that are "designed" [tm] for DC use on AC may cause problems. At 50 Hz flicker may be an issue. IF the lamps have intelligent and/or active internal regulation to a lower DC voltage or to constant current then conceivably the need to provide a pulse of power input twice per cycle (or quite possible once per cycle if they are polarity dependant ) may cause massive converter overload and may even cause core saturation problems upstream in the 230:12 VAC supply. For extra points they may have placed a reverse diode across the supply input for "protection" and this may be causing a massive current peak every unused reverse half cycle.

SO too many guesses needed, not enough data.
A much fuller description should help.

IF you take the 12 VAC, feed through a full wave bridge and then add a LARGE electrolytic cap so there is well under 1 Volt ripple on the DC, and drive the lamp from that, what power does it draw?

Any web links for transformers, LED lamps, ...?

  • \$\begingroup\$ I edited the post to be more clear that these are drop-in replacements meant to work off 12VAC. I suppose I could just remove a transformer from the roof and check it with the low-voltage side open circuit. \$\endgroup\$
    – izak
    Commented Feb 5, 2014 at 10:22
  • \$\begingroup\$ I use one of these: theowl.com/products/owlmicro.php Of course that does not take into account power factor, since it only has a clip-on CT on the whole house line, but it shows a 50W increase when I turn on the 3 lamps in the bathroom. Of course the measurement might be wrong, but the rate at which the numbers opn my prepaid electricity meter drops seems to mirror what the OWL is saying. I have an plug-in efergy that can measure and correct for the power factor. Will simply have to test it. \$\endgroup\$
    – izak
    Commented Feb 6, 2014 at 13:40
  • \$\begingroup\$ I'm marking your answer as correct, because I think you likely provided the most info to help me figure this out. \$\endgroup\$
    – izak
    Commented Feb 6, 2014 at 13:58

You could indeed be losing somewhere between 5-10W in each of those 50W transformers. Cheaper designs economise on iron and copper at the expense of decreased efficiency. However, depending on how you are measuring current on the AC side, you may be seeing reactive current which lowers the power factor but does not actually consume power. If I had to guess, I would expect about half the "power loss" is real, and half is inductive, but it does depend on the characteristics of those transformers.

I would suggest running LV cable from one transformer to several adjacent lights, thus sharing the loss from one transformer among several LEDs. Whether this makes sense depends on the switching arrangements and the difficulty of adding the wiring...

  • \$\begingroup\$ You're saying what I've been suspecting. I think the only thing for this is to remove one of those transformers from the roof, rig it up to a plug, and use my efergy plug-in meter to check it. The efergy actually measures true power, takes into account the power factor (and can even tell you what it is). Then I'll know for sure. It would not surprise me one bit if the problem is cheap transformers. Everything in this place has been done on a budget. \$\endgroup\$
    – izak
    Commented Feb 6, 2014 at 13:42

Although this post was some time ago, I’d like to give some comments. 1. It would be nice to test each of the LED lamps individually with a bench power supply, perhaps initially with DC12V, to measure all the V & A figures. 2. Then, test with AC12V 50/60Hz and do the measurements. 3. If everything is fine, connect to the original lamp transformer. 4. One reminder is that if the wire resistance is large, hence the supply voltage to the LED will drop. The circuit inside will actually try to draw more current to maintain the same output power. Therefore, total power will increase including the loss in the wires and also a drop of efficiency in the LED driving circuit.



I had a lamp in the kitchen that needed replacement anyway. Turns out it has a loose wire, so while I was up there I removed the transformer and plugged it into the efergy plug meter.

The actual consumption is 6.5W. The power factor is a whopping 17%.

No wonder the readings are so totally out of wack.


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