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I am building a wall cabinet with downlights in the shelves, for which I would like to use MR16 LEDs. However, my first attempt was obviously deficient, since two of the LEDs blew. I'd appreciate some help to figure out a) what went wrong, and b) what I need to install them properly.

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As you can hopefully see in the image above, I have a single transformer, which has the following specifications:

Electronic Transformer for LV-Halogen Lamps
PRI: 220-240V ~50Hz 0.27A
SEC: 11.5V~ 10-60W λ=0.99

I have four 12V, 4W MR16 LEDs wired in parallel to the transformer. When I connected them all up to the power, they seemed to burn fine, without any noticeable flicker; after about 30 min, one of the lights went out, and I thought it might have been my wiring. After another 10 min or so, another light blew with an audible pop. At that point I realised there was something wrong with my setup and switched it off.

When all four LEDs were burning, and when only three were working, I took some voltage readings, which were around 4.5-4.6V (nowhere near the expected 12V).

I have considered 4 possible issues here:

1) The LEDs are expecting DC input, and are getting fried by the AC current. When I bought the transformer (from the same store that sold me the LEDs), I specifically asked if I needed a DC transformer, and the assistant replied that they didn't have any other kind of transformer, and that this was what they used in their in-store demonstration arrays. The LED packaging makes no mention of whether they need AC or DC current.

2) The LEDs are wired in parallel, when they need to be wired in series to give the necessary resistance for the transformer to work.

3) The LEDs are too far from the transformer, resulting in voltage drop (the maximum distance is about 1.5-2m).

4) The transformer is faulty.

I don't have the knowledge to diagnose which of these issues (or combination thereof) is at fault here, so any advice will be gratefully received.

Edit: I am in South Africa, with 220V AC mains electrical supply.

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  • I suspect if you use a RMS meter you would see that the actual reading was not 4v
    – Steven
    Dec 30, 2012 at 2:11

4 Answers 4

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Electronic transformers designed for Halogen lamps will kill LEDs.

Electronic transformers are very simple switched mode power supplies. They "chop" the input 230 V/125 V sine wave in a way that will yield an average output voltage of 12 V. However, the peak voltage will be much higher and will eventually kill LEDs (that's no problem with Halogen lamps because they react more slowly). In addition, the chopped waveform may cause radio interference if the cables to the lamps are too long. The waveform may also be the reason why your voltage readings are off.

Instead of an electronic transformer designed for Halogen lamps, you should either use:

  • a conventional AC transformer (the heavy kind); or
  • an electronic DC transformer designed for LEDs.
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  • Just a comment on this. I think it's still a valid and correct answer, but switching to a conventional transformer did not solve the problem for me. Eventually I had an electrician come in, and he replaced my thick solid-core wiring with much thinner wire that he called silicone wire. That seemed to do the trick, although I also switched back to halogen at the same time, so it's hard to be sure what made the difference. Oct 23, 2013 at 14:11
  • The only reason I can think of that this "silicone wire" (as the electrician called it) might have made a difference is if it introduced enough extra resistance to lower the final load voltage a little and thereby protect the LEDs. Aug 21, 2014 at 12:19
  • Copper is copper. "Special wire" is not used to perform any function, that would invite mistake and hazard. That function can be performed inside a device, where it can be UL listed. Special wire usually relates to special insulation with fire rating (no toxic fumes), water or chemical resistance, mechanical durability or features (the easy-pull coating on THHN) etc. Silicone is insulation, i.e. that makes it rated for your application. Mar 13, 2016 at 19:44
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We can determine by the specs you gave that the transformer is indeed AC

Electronic Transformer for LV-Halogen Lamps
PRI: 220-240V ~50Hz 0.27A
SEC: 11.5V~ 10-60W λ=0.99

The ~ sign indicates alternating current. Also, the minimum load is indicated as 10W. This is important when using LEDs

When all four LEDs were burning, and when only three were working, I took some voltage readings, which were around 4.5-4.6V (nowhere near the expected 12V).

This is a common error even with pros. A Electronic transformer chops up the load side freqeency between 20k-100k Hertz. The common digital multi-meter is set to read between 50-60 Hertz. This makes reading or testing electronic transformers practically impossible if relying on a multi-meter. Instead, just take a basic working MR16 halogen and test it directly to the electronic transformer. If it works the transformer should be okay.

1) The LEDs are expecting DC input, and are getting fried by the AC current. When I bought the transformer (from the same store that sold me the LEDs), I specifically asked if I needed a DC transformer, and the assistant replied that they didn't have any other kind of transformer, and that this was what they used in their in-store demonstration arrays. The LED packaging makes no mention of whether they need AC or DC current.

MR16 lamps whether LED or halogen will usually work on either AC or DC voltages.

2) The LEDs are wired in parallel, when they need to be wired in series to give the necessary resistance for the transformer to work.

All wiring in homes is done in parallel, never series.

3) The LEDs are too far from the transformer, resulting in voltage drop (the maximum distance is about 1.5-2m).

While this could possibly be a factor for runs over 4m and incorrect wire sizes, 2m should not.

4) The transformer is faulty.

This seems to me to be the actual problem. Even though the bulbs used did meet the transformers minimum load requirements, I suspect the transformer was faulty.

Using a magnetic transformer will have several benefits to electronic.

1. Will solve the minimum load requirements (when using LEDs).

2. Be more reliable for longer runs.

3. Can be tested with a common multi-meter.

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Its the transformer for halogens that did the LEDs in. The wiring had nothing to do with the problem you mentioned. Its best to use a driver designed for LEDs to power them. If you can share the specs on the LED lamps and weather there is a circuit in the lamp and how are the LEDs connected (in series back to back or in parallel) I can help you with the specs for the optimum driver. Noor.

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You MUST use with Low Voltage LED that has MR16 base (two thin pins) special designed LED DC Driver. The other way just to rewire (remove the transformer for the Halogen lamps and using the mains supply side connect the GU10 (two thick pins ) socket. After this you can use LED that has GU10 base. Those LED with GU10 base have build in driver and designed to work with wide range of AC mains. In both cases you MUST use licensed Electrician.

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