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Total noob question.

I have MR16 halogen downlights and want them replaced with LEDs. Someone told me that, if I simply replace the bulbs but not the transformer, then the power draw will still be the same (or perhaps not as improved as if I get new transformers or replace with newer GU10s with an integrated transformer).

Is this correct? Or nonsense? Am I over simplifying it and more information required?

Thanks.

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    \$\begingroup\$ It's nonsense, but there is no guarantee your LED lamps will function well with the existing "transformer", which may well be some kind of electronic switching supply. \$\endgroup\$ Jul 10 '17 at 1:30
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    \$\begingroup\$ Here and here \$\endgroup\$
    – Jeroen3
    Jul 10 '17 at 7:22
  • \$\begingroup\$ Unless the voltage from your transformer rises significantly as the load decreases (due to higher efficiency of the LED) and the LEDs have very little margin for increased voltage, you will be fine. \$\endgroup\$
    – winny
    Jul 10 '17 at 12:36
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    \$\begingroup\$ Possible duplicate of Using low-voltage halogen transformer for LED? \$\endgroup\$
    – winny
    Jul 10 '17 at 12:37
  • \$\begingroup\$ most of the LED replacement "bulbs" i've see have an AC/DC converter (FBR) on-board, and will draw whatever the built-in resistor allows, hence the watt ratings given at time of sale. \$\endgroup\$
    – dandavis
    Jul 10 '17 at 20:18
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The answer is, "maybe".

The issue is that MR16 halogens can come in one of 3 voltages - 12, 24, and 120 volts. You need to check to determine what your system uses. Then you must check to make sure your LED bulbs use the same voltage. If not, you have a choice: change your choice of LED bulb, or change the voltage. The latter may involve changing transformer, or possibly eliminating it altogether. It all depends on what you've got.

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    \$\begingroup\$ Also, some 12V LED bulbs are DC only while some have a built-in bridge rectifier for AC. And some tolerate slight over-voltage from poorly regulated transformers while others expect ... 12V. You need to make sure the LED bulbs you're buying are compatible with transformers for halogen lighting. \$\endgroup\$ Jul 10 '17 at 9:56
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    \$\begingroup\$ @BrianDrummond - As far as I know, MR16s are unpolarized, so that particular consideration doesn't apply. Good general point, though. \$\endgroup\$ Jul 10 '17 at 12:01
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In addition to the voltage, AC/DC characteristics of your PSU there are also transformer based PSUs and electronically regulated PSUs (SMPS).

While transformer based PSUs primarily suffer in terms of power efficiency with too small a load applied, SMPS units show multiple issues: not only do they exhibit decreased efficiency with too small loads but they also may not work at all from the beginning, cease to work after initially working for minutes or even couple of days (depending on power-on time, load etc.) or prematurely fail fatally. In other words, some PSUs need a minimum work load to operate at all/within specs/reliably.

For example, with one transformer based unit we needed to provide a certain minimum load for it to operate silently and within tolerable temperature range. Too little load and they would angrily humm and become quite hot in a short time span.

In another example, once we encountered an SMPS unit that simply would refuse to start up with two LED devices attached, one LED and one Halogen bulb would make it run however. Then another SMPS unit would only operate intermittendly with kind of a stroboscopic effect (est. ~1Hz).

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A lot of transformers for (e.g. 12V) halogen lamps state a minimum load, e.g. 50W. If you just replace e.g. three 35W spots with three 10W LEDs, they will not work, or not work well. Best in this case is to replace the transformer with one suited for lower minimum loads (sometimes called LED light transformer). Even branded ones should not be very expensive.

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