# Using low-voltage halogen transformer for LED?

During a recent kitchen renovation, the electrician installed LED under-counter "pucks" and drove them with a 120VAC-to 12V, 40 kHz transformer labelled "low voltage halogen supply." I'm fully aware that LEDs are happy operating at 40 kHz (one transformer blew & I have to replace it), but is there any reason to change the driving frequency? Obviously, if I were to change, I'd stay above 100Hz or so to avoid any visual flicker, and there may be a complete death of commercial devices at other output frequencies.

FWIW, the 40kHz drivers cause two minor problems: AM radios nearby are not happy, and the fancy circuitry in my exhaust hood's lighting system tends to flicker (even though normally turned off) when the LEDs are on. I'm guessing a low-frequency LED driver might mitigate these side effects.

• FWIW I'd guess your guess is right. – RedGrittyBrick Jan 28 '14 at 16:32
• Is there some reason you need to drive these LEDs with a frequency? Why not a more conventional AC to DC power supply? – JYelton Jan 28 '14 at 17:17
• @JYelton In general, LEDs are happier if pulsed, and the perceived brightness remains high while the heatload is reduced. Further, using AC allows for a "future upgrade" to pulse-width modulation dimming. – Carl Witthoft Jan 28 '14 at 17:45
• Makes sense. In my hobby lighting circuits, I drive LEDs such that the flicker isn't noticed, and mostly concern myself with duty cycle when determining how much current to drive them with. That aside, I would probably select the lowest frequency that did not have any noticeable flicker. I'm curious what reasons more experienced EE's might offer as to why 40 kHz would ever be desirable. – JYelton Jan 28 '14 at 17:50
• 40kHz might be a value that works well with low-voltage halogen lamps for some reason, and the electrician just grabbed an available part(transformer) to use w/ LEDs. But I don't know either. – Carl Witthoft Jan 28 '14 at 17:54

12V and 40kHz actually sounds like a Halogene transformer for MR16 or alike.

You can run LED retrofits with such transformers and that is what I suspect you have in your kitchen.

For more information on these look at FAQ - 12V Transformers and LED Compatibility

Got little time? See TL;DR!

It is unspecified whether you are driving LEDs as in plain LED components (some LED-strips are plain LED in series-parallel combinations, others are not) or LEDs as in LED appliances like MR16 socket based ones.

The latter include additional electronic circuitry (gosh, some even include fans!) usually not so happy to live on anything else than 12V sinewave or DC. This circuit usually does rectification, filtering, DC/DC conversion, etc. and provides a constant current source being the preferred method for driving LEDs.

Plain LEDs should be fine with a 40kHz supply. Appliances usually aren't happy. Flickering, out of spec temperature ranges, etc. can happen. For effects that can be observed, albeit for different reasons, and why or why not to use legacy transformers, see halogen transformers for LED.

This 40kHz signal is very likely some sort of squarewave or other signal (the very short rise/fall time is the main culprit) which is a huge source of interference, even more so the more power is driven and the longer the wires. Unshielded 40kHz signals in 12V appliances could easily drive your amateur radio neighbour up the walls, the interference usually goes unfiltered and propagates easily so in the end your entire electrical installation constitutes a big antenna. Not to mention degrade your own AM radio reception. As noticed it also deeply disturbes the electronics inside your exhaust hood's lighting system. NB, similar situation arises with powerline-communication (PLC) which sometimes even is held responsible for avation-grade interference.

It is very easy to buy LED appliances even from local stores, name brand labeled and nonetheless they're produced overseas which in turn renders it quite likely that the circuitry inside -- being of high volume, low quality design and components -- is itself such a source of interference (the web is full of such references).

TL;DR:

Plain LED parts (as opposed to LEDs with circuitry) are fine with 40kHz (they're all that often driven by PWM). Everything else i.e. surrounding electronics from other LED lights to amateur radio is rather turned off by 40kHz squarewave radio interferences.

Flickering: Either drive plain LED parts by current controlled plain DC or use appliances (with plain sinewave or DC). And those may provide their own switching frequency which are out of user control and it may happen they internally use 40kHz or something in that range, are effectively unfiltered and therefore constitute a source of interference themselves. Difference: It's the appliances 'fault' instead of user's decision. Pick your favourite, that being said the appliances seem the mass market's solution.

edit: @blablubbb's answer above includes a link visualizing some types of interference waveforms (aka noise).