The most efficient LEDs?

Atm. im converting some outdoor lightning from fluorescent to LEDs, and im in a need of some good LEDs.

My power source is a toroidal transformer, delivering 15VAC @ 15VA.

I want to make it as efficient as possible. This means, either running it with a full-wave rectifier, with as many LEDs in series as possible, to mininize the power dissipation in the current limiting resistor.
Or.. running it at AC, limiting the current with a capacitor (probably the most efficient solution there is).


Anyhow.. which LEDs can you recommend? Maximum total power used is 10W. I was thinking about either using around 6x 1W in series, or a couple of high-power LEDs. The problem is, as i see it, is that efficient high-power LEDs are expensive, and that cheap ones tends to burn out after a while.

Another solution is to use 120x 5mm LEDs - but i have no idea how their light output is pr. watt, compared to high-power LEDs.



Can anyone enlighten me? lovecpf Thanks in advance!

I would use a full wave bridge and a filter cap. You can use 5 power LEDs in series for about 16 volts. Small transformers have poor load regulation and in some cases you can avoid the dropping resistor altogether for as long the current draw is less than maximum for LEDs and PSU. If you do this, the LEDs must always be connected when the PSU is powered to avoid a surge in the LEDs because of the high open load voltage.

I'd recommend any power LED with a built in ESD diode if they are spread out with several feet of wire as in landscape lighting. The Cree XP-E has this while the XR-E series don't seem to have this.

JohnR66 said:

I would use a full wave bridge and a filter cap. You can use 5 power LEDs in series for about 16 volts. Small transformers have poor load regulation and in some cases you can avoid the dropping resistor altogether for as long the current draw is less than maximum for LEDs and PSU. If you do this, the LEDs must always be connected when the PSU is powered to avoid a surge in the LEDs because of the high open load voltage.

I'd recommend any power LED with a built in ESD diode if they are spread out with several feet of wire as in landscape lighting. The Cree XP-E has this while the XR-E series don't seem to have this.

Click to expand...

My current setup consists of 46x 5mm LEDs - for this i use a full-wave bridge. This ensures a minimum power loss in the resistors.

However.. Five Power LEDs dropping 16V @ 350mA, would require a 14.28 Ohm resistor, resulting in 1.75W dissipated. If its 1W-LEDs, then its only ~74% efficient.

If efficiency is paramount, I would get a constant current electronic LED driver. You can use more or less LEDs in series and the driver adjusts to minimize losses. You really want to spend money on good LEDs, so put a good driver behind them.

My suggestion is a few XP-Gs from Cutter and a low voltage AC driver from DX. http://www.videofoundry.co.nz/ianma...nstock&type=buck&v_in_max=15&ac=1&order=price

JohnR66 said:

If efficiency is paramount, I would get a constant current electronic LED driver. You can use more or less LEDs in series and the driver adjusts to minimize losses. You really want to spend money on good LEDs, so put a good driver behind them.

Click to expand...

Yea, if im gonna choose high-power LEDs, i probably have to do that

I found these: http://www.led-tech.de/en/Light-Emi...er-Flux-LED--1-Chip-Technic--LT-1533_1_7.html

20 lumens each, resulting in 100+ lumens/watt @ typ. ratings. Thats quite impressive.


TorchBoy --> Got a link to those XP-Gs? Prefer within EU if possible.

I just ordered 50x 1W LEDs, so im gonna give those a try, and see how they perform.

Since you've just bought some LEDs I'll post this link for reference only. http://www.cutter.com.au/proddetail.php?prod=cut937

TorchBoy said:

Since you've just bought some LEDs I'll post this link for reference only. http://www.cutter.com.au/proddetail.php?prod=cut937

Click to expand...

I often buy LEDs, so its good to know which to look for the next time

Ive looked alot at CREE, but imo they are too pricy.. And the high-current for just one LED, produces a problem - i would need a driver for it. These other cheap LEDs, can easily be connected in series and anti-parallel, and driven @ AC.

"Anti-parallel"? Whatever do you mean?

TorchBoy said:

"Anti-parallel"? Whatever do you mean?

Click to expand...

This is my test-setup for driving LEDs @ AC:


For it to work, you need either one LED and one diode facing in opposite direction, else the reverse voltage will kill the LED. Or.. you can put two LEDs in anti-parallel - thereby using both half-waves of the sine.

In the shown schematic, a capacitor is used as an active component. Its reactance is used to limit the current - this eliminates the use of a resistor (almost..), which lowers the power loss.

Ah, I see. Thanks.