Luxeon/Cree/etc. On 12 VAC - Landscape Lights

[Froggy]

I'm opening this discussion because I'm in the process of planning some landscape lighting. I'd like to get opinions on using a 12 VAC landscape transformer as a source of power for LED modules, whether they be Luxeon, Cree or others.

Has anyone used these LED's with an AC power source (along with the appropriate resistor)? I don't believe that the unit will be "killed" simply from being sourced with AC as it is a diode after all. I would suppose that the LED would "pulse" at 60 hertz.But what about the 12V waveform that the LED allows through itself to operate? As you know, 12 VAC is seldom 12 V. Sometimes it is zero. Sometimes it is 12 V on the way up or down, and sometimes the waveform may/will be above 12V. This is all very confusing to me, but I trust that the LED will sort it out and provide some lighting during the phase of the cycle that it can use.

I'm prepared to hear a number of opinions on the use of VAC for powering these units, all the way from, "You'll blow the units up!", to "Since it is only allowing half of the waveform through, it is only operating at half the duty cycle of a DC powered unit.". All comments are welcome.

What's the opinion/experience of the group? I'm going to give this a try. I'd just like some "emotional reinforcement" on using/abusing these lights. Using the buried 12 VAC cable that is already in place will really save a lot of trouble. I'm just wondering how to "massage" the resistor value to maintain the life of the units.
​

 

[Ken_McE]

Hi Froggy,

I am partway through replacing some garden path lights. They run on 12 V AC. The old system had little incan bulbs that had a life expectancy, of about long enough for me to go to the store and get the next bulb. They did have nice color though.

The new system uses MR-16 bulbs and I have four different types of LED bulb coming in later this week. All of them are around 100 lumens in warm white and are billed as working equally well on AC or DC. I plan to install one of each and run them for a while to see what I think of them. Then I'll get more of the winner and finish the job. Right now I have some 20 watt halogen bulbs in there as place holders.

I believe there is a little circuitry built into each bulb to make it AC/DC friendly, but I don't know what it is. I know that the one 21 LED MR-16 that I have now is not polarized, so it must have something inside that directs the voltage in the right direction.

If you are planning on just using naked LEDs fresh from the roll I would expect them to have a 60 hertz flicker, and I would expect some people to hate this. I suspect that a capacitor between each set of leds and its power could smooth this out, but don't know for sure.

I strongly recommend that you try one of the color and size unit you are thinking of and see for yourself how it looks. I have tried amber and cool white and found them disgusting in this setting.

 

[dave w]

I'm opening this discussion because I'm in the process of planning some landscape lighting. I'd like to get opinions on using a 12 VAC landscape transformer as a source of power for LED modules, whether they be Luxeon, Cree or others.​

Has anyone used these LED's with an AC power source (along with the appropriate resistor)? I don't believe that the unit will be "killed" simply from being sourced with AC as it is a diode after all.
​

FWIW
All diodes have a "peak inverse voltage" (PIV) limit. The highest reverse voltage they can take before the smoke comes out. As long as the LED's you plan on using have a PIV greater than about 20V (I'm figuring a safety margin of 8V) you will be OK, but most LEDs do not have high PIVs.

MR16 bulbs, bulbs designed to replace Malibu bulbs, etc. (bulbs designed to work on AC) all rectify the incoming AC with power diodes or full wave bridges which allow the LEDs to see only one half of the AC sine wave. The LEDs never see the reverse or inverse polarity portion of the AC. Check the specs on your LEDs or LED driver boards for a PIV before connecting to AC.​

 

[LEDite]

Froggy;

Just put a full-wave rectifier inline before the LEDs.

It will change the AC to pulsating DC.

Check the current in the LEDs with all of them hooked up.

You may need a low value resistor to limit the current in the LEDs.

Larry Cobb

 

[LED-FX]

Use a current regulator per light in the light, for high power LEDs resistors end up uncomfortably big and hot.Very sensitive to volatge fluctuations as well. Over long runs 24V is more common because of V drop.
Number of Chinese vendors offer cost effective 12V buck regulators.

Cheers
Adam

 

[FrontRanger]

Number of Chinese vendors offer cost effective 12V buck regulators.

Cheers
Adam

Hi Adam,

Can you post links to some of these, please? I've seen a few inexpensive current-regulated drivers on DX and KaiDomain but most do not seem suitable for 12VDC input.

Thanks,
Stephen

 

[LED-FX]

Heres one, at the cost its probably cheaper than a power resistor and heatsink:

http://cgi.ebay.com/1W-LED-Driver-f...ryZ36323QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Cheers
Adam

 

[Gunner12]

Kaidomain has a buck driver that will take up to 18v but I doubt that it will work with AC current. Click here.

 

[LED-FX]

Either just stick a bidge and smoothing capacitor in front of transformer, remembering output DC will be abou 1.41 times AC input, or use a nice compact switch mode PSU in, Meanwell are competitvely priced and very reliable in my experience.

Cheers
Adam