LED Power Supply

There are other ways (such as switch mode power supplies, capacitive voltage divider networks, etc) BUT a step down transformer is much safer, easier and most likely cheaper for this particular application. Step-down transformers are relatively inexpensive (especially when compared to a 20W LED!), have a look in your local electrical store.

For this application I would still recommend a step-down transformer along with a bridge rectifier, DC voltage (or current) regulator, capacitors, etc and a BIG heatsink for the 20W LED!

Additional note - Did I mention a big heatsink, yes I did! For recessed lights be especially careful to ensure that the excess LED heat does not cause a fire hazard! You may not be able to safely fit the assembly into an enclosed ceiling recess because of this!

Consult your LED data sheet for more information (including heatsink sizing) and test your completed LED light for extended periods on your workbench to ensure you don't accidentally set your ceiling alight. Don't skimp on the heatsink and cooling or your LED will likely fail prematurely!

hi Jack of all trades. read and under stood what you are talking about.The 20 watt led is about as bright as the new small energy efficient I think 65 watt bulb,they look like a curled up floresent bulb.... Yes you are also right about the heat, it dose get hot. this led is built on a Flat peice of metal and the led is also Flat.I know that I would have to use a large about 5 or 6 inch round heat sink what ever fits into the light housing.

like i said I just ran this on 12 vdc, Not what it is made for [ 13 to 15 ] also with the higher ma. load.

I would like to run these all off a 12 deep cycle battery with a small solar panel, A lot of rewiring or just new wiring to the existing house.It also has to look good,for the WIFE. Most would have to be custom made.[DIY] That's not too much of a problem for me. oh well just a lot of fun anyway.

Thanks for the reply.

John

I wanted to add to this comment. I recently learned that the automotive style, cigarette lighter outlets do not meet NEC code requirements! Install them in your house at your own peril. Don't expect your insurer will cover you if you do.

The only reason these sockets are still in use in cars is because they became standard, and no-one has the will to update to any other device because they will be seen as breaking the mould.

They are also capable of supplying up to 20A, according to 1960s standards.

To use these LEDs in a house, there are a multitude of low voltage connectors available - see those which come attached to transformers (the small ones on the right are acceptable).

Since you are experimenting, try using a series resistor or an accurately adjusted electronic fan regulator.

One place to try might be here: http://parts.digikey.com/1/parts-kws/power-supply-15v-dc (Usual disclaimer.) Not likely to be very costly.

Thanks to all.I do understand about all the down falls of using led's. and Mostly about the heat.I also understand about effiency . Trying to save on ele. and cost,. one must understand the Waste from all componets used in the cir.Then is it worth it after all that. Yes the led would give off light, But are you saving money,by doing all this./ The best way is with solar cells.[I think] you cant depent on wind,. But now the High Cost of the solar cells. 36 to start with for only 12 vdc. hummmm how long is pay back? is it worth it? or is it just for fun,and to say that you are lighting for Free energy from the sun.[But really not Free]you paid for it.

Im doing the last one i guess

Thanks to all.and it is fun and interesting to hear from everyone here.

John B

John,

The guys at 1000 light bulbs did this for me. You can find them on line. they seem very capable and not expensive

rochelle

Next what I was thinking of seeing that LED's are so much in different voltage most I see are 3.5 -12-and 18vdc. If you use three 6volt batteries and wire them in parallel with diodes between each battery, you then could get 3 different voltages , but just thought of it,you wouldn't be able to use the amp's from the other batteries because of the diodes. hummm

I know all about the voltage regulators. Just thinking about the loss of power.

what's your thoughts? Don't yell at me too much.

tks John

In my experience, LED light output is around seven times that on standard halogen lamps. I used a pair of 17W LEDs to replace 250W halogens quite successfully. There are commercial LED dimmers available which run on mains voltage, and use 1-10v input to dim via PWM. Expect the driver to be around the same price as the 17-20W LED.

http://www.power-one.com/board-mount/

From Sleepy.

Remember that LEDs are diodes.

You need to drive them with an accurate current, not voltage.

If you look at a diode curve you will come to appreciate that a 1% variation in current can be caused by a 0.1% variation in voltage. Runaway is not far away as more current causes more heating which can provide less resistance which can provide more current and hence more heat.

be aware be very aware .

Sleepy.

Sleepy. on the 20 watt I did go and get a 1ohm res. for using it on my 14 amp power supply. And it did work well on a 12 motorcycle battery also with the res. that was at 12.6 vdc..The only bad part about led's now that I find out is that the angle of light is small ,i think this one is 140 deg. Oh well. And with the res. it did seem some what cooler.

Johnny

What's wrong with 140 degrees? A halogen will be aimed at between 13 and 60 - more of a spot than a flood!

Hi GM. Nothing is wrong with the 140 deg.I could use about 180 deg.For the rec. lights over the kit,counter.But right now Im going to play around with putting them under the cabinets to shine on the counter.Im just going to order 4 more 20 watt at only $10.00 each that includes ship.Now for in the living room for like a table lamp, I would like 360 deg.Just having fun with the LED'S.

Have you seen the LED clusters intended for car headlights? Illegal for use on cars this side of the pond, but design gives 270+ degree output and they are 12V as standard.

contaxman

Hi, what I was trying to get to is that if you look at the V/I curve for a diode ( which is what an LED is) you will see a roughly exponential curve where a small change in voltage will provide a large change in current and hence power.

So ideally you would want to drive LEDs with Constant current power supplies to minimise the risk of changes and possible over-dissipation and burnout.

An approximaton to a Constant Current supply would be provided by a High voltage, say 120 volts and a large series resistor; this is not economic but produces the required effect. Constant Current supplies (electronic) can be obtained from similar sources to the bulbs and to the transformers tha we use to drive Halogen and other low voltage bulbs.

Good Luck

Sleepy

Hi Sleepy. I do know what you mean about the v/a. I want to run the leds off a 12vdc auto battery.I know that voltage If charged from an alternator can get up to 15 volts.If im using a smaller voltage led I do your a volt/reg, and resistor for the current.And in one place I will be using I bought will say like a wall wart, constant v and then I add the resistor. and that worked out fine.

I built some other cir. for the 3/4 volt 700 ma to were as I can hook up three 3 watt leds in series and that also is working out well.

I know you can buy the drivers for all these also.

I know that they burn out EASY. I did that a few times. ha ha ha

Thanks for all the help, which is always nice to have.

Johnny

Sleepy,here is a cir that I figured out to replace a bulb in a 115 volt light.20 watt led.FV.13.5-15 -FC 1400-1800ma. I use a step down trans.Pri.115 sec 18volt. connect a full wave rec to the 18 and then connect the 18vdc to a L7815CD2T-TR from Mouser.

Out put of that is 15 volts . Also i will put in a 1.8 ohm res. to the pos.side of the led.

I think that would/should work ok.I have it regulated both current and voltage.

what do you think? I don't build cir.

thanks

John

Question? I was comparing my 20 watt led to a 25 watt spiral fluorescent bulb. I first ran it on 12vdc with a 1ohm res. and it was not as bright as the Fluor. so then i remembered that I have a 14vdc adp 3 amp. used for labtops. So i pluged that in and used the 1ohm res. and connected the led.It was as bright or brighter as the fluor. but then I turned the led to face it in a different direction and it got dimmer a little bit. so I unpluged it.I had it hooked up for about 1 min.It has the small 1x1 metal heat sink that it came with,.It looks like all the small leds that look like "dots" are still lighting. Did it get dimmer because it got hot? I didn't try it again.Ill let it cool down.It looked like it went from a 20 watt to about a 15 watt.

any help, thanks. first time this ever happen to me with a led.

Johnny

These high power LEDs need to be put on a heatsink just like a CPU - and a fan can be useful where airflow will be limited otherwise.

I understand the heat sink. But why would this 20 watt led go bright [white color] and the about 30 to 45 seconds Dim down and be a yellow white color?None of the tiny leds [20 of them] on the led are not working.They all still light up.I still can put the same 14.5 volts to it and it will light. Is their a break in period? lol. I know their isn't.

tks

John

You may have a slight overheat situation.

I had not realised you were using a cluster - the LEDs I use are these (17W). Drivers from RS.

Check the colour of the light you have - it may be one of the duller shades (they come in cool, warm, daylight............)

Thanks for the reply,that is what I thought had happened. Over heated it.But i thought it the leds were over heated that they would burn out and Not work at all.Oh well.

John

They will after a while. Slight over-voltage gives more light until they heat up, then poorer until something gives.