LED power supply help

[Steviecuk]

Hi. I would like some advice on an LED fish tank light that I am trying to make. I am not sure if to use the CREE style 3w SMD's mounted on a 20mm star PCB or the larger square 10w LED's, I will decide later. But either way I need help working out the PSU if anyone would kindly steer me right.

The LED details are as follows.

10w, 0-12v, 1400mA.

Or

2.3w (CREE), forward voltage 3.3v, 700mA.

What kind of power supply would I need? Is it as simple as just one that is rated at 5x the voltage, current and wattage above? Or something else?

i have seen this driver with the following spec:
Model:LA50P-700
Input: 100-240Vac
Output: 36-72Vdc 700mA
Rated Loading: 11---20LED
Efficiency (at full load): >87%
Dimensions (LxWxH): 148x40x30mm

http://m.ebay.co.uk/itm/131346072951?cmd=VIDESC



Would I be right in thinking that this driver could be plugged into the wall at 240v (uk mains) and then run out to the LEDs in series. For example say 20x 3.3v, 700a, 2.3w LEDs as this should total 66volts.

In essence, am I also right in thinking that the current of the LEDs must match the driver, if it's less then a resistor must go in (for if I say have 6 blue at 700mA and 6 white at 900mA) and the total voltage of all LEDs added together may be within the drivers operating range?

if that bits right, I'm also thinking, if I have 900 and 700mA LEDs, get a 900 driver, run to the 900 leds first, then add a resister before going on to the 700 ones? Resister being V/I=R (2.3v/0.2A=3 ohm resister to bring it down 200mA to 700? Or is it the calculation of total voltage pulled/current? 66v/0.2A=330 ohm. Or then again, is it 66v/0.7A=94 ohm?

Oh it all gets very confusing.

I look forward to your responses.

Thanks, Steve.

 

[tazz]

First of all, I would suggest keeping to a safe 230v to 12v driver, after looking at your calculations..
If an led requires 700mA to drive it then a 12v 700mA minimum driver is required. Would suggest using a larger driver.

If you want to wire single leds in series again stick to low voltage, and use calculation
Resistor= (voltage - led forward voltage) / led current in Amps

 

[Steviecuk]

Hi Tazz. I'm obviously not saying that you're wrong by any means, but can you help me understand why 12v if one led is 2.3v? Won't that blow it? On the other hand, if I had say 10 LEDs at total of 23v, wouldn't the 12v driver be insufficient? ( I could just do what you say but like to understand why it does what it does. )

I thought you would have to add the voltage requirements together?

So Does the current of the driver stay the same? Ie 1x led @ 700mA = 700mA driver. 6x Led @ 700mA each = 700mA driver?

 

[tazz]

If your using separate leds, then each led will require a resistor to drop the voltage from 12v to 2.3v. Using the formula I posted earlier. R= (12-2.3) /0.166..or R= 9.7/0.166...So R=58.4 ohms.
So to power the led, you would need a 58.4, or 60 ohm resistor.
If you required more leds, you would wire in parallel.
if each led requires 700mA then every led wired will increase the current by 700mA so 2 leds will be 1400mA or 1.4 Amps.
For a fish tank, I would suggest 1-2m led strip light powered off a 12v driver, you can get any colour, or even rgb colour changing....there cheap, safe waterproof,and less hassle

 

[Steviecuk]

Cheers again. The only problem I have with led strips is they're not bright enough or the right wavelength. It's for a sal****er tank so need many different frequencies at high par levels which strips can't reach.

I mi didn't realise resisters limit voltage and not current.

So Think I've got it..

the voltage will stay the same, ie don't have to up the voltage proportionately to the voltage of the LEDs, ie 3 LEDs at 3v each won't require a 9 volt driver, essentially a 3v driver would power them equally.

The current will need to be multiplied. Ie 3x LEDs at 700mA each will need a driver of 2.1A?

So to run 3x 3v, 700mA LEDs the perfect one essentially would be a 3v 2.1A driver/PSU?

What is the benefit of getting a 12v one and then having to resistor everything down to 3v? Would it not be better to find a 3v one?

 

[tazz]

Will need more info on what you trying to achieve ...I can see a problem of a lot of heat being generated here....so How many watts is required.? What wavelength is required...?
It may be better to use say 50 small leds wired in parallel / series config over larger area

 

[1Justin]

Hi Stevie. You need to understand the difference between a voltage source and a current source. One has low internal resistance
(The voltage source) and one has very high internal resistance , (The current source). Bare LED's will be current driven. A current source driver provides it's rated current into any load, and the output will just keep on rising in voltage till it does, or till it runs out of compliance, hitting it's voltage ceiling.

-A high voltage battery in series with a resistor approximates to a very rubbish quality and lossy current source, the resistor limiting the current into the LED which has an essentially fixed forward voltage.

The Cree LED requires 700mA to run at full brightness. At this current it will have approximately 3V forward voltage. To drive this you ideally would select a 700mA constant current driver which has voltage compliance to include the lowest and highest expected forward voltage which is dependent on how many you put in series.
The driver also won't work below it's minimum output voltage (in the case of the one you suggest, that's 36V or about 11 LED's).
If you mount your 3W SMD LED's on a PCB close together with no heat conduction precautions, they will get very hot and fail. You
will need a reasonable heatsink. LED's don't like hot.

The spec you provide for the 10W parts only makes sense if there are already two in series within the package. 2 in series at 10W and 1.4A would run at 10/1.4 = 7 V which is about right. The 700mA driver you highlight won't run these very well, or if they do produce anything, will be very dim.

You'd have more flexibility about No of LED's if you chose a driver which didn't require 36V minimum voltage output.

Re:"if that bits right, I'm also thinking, if I have 900 and 700mA LEDs, get a 900 driver, run to the 900 leds first, then add a resister before going on to the 700 ones?" That, is nonsense. You are talking about a current source driver here. They will all get the same current in series. Adding resistors just gives you very hot resistors and raises the output voltage of the driver! If you use a 900mA driver on 700mA LED's they will fail very fast.

I hope that helps a little

 

[Steviecuk]

Thanks Tazz and Justin and I thank you for your patience with me whilst I try and learn the difference between current and voltage.

Main relation to the heat issue caused by the Leds, the design will include them being affixed to a heatsink which will be fan cooled to dissipate the heat generated.

I I will need a mixture of Leds of differing colours and wavelengths. I will do a quick diagram tomorrow (just on way back from Xmas Doo) and post on here so you can see exactly what I am trying to achieve.

Justin, the 'nonsense' that I wrote I apologise for. I misunderstood and thought that resistors are used to control the level of current, not voltage. I stand corrected.

Whilst i i wait to be able to upload the diagram, when we say driver, is this the same as a PSU that I can plug straight into the wall? Like a transformer plug?

 

[1Justin]

Go for well known brand CE marked driver. The "Driver" being the step down transformer from 230V input mains to whatever output current you need. These will be SELV (look that one up if it's too much jargon)

Take care if selecting unbranded off Ebay, you don't want to pick up an unreliable/untested/dangerous etc grey import. There are plenty of good ones, Collingwood, Aurora.. many others.

 

[Steviecuk]

Ji. Below is a diagram of roughly what I am going to achieve. It will have more less but once I understand the workings, I should be able to adjust it accordingly.

I will have two channels, each operated by a separate power source. (so I can turn them on and off individually)

What would both power supplies need to be rated at in the diagram to successfully run the LEDs linked to it?

From trying to digest what you have taught me so far, I'm thinking...
PSU A= 18v/700mA (I'm thinking this one would work http://www.johncullenlighting.co.uk/site/wp-content/uploads/2014/01/DRMV24700.pdf)

PSU B= 9v/700mA

[/IMG]

 

[Steviecuk]

Just found these on a driver manual... looking at this the above should be right? Would you agree?

 

[Steviecuk]

Whats MeanWell like? Found this which I thought was ok for running 8 LEDs??

Mean Well MW 9~24V 700mA 16W AC/DC LED Driver APC-16-700 TUV Brand New | eBay

 

[1Justin]

Stevie,

Your JohnCullen supply would work for the first circuit but not for the second (only 3 LED's so ~ 9..10 V, and the driver supplies 12..24V)
A fouth LED in the second should fix it, or (the wasteful option), add in a 10Watt 5 Ohm power resistor to take the place of LED # 4. (Resistor will get quite warm so bolt that down to metal as well)

The MeanWell looks like Ebay tat, (Someone may correct me there, but I've never some across them). And it's also not a constant current driver! (The one you highlight is a 24V output voltage source, with MAX output of 700mA ).

Rgds

 

[Steviecuk]

I'm getting there Justin!! Woohoo. The diagram was only a rough one to understand the workings out. There will be more leds than that on the finished article.

I want to add dimmers to each colour too so I can adjust the brightness of each set. I'll do another diagram and upload later and get your opinion if that's ok? Cheers mate.

 

[Steviecuk]

I have been speaking to somebody briefly about the light unit that he has recently made. (It was only brief) he said that he used a PSU from a PC to run all his LEDs. He bought some 12dv dimmers to dim the channels and basically went from...

240v socket -> PSU -> split into 4 DC time switches -> 4 strings of LEDs.

Any my ideas how he would have done this as looking at the ratings they are several outputs on the PSU but they are only 12v each. I'm confoosed again. :-/

 

[1Justin]

Two possibilities.

He was using LED's which were packaged with their own buck mode SMPS driver chip, hence could individually be connected directly to 12V, in the same way as all MR16 spot light LED's are. Had he done this he'd have needed to connect the LED's in parallel to the 12V (not, as you say in a series "string").

Or if using "naked" LED's it could be that with four in series, he happened on the "sweet spot" whereby the back EMF of the four LED's in series just about matched the 12V output, and the natural current limiting of the PSU prevented runaway current from the LED's and kept them running at the right level. Messy and not a nice engineering solution, but it could work out. Experimentation needed, possible risk of damage to power supply, or LED's.

 

[Steviecuk]

Hi Justin. Just when I think I'm getting the hang of it, something frazzles my logic. Thanks for bearing with me. I've just emailed him and asked how he did his. All I know is that he used a PSU from a computer, and about 26 of the square 10w naked less. (like these AQUARIUM 10w LED ROYAL BLUE 450nm - 455nm, LIGHT UNIT, DIY, MARINE, MULTI CHIP | eBay)

I also went into a shop and looked at my mates proper bought light and looked at the PSU for them. These units consist of 2x 35w square LEDs in white, and about 30 of the 3w CREE SMD's on star PSB's (various colours) I looked at the supply thinking right, it'll be something like 700mA - 1A and something like 110v (30x3v (small) + 2x10v (square) yet I was surprised as the label read OUTPUT 24V, and I think 2A. Which kind of confused me as I thought the voltage got added together and the current remained the same throughout. :-/

 

[1Justin]

Which kind of confused me as I thought the voltage got added together and the current remained the same throughout. :-/

.. Is correct. Sorry, so now you have lost me too with your last post.
A picture tells a thousand words.

 

[Steviecuk]

This is what he drew me, however he did it wrong and said the timers were AFTER the 12V PSU.



This is the write up that he gave... (using the 10w, 9-12v, 900mA LEDS)