hi all

i am on my way to make a led lights for my shrimp tank

well leds on its way by mail

i am trying to Prepare everything so i have a spare laptop driver with 19.5 volts 4.7 A
and this cree XTE runs with maximum 3.4 volts

so can i connect 6x of this leds in a series and connect them directly to that Driver so every led will be taking about 3.25 volts

so will it work and be safe ?

If you have to, I suggest 7x LEDs since XTE is rated at 2.85V. Their internal resistances alone will limit the current to 0.35A.

For only 6 LEDs there is a risk in assuming the LEDs will be fine at 3.4V since LEDs aren't linear loads. At 3.3V versus rated 2.85V the current is almost quintupled (I checked the data sheet) to 1.5A. This risk comes from using a constant voltage source rather than constant current one.

So a 6x string will draw 29W while a 7x will draw 6.8W. In the former case you are running XTE's at the limits of testing they were subjected to, and IMO will die very quickly, if not be very sensitive to voltage fluctuations in the power source. The latter case is perfectly safe, but is very inefficient use of a 90W adapter.

P.S. You should be using XM-L anyway...

Don't do it. Leds's require constant current not Voltage. Unless you've tested the power supply under differing loads, there's no way to know how it will behave when connected to the leds. Chances are good that your leds will be destroyed. Do yourself a favor and wait until you have a proper led driver in hand, before trying to power your leds.

Get a buck puck for the driver and use the line wort you have. Dont go directly from the charger to the leds like mentioned above.

Ok well this is a new information for me i have been studying led lighting for about 2 years now and this is going to change a lot of things to me

i did use a laptop charger with 19v to power up a 20x 3w chines Leds and it was doing fine i did a 4 rows of series of 5 leds

all i did i divided the 19V on 5 it gave me 3.8 so i used 5 leds in series and then i tested it , it only consumed about 700mA so i used another 3 rows

and at this time i use a computer power supply to drive 3x 10w chines Leds
for my shrimp tank and it is doing great

well after your info about constant current Driver i got kinda confused

so could u please explain the different between constant current and constant voltage and how constant current Driver Work ?

and about the buck puck do u mean something like that will solve my problem ?

http://www.aliexpress.com/item/5-PCS...550721071.html

or this

http://www.aliexpress.com/item/DC-Bu...931703377.html

Forget about the driver. What you need to understand is how the LED works. Once you understand the LED, it will be obvious why you need a certain approach for the drivers.

Any time you place a load on a voltage source, the properties of the load determine what happens, i.e. how much current it pulls from the voltage source. Many loads people are familiar with (i.e. a normal incandescent lightbulb) behave in a way that is linear. What this means is, change the voltage by x amount and you can expect a change of the same scale in the current. If you were to graph this it would look like a straight line at some constant angle.

LEDs do NOT behave linearly. Basically, they act like floodgates. As you increase voltage, very little current flows, until you hit a certain minimum point, at which case LOTS and LOTS of current starts flowing. The I/R curve is almost horizontal, and then in a little narrow range it switched to almost vertical:



The plain English message here is this: once an LED is lit, VERY small changes in voltage will translate to VERY large changes in current.

Next lesson:

When you're driving a LED, what you're trying to do is basically control the total power through the LED. LEDs fry when there's too much power put through them. Because the I/R curve gets very, very steep once the LED is on, small changes in voltage mean very large changes in power (following an exponential curve), while small changes in current mean very small changes in power (following a linear curve).

Hence, if you're trying to precisely control the total power, it's very hard to do that by controlling voltage. But, it's very easy to do that by controlling current. Hence, most approaches to driving LEDs are constant current (meaning, the driver is regulating to a nominal current) instead of constant voltage (meaning, the driver is regulating to a nominal voltage).

That's the most important part of the concept, but there are really two other factors that make it even more critical.

First, look at the datasheet for those LEDs. Typical voltage at 350mA is 2.85. Max voltage at 350mA is 3.4. When Cree publish those numbers, they're not telling you a specific LED will happily burn 350mA at any voltage in that range. Instead, they're admitting that their product is HIGHLY variable. One XT-E might require 2.85v to pull 350mA, while another XT-E might take 3.4v. This is why it's dangerous to run on a constant voltage source without experimentally measuring conditions for every single LED in your array. If you plan your array with the assumption that the LEDs will run at 350mA with 3.4v but then you end up getting LEDs that only need 2.7v to run at 350mA, you'll be thousands of milliamps above your target current (remember, it's a very steep curve!)

The second additional factor is that LEDs are not very stable. The performance curve shifts under different conditions (mostly, temperature) and as the LED ages it'll shift permanently. So if you build an array and measure that a given LED is running at 350mA with only 2.85v when it's cold, you might find it's running at 400mA with 2.85v once it's heated up.

The inaccuracy of the voltage spec and the fact that it's not constant under different conditions both reinforce the approach of using constant current. If you're driving with constant current, neither of these factors are a problem, because the driver just adjusts it's output voltage to maintain the current you want.

All that said, there's nothing that says you can't run LEDs on a constant voltage source, as you discovered with your knockoff LEDs. When you experimentally measured voltage and then adjusted the array, you were basically accomplishing a similar task to what a constant current LED driver would do. Except, when doing it, you're risking blowing the LEDs and/or finding out that things have changed over time. Since most people who use HP LEDs on fish tanks are using very expensive, high end LEDs and running them close to their designed limits, this approach is very risky. Possible, but risky. When you combine that with the fact that a "proper" constant current driver isn't really that expensive, it's an easy choice for most people.


A buck DC-DC LED driver basically requires an input DC voltage higher than what you expect the LED array to use. It steps down ("bucks") the voltage to keep the current at the preset value. The two you linked are indeed buck drivers, but when most people in the hobby hear "buckpuck" they're thinking of the LuxDrive product:

http://www.ledsupply.com/03021-d-i-700.php

The two you linked would probably work, but you'd need to be very careful when setting them up as they both have extremely high maximum currents - you'd want to ensure you had the current turned way down and adjusted it up carefully while testing.

You could place the appropriate resistor in series with the leds. It costs you a few watts but it works well. It will limit the current.

^ very informative!

Okay I just realized that was probably a lot to swallow. Cliff's notes:

1) Constant current drivers ensure that the LED is always being run the way you want it, regardless of manufacturing variability or changing conditions or the LED getting older.

2) Constant voltage CAN work, but you need to determine the voltage you're running at experimentally, because of variations in the LEDs and the environment. Then, you need to cross your fingers and hope things don't change. Most people think this is too much hassle AND too much risk, given that constant current drivers aren't THAT expensive.

3) The links you posted are probably suitable but aren't the products most people in the hobby are familiar with, so you'd be out on your own a bit.

Using an approach that limits current in a static manner (a resistor) is honestly not much better than constant voltage. The only real advantage is that you can easily change the resistor value to get the current you want, which accounts for manufacturing variability (but not changing environmental conditions). And when you're talking HP LEDs, these have to be some meaty big resistors, the little fractional watt resistors most electronics hobbyists are familiar with are OK for gumdrop LEDs but will vaporize at HP LED currents.

Considering a good DC-DC buck driver is cheap these days (meanwell LDD comes to mind, or the china-products the OP posted. Or make your own driver) IMHO it's not really worthwhile to use a resistor.

These little boost converters are $5 shipped online.


You can get a 10w resistor at radio shack for about the same price.

Listen to All Your Base. He know what he is talking about.
But like many diy'ers you may have a surplus of parts laying around. I just happen to have a large 10w resistor laying around that worked out well for me. At operating temp good resistors don't vary that much.

I ended up not using the boost converter since I had the resistor. I used one 2.7ohm 10w resistor for 3 10w leds in series. I think it ended up being a little over 700ma.




Ps. The black tape was only temp until I was happy with the resistor.

omg thanks every one specially ( All your base ) really thank you man that was a peace of info i have been suffering to drive my LEDs

well i have one last question about constant current drivers

for an example, lets say i have 6 cree XM-L and i have this driver ( Mean Well ELN-60-27D dimmable driver ) that has 0 ~ 2.3A current range and 13.5-27V DC Voltage

and i want to drive this 6 leds at 1700 mA

is all what i am going to do is to connect them directly to the driver after i adjust the current to 1700 with a multimeter, and the driver will operate with the exact voltage the leds will require or i will have to adjust the current and the voltage ? in other way of question those dimmable constant current drivers have a 2 switches to adjust voltage and current or only one switch to the current ?

sorry for all my questions but this is totally new for me

It isn't a good idea to have the driver putting out a lot more voltage than the LEDs need, but it does have to put out enough to start the LEDs when they are cold. You can control that voltage with a pot inside the case of the driver. And, the recommended way to start up the first time is to have the pot that controls the current, inside the driver, turned all the way down, then turn it up until you get the 1700 mAmps, using an ammeter in series with the LEDs, or a high wattage 1 ohm resistor with a voltmeter across it.

The dimming circuit, for the "D" drivers requires a 10VDC supply with a pot connected as a voltage divider so you set the dimming by adjusting the voltage applied to between about 3 and 10 volts.

1)first you consult the datasheet to see what kind of Voltage is required by the LED to run a desired current through it. look for the I-V curve.

2)then you multiply this voltage value by however many you want to string in a series and then check to see if your driver can produce that voltage.

3)if the driver's max voltage is higher than required, it means the driver can develop your desired current through that series of LEDs (your setup works as intended). otherwise the driver will run the series a lower current or may refuse to start.

Scenario A: XM-L is roughly 3.15V at 1.7A (consult datasheet for other current you desire). 6x of those will require ~19V potential difference, which is within range of the driver you mention. power draw is 32W, well under the 60 which that driver can do.

Scenario B: what some people might do is run 1.6A instead, and have two parallel series of 6x LEDs to maximise the driver. these things reach their best efficiencies at 75% load and higher. you can just get a lower duty driver.

Scenario C: if you try 1300mA, XM-L will need 3.05V. 8x of them at 24.4V for a power draw of also 32W.

scenario C vs A. I recommend C. you spend a few bucks more to buy 2 more LEDs but all 8 will have more than double the lifespan of the 6 in scenario A. people forget that the legendary lifetimes of LEDs only apply at rated currents (700mA for XM-L)...not at maximum rated current (3000mA for XM-L).