How to wire 5x 3W leds in series with led driver.

Thread Starter

semmyroundel

Joined Oct 28, 2014
17
Hi all, I'm trying to make an algae scrubber for my marine aquarium, this needs 5x3W 660nm deep red leds wired in series.
The tech says forward voltage 2.4-2.7v, so would I be right in saying that a 12v 18w driver would be sufficient to power these in series?
Also, what if I use less, say two, three or four leds? Do I need to introduce a resistor to each led, and what about 6 leds, do I need to get a driver that delivers 18 or 24volts?
Many thanks.
 

MikeML

Joined Oct 2, 2009
5,444
5x2.7 = 13.5V. So you either need a 15V+ constant-voltage supply with an appropriate wire-wound or ceramic power resistor, or you need a constant-current Led driver power supply that has a minimum compliance of 15V adjusted to deliver P/E = 3/2.7 = 1.1A. Either type of supply would have a minimum power rating of I*E = 15*1.1 = 16.5W.

Or you could use two series strings of three, in which case halve the voltage and double the current...
 

wayneh

Joined Sep 9, 2010
17,496
...would I be right in saying that a 12v 18w driver would be sufficient to power these in series?
Yes
Also, what if I use less, say two, three or four leds? Do I need to introduce a resistor to each led, and what about 6 leds, do I need to get a driver that delivers 18 or 24volts?
Only one resistor is needed to limit the current to the series string. An 18V driver should be OK, as this exceeds 6 x 2.7V.
 

mcgyvr

Joined Oct 15, 2009
5,394
Do you want dimming?
Do you have a reef controller that you want to control this LED with? If so whats its dimming signal?

A few choices..
1-Meanwell LDD + AC/DC power supply (can accept 5V PWM dimming)
2-Meanwell LPF (can accept 0-10VPWM or 1-10V analog or 100k pot dimming)
3-AC/DC power supply + high wattage resistor (which also equals heat + instability if Vf fluctuates)
4-Other constant current drivers.

An LED driver needs to have an output voltage greater than the sum of the Vf of the leds in the string and a current rating at or below the maximum current rating of the LED (700ma is good for those 3W LEDs.. (350mA if using a blue/sub 450nm LED)
 

Thread Starter

semmyroundel

Joined Oct 28, 2014
17
Hi all, thanks so much for your prompt input. The reason that I was going to wire in series was that the commercial versions of this project are done so, in fact on the forum dedicated to making these scrubbers, they frown upon wiring in parallel, I don't know why, I assumed it was something to do with the nature of the way the leds are driven so I didn't question it.
I bought a 12v 18w driver and now I'm wondering because of the smaller size of the scrubber than was originally intended, due to my sump size restrictions, I will use 4 leds.
So my questions should now be:
Should I wire them in parallel, or series?
If I use this driver, what resistor do I need (if any at all) and do I need one of them (I'm assuming this to be the case if in series) or four individual ones if in parallel?
What does 2.4 to 2.7 forward voltage mean? I'm a qualified electrician and understand va ratings for lamps and power factor in discharge circuits, but have never heard of these terms.
I'm not dimming.
 

wayneh

Joined Sep 9, 2010
17,496
What does 2.4 to 2.7 forward voltage mean?
It means that, when they are operating on the specified current, they will drop that voltage across themselves. For instance you might say that an automotive bulb has a Vf of 12V, since it drops 12V when operating normally.

The Vf is not a terribly reliable number for design purposes, but it helps you match a power supply voltage to a series string.

Current flow is a far more important design consideration. Unlike the auto bulb, an LED does not self-limit the current it draws and can be easily damaged if current is not controlled externally.

Should I wire them in parallel, or series?
Most likely - depending on your driver - you will use a series arrangement. Then you need to control the current in just one string. Otherwise you need a current controller for each parallel string.
 

MikeML

Joined Oct 2, 2009
5,444
If you wire several LEDs in series (so that the same current flows through all of them), and you use your DMM set to DC Volts to read the voltage across each individual LED (one at a time), when operated at the rated current, the voltage across each LED will be somewhere between 2.4V and 2.7V. If you tested a 100 LEDs, and plotted their Forward Voltages, they would distribute on a Bell Curve, just like grades...

If you grab four LEDs at random, wire them is series, hook them to a variable lab DC supply, and very slowly and carefully turn up the voltage (while watching the current meter to keep the current below the LED's max. current rating), you will find a point where the four LEDs light up, and the current is near the rated max. The total voltage could be as low at 4x2.4= 9.6V or as high as 4x2.7V = 10.8V, or anywhere in between, based on the random selection of the LEDs. You cannot operate them this way because as the temperature changes, the curent will increase, and you will burn them up.

Now, if you hook the four LEDs to a 12V high-current constant-voltage power supply, you will instantly vaporize them. You need to put a resistor between the 12V supply, and the four LEDs to prevent that!

Unfortunately, the specific value of resistance required to limit the current through the LEDs (think of ballast in the context of a florescent tube) is dependent on the exact total of the forward voltage drops of the LEDs. This is not the preferred way of doing this.

It is much easier to buy a LED driver constant-current type of power supply, which has a pot on it to set the max. allowed current for your specific LEDs, and the power supply voltage magically takes care of itself with no resistor required...
 

MikeML

Joined Oct 2, 2009
5,444
Geez, not enough current capability, shoulda looked more closely (4x700ma =2.8a)
That thing is not even a "LED driver" in my understanding.

What a dumb set of specifications...

I make it out to be a constant-voltage 12V supply, designed to drive a "LED lamp replacement", NOT for driving series-connected individual LEDs.
 

Thread Starter

semmyroundel

Joined Oct 28, 2014
17
No, that was my mistake, I just added up the wattage of the leds, doh! The reason I thought it might work is that I routinely instal self adhesive led strips to under-cabinet lighting with many leds on it and just wire in a regular driver such as this.
Spec:

2x 3W Deep Red Led for DIY lighting fixtures, aluminium flat bars and heatsinks.

High energy efficiency, long life, high reliability and low maintenance costs.

Applications: Plant Growth, Increase Flowering, Autumn, Sunset, Chlorophyll A, Food Production, Antioxidants, Aquarium, Tank, Cabinet, Increase Poultry Reproduction and Reduce Feed and Aggressiveness.

Technical Parameters:Quantity: 2 pcs
Emitted Color : Deep Red
Intensity Luminous: 70-80LM
Viewing Angle : 120 Degree
DC Forward Voltage (VF) :DC2.2V~ 2.6V
DC Forward Current (IF) : 750mA
Wavelength: 660NM
 

Thread Starter

semmyroundel

Joined Oct 28, 2014
17
Thanks for your reply, I don't understand how it's 550mA short though, I've done the math and can't get to 550mA short (once I posted the spec I can see they're 750 mA) so this driver says 1.5A
 

adam555

Joined Aug 17, 2013
858
Yesterday I was reading the datasheet for that type of LEDs, an those 2 voltages -at least in that model- where for the typical voltage and maximum voltage drop; not the minimum and maximum.

So I was thinking, if 2.2v is the typical voltage, 12v for the 5 LEDs in series you had originally intended would come to 2.4v each; which shouldn't be too dim.

Have you given it a try yet?
 
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