DIY string of LEDs - 12v power

[howardb]

Hi all,

Been doing a lot of reading this past weekend on the subject, but cannot get a definitive working of how I should do this. Any comments/suggestions/guidelines would be welcome.

I need to string 6 LED's approx. 30-40cm apart - the LEDs are different colours (Blue, Green and Red). When I purchased them, the seller reflected the following specs for each:
Blue/Green = 3v-3.5v, 350mA, 1W, 8mm epistar
Red = 2v-2.4v, 350mA, 1W, 8mm epistar
I have tested each LED and confirm I'm happy to run the lower voltages of 3v and 2v.

To keep the wiring as simple as possible, I'm wanting to use 2-core for the string, parallel LED connections to +/-.
Now the power supply being used was to be a 12v constant (10A max output; can adjust the PSU output to 10V if needed), however I then decided to maybe step 12v down to 3v using a LM2596S DC-DC Step Down CC-CV Adjustable Power Supply Module - I thought it may save me having to possibly fiddle with resistors on all LEDs, etc.

My question is:
I want to string these LEDs in this sequence - Blue, Red Green, Blue Red, Green - if I connect them in parallel from the 3V supply (seems easier than series/parallel on 12v), the Blue and Green would run fine on the 3v, but the Red would burn out as it's 2v rated. According to the different online calcs, I should be using a 3.3ohm resistor on the Red LEDs.

3v+ ___ Blue+ ___3.3ohm/Red+_____Green+__repeat
3v- ___ Blue- ___Red-____________Green-__repeat

Based on R= V1-V2/I
R= 3v-2v/0.35a = 2.8ohm (3.3ohm resistor is the closest for the Red LEDs)
To me, the 3.3ohm seems too little due to the 350mA rating? Should the mA of all 6 LED's be calculated instead, i.e. 3v-2v/2.1a = 0.5ohm? Also seems incorrect...

If it's easier to run directly off the 12v, I'd appreciate some input, guidelines.
To be honest, the resistance is the confusion for me, given I want to connect in parallel... maybe I'm just not grasping the formulas...

 

[agentrfr]

The LEDs can not be considered straightforward resistors themselves. They act differently according to the voltage dropped across them. In fact it's better to work with around 20ma of current to go through the series component of the LEDs to tell you their forward drop than start to work with their forward voltages.... It gets very messy.

The LM2596S is a buck converter with a minimum output of 3v3, so you already cant use that.

How concerned are you with efficiency? If the whole setup is only a few watts rather get some linear regulators for the 2V and 3V rails. Have a two core wire for the 2V rail and another two core wire for the 3V rail. It's much easier this way - promise. I recommend not running the LEDs on their maximum ratings (2.5 and 3.5V) and rather stick to the lower rating (2V and 3V) to improve lifespan.

http://za.rs-online.com/web/p/products/7140792/?tpr=2

Be sure to read the datasheet. You can see examples on how to set up a simple rheostat configuration (Figure 6 on page 9) to adjust output voltage easily yourself "in the field"

Good luck!

 

[howardb]

Thanks agentrfr.

The LM2596S I bought shows the following specs:
Input voltage: 4.75 - 35V - my input supply voltage is 12v
Output voltage: 1.25 - 33.5V (adj.) - I currently have it set to 3v output, calibrated via a multimeter with no load.

The LED's I bought are similar to these ones (same LED shape and specs): Red, Blue, Green.

Efficiency is not a problem and the string is only 6x 1w LEDs, therefore max 6w, although probably closer to 4w-odd.
Will see if I can use the 2x 2-core wire setup on the 2v and 3v rails. Will also be "under-powering" the LEDs to 2v and 3v only, so it should be on the lower end of their ratings.
I can't access the data sheet on the Linear Voltage Regulator at the moment; think my work is blocking some web components, but will check it out later when home.

 

[howardb]

Would something like this make life easier? String all LEDs together in series/parallel with this in front - would it then matter if the voltage is 12v?

 

[agentrfr]

Hi Howard

That one that you linked to doesnt say what type of driver it is. It is possible it is a PWM type driver, which will definitely blow your LEDs (to get 3V out it spits the full 12V but only 25% of the duty cycle - still enough to blow your lights).

You have a choice really, you can use the LM2596S and get another one for the 2V rail, or you can use two linear regulators to make a 2V and 3V rail. TBH I don't know what you paid for the buck switch mode power supply, and normally the linear regulators are way cheaper but if your LM2596S is an assembled unit it might be worth getting a second one for your second rail.

 

[agentrfr]

In fact I'm 95% confident that whenever someone uses "LED" and "dimmer" in a single sentence then the driver is a PWM - it will not work for your light configuration. You'll expose them to the full 12V, even if it is only 25% of the time.

 

[howardb]

Many thanks agentrfr.

Thinking about it more, if I series connect two seperate sets of blue/red/green to total 8v each, then parallel the two series'd sets and set the LM to 8v output, that should also work? They'll all be pulling the same mA in each set/led.
The 8v would be constant supply in the parallel set and the series sets would also pull 8v.

I only paid R20-odd for the adjustable LM, so can always get a second one as you suggest, to split the 2v and 3v strings.

 

[agentrfr]

Many thanks agentrfr.

Thinking about it more, if I series connect two seperate sets of blue/red/green to total 8v each, then parallel the two series'd sets and set the LM to 8v output, that should also work? They'll all be pulling the same mA in each set/led.
The 8v would be constant supply in the parallel set and the series sets would also pull 8v.

I only paid R20-odd for the adjustable LM, so can always get a second one as you suggest, to split the 2v and 3v strings.

Nope. You can't do that with LEDs unfortunately. They're technically diodes, not resistors, so they don't behave like you'd expect resistors would unless a whole bunch of requirements are met. Definitely get yourself another switch mode power supply if it is only R20. Where'd you get it from anyway? That sounds suspiciously cheap...

 

[thehuman]

what if he string all leds in series and use constant current mode on the lm .

 

[agentrfr]

what if he string all leds in series and use constant current mode on the lm .

That'd work sure, but for R20 I dont know if his LM2596S has a programmable CC mode

 

[howardb]

Thanks agentrfr, noted.
Will have to check where I got it from as have had it a while - the same one is going for about R40-odd on BoB...

In order to accommodate a 2-core-only-string (unfortunately cannot use more than 2-core as I'll have to then re-work the rest of the project; hole sizes, etc...), I've resigned myself to the fact that I should rather use the 12v power source and place 27/33ohm resistors before each respective LED in the single string (Blue/Green - 27ohm and Red = 33ohm) - was hoping to avoid this, however will allocate a bit more time and do it properly

 

[howardb]

FYI, my LM looks exactly like this one on BoB; same specs/markings too.
Not sure about the programmable CC though. The potentiometer adjusts the CV, so assume that the CC is also proportionally adjusted when CV is adjusted - will do a few tests this evening on the current vs voltage.

 

[thehuman]

FYI, my LM looks exactly like this one on BoB; same specs/markings too.
Not sure about the programmable CC though. The potentiometer adjusts the CV, so assume that the CC is also proportionally adjusted when CV is adjusted - will do a few tests this evening on the current vs voltage.

hmm cc capable one would have had a adjustable preset for voltage and current
so constant current is out
remember risistors values add up if in series so you may wire string of leds in series and one risistor for current limiter