Power supply for 90 LED's

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eggie

Member
This is not strictly Picaxe, but is a subject that is appropriate to picaxe designers. Please could one of you EE's advise me on the best way to power 90 LED's

I am trying to build an animation studio for my son - basically a large open fronted box with a green screen at the rear and very regular lighting across the top - which is where a regular array of LED's is proposed.

Which is best - to power them directly from a regulated 3V supply or feed them with a regulated 5V supply but with a current limiting resistor on each LED. 90 LED's will be drawing just under 2 amps so I need a significant supply so I will probably limit the supply by dividing it into 3 sections.

I suspect the latter will be best as it will avoid taking out the whole circuit if one LED shorts out, but I wonder if there are any other considerations before I put pen to paper to design the circuit.

Thanks

eggie
 
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srnet

Senior Member
The first consideration is that you need check the data sheets for the LEDs in question, the forward voltage parameter in particular.

White and Blue LEDs for instance typically need around 3V or more to conduct properly, so a 3V regulated supply wont be much good .........
 
J

John West

Senior Member
Please could one of you EE's advise me on the best way to power 90 LED's
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That leaves me out. No formal education beyond secondary school, but I'm sure there are a few EE's around who can explain series-parallel hook-ups and current limiting circuits.
 
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eggie

Member
srnet

I am just using the 3V as an example - I still have to choose an LED to use, and would regulate the supply as appropirate if I went for a supply at the same voltage as the LED.
 
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Haku

Senior Member
I would look to use a higher source voltage and put the LEDs in sets of 2 or 3 in series with a current limiting resistor for that set.

Take a look at this very useful webpage for calculating resistor usage for multiple LEDs:

http://led.linear1.org/led.wiz

White LEDs are typically 3.0 to 3.6v (I have no problems sticking to 3.2v during calculations for all the white 3mm & 5mm LEDs I've encountered) with 25mA diode forward current, so for an example, put 12v source voltage, 3.2v diode voltage, 25ma diode current and 90 LEDs.
You'll be presented with a nice wiring diagram for the LEDs + resistors and an approximate power draw for the entire array.
 
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eggie

Member
Thanks Haku - I will have a play on that webpage tonight ( I am currently at work so have only had a limited look).

eggie
 
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eggie

Member
eclectic - thanks for the suggestion.

My son is needing a very general lighting to do the animations and so an array of smaller LED's is more use than a few larger units. I would also have the problem of heat dissipation from the high power LED's

eggie
 
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MartinM57

Moderator
I wouldn't even be thinking about voltage in this scenario - you should be thinking about driving the LEDs with current (which happens anyway of course, but once you start thinking current only, it all becomes very easy)

...as done by "LED drivers" - and there are '000s of them to choose from (fixed and adjustable) .. see EC's link for a starter

Or you can make a really simple one from a LM317T regulator and a resistor e.g. http://users.telenet.be/davshomepage/current-source.htm

...and yes, they will get hot and you might have to deal with that ;)
 
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Goeytex

Senior Member
How you approach this may depend upon the budget for the project.

If it were me, I would get something like a Mean Well 24v 2A power supply and then something like an LDD-350H LED Driver
to efficiently control the LED strings. All the Picaxe has to do is provide a PWM to the LDD- H series LED driver module.

Another decent Driver is the LED Dynamics "Buck Puck". These are adjustable with no PWM needed.

Simple and no fuss solutions.
 
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boriz

Senior Member
PWM might be problematic if using a video camera, as the PMW frequency and the video frame-rate go in and out of phase.

I'd plumb for a simple series resistor in line with several LEDs, repeated in parallel until you get the full 90. It's not the most electrically efficient solution, but I suspect that's not a major factor. The number of LEDs in each series string will depend on the voltage supply chosen. Higher voltage, longer and fewer strings will be better since that requires fewer resistors.

One other point. Photography under these conditions will be very sensitive to the colour temperature of the LEDs. Most 'white' LEDs either have a blue tinge, or in the case of 'warm white' an orange tinge. For best results, you may end up using more than one type of LED. Best to experiment with small numbers of LEDs to make sure you're happy with the colour.
 
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eggie

Member
The animations are created with a standard still camera (stop motion technique), but flickering would be a serious problem - he needs a continuous and steady light source. Colour temperature is not a problem - he adjusts his camera white balance to compensate. So long as it is reasonably white, he is not bothered. My son is 13 and so I am not looking to spend huge amounts on the project. Efficiency is not as important as reliability and safety. Stage size is 767 x 383. Not bothered about the tedium of soldering lots of joints - I will enlist the help of my son on that one - it will keep him quiet for an afternoon !!

eggie
 
premelec

premelec

Senior Member
Martin's LM317 good...

I'd go with Martin's LM317T current regulators and strings of 6-8 leds in series from 24 VDC power supply - you could parallel several such strings for a single LM317 - also I'd look into cheap multi LED flashlights - I bought a 95 LED [3 D cell] flashlight for USD15 from an auto parts store here in USA - draws about an ampere but puts out a lot of light [and kills batteries]. I've used it with 5v 1 amp plug in supply and series resistor OK... Look around and see what's available in units and power supplies...
 
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Haku

Senior Member
I would go with the LED+LED+LED+resistor type setup as it's easy & cheap to do, I've soldered well over 600 LEDs like that and not had any major problems. My bike lighting setup consists of 360 3mm LEDs I soldered myself, works a treat and gives me 'daylight' at night at the touch of a button ;)

Another thing to consider is wether you buy 3mm or 5mm LEDs, I chose 3mm LEDs because they use up less space and the beamspread is I think nicer:



What you see above is why many sellers state that 5mm LEDs are brighter than 3mm LEDs, 5mm typically have a tighter beam and look brighter when projected at longer distances whereas the overall light given out is between the two about the same.
 
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eggie

Member
Thanks for all your comments - lots to consider and I will now go and look at the alternatives.

eggie
 
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eggie

Member
I spent last night pouring over the various recommendations and comments and also doing further research of my own. Martin M57 hit the nail right on the head saying that I should be thinking about current rather than voltage. That is where I was totally barking up the wrong tree.

I (and I am sure lots of other novices like myself will have done the same) thought that an LED was just a very efficient light. I now realise that even the humble (or so I thought) LED only resembles a lamp in two ways - it has electricity at one end and light at the other. The bit in the middle is totally different and if there are any forum members who think like I used to do, then do start some research before deviating anywhere from the designs shown in the manuals.

I am still working on the exact design but it looks likely that I will be using 30 arrays each of 3 LED's and a 120 ohm resistor fed from a 12V supply. This only draws 600mA and so I can safely use a standard retail plug in adaptor and this keeps the 240V away from the unit and my enthusiastic son.


Again, thanks to all the contributors who have assisted me.

eggie
 
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gengis

New Member
I am still working on the exact design but it looks likely that I will be using 30 arrays each of 3 LED's and a 120 ohm resistor fed from a 12V supply. This only draws 600mA and so I can safely use a standard retail plug in adaptor and this keeps the 240V away from the unit and my enthusiastic son.
Click to expand...
That is your best bet to keep cost low. I just recently bought one of the 5 meter LED strips and they use a resistor per three leds and specify 12 volts. I mounted 40 inches of the stuff over my kitchen sink for lighting. Works pretty well. I did some experimenting with it first: the ~120 leds dissipate .9 amps which I'm sucking from a 12VAC transformer with a diode bridge and no filtering (which may not work for the fast speeds of an electronic camera - but my eyes don't notice the flicker) using a half wave rectifier the flicker is noticeable (60 cycles here) They use surface mount led chips mounted on a flexible plastic substrate with printed copper wiring and it is divisible every 3 leds ~ or about an inch. Not having any lens on the leds means the light spreads >120 degrees. If I ever redo it I'd mount it to inside surface of a polished aluminum angle to double the light inside the room. The heat dissipation seems negligable - almost 12 watts but there's a lot of surface area and the resistors and leds are not all in one small place.
 
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