Eye doctors strongly caution the experimenter/hobbyist against looking directly at a lit LED. The risk of retinal damage is very much there even with low power LEDs on account of their highly focussed and coherent beams emanating from a near-point source. The danger is VERY REAL with higher power LEDs.
The reader is STRONGLY REMINDED of the need to exercise great caution.
The easiest way to ensure eye protection is to fix a small piece of plastic diffuser in front of the LEDs with adhesive tape as the very first step-- I had good results with the speckled plastic that is used to make flexi CD pouches and file folders-- until the assembly is safely fitted into a good diffusing fixture.
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Of late LEDs and LED lights have been, how does one say it, "very much in the spotlight"!
Long service life, reduced power consumption and the plus point of 'going green' were some of the factors that favoured the minuscule semiconductor 'light bulbs'. Increased R&D activity in recent years have raised the bar of Lumens (light output) to higher and higher levels, along with a lessening of costs, so much so that high power LEDs now offered the enterprising hobbyist opportunities for replacing many of the lamps in his/her home with DIY LED lamps.
Seoul Semiconductor (http://www.seoulsemicon.com/en/) is a comparative newcomer in the field, but that has not prevented them from reaching the very cutting edge of the technology in a short time. (The company is an innovator and their latest product 'Acriche' is a case in point. It has no usual --and unreliable-- components like resistors and capacitors and transistors that make up the conventional LED driver, but relies on just a custom IC integrated with the LED chips on a ceramic mount offering DIRECT AC connectivity with NO OTHER driver requirements.) (http://www.seoulsemicon.com/en/product/prd/acriche.asp)
Recently when some higher power yet cheap Seoul Semicon LEDs in the 1W and 2W range came into the market, I thought it was time to wire up a practical spotlight.
Luckily I had an old LED holder and lenses lying about. Many holders (for old 5mm LEDs) and lenses are available in the market and the good thing is that the Seoul Semi LEDs will fit those like a glove.
The 1W LED has a single 'chip', while the 2W one has a couple of chips--easily viewed with a magnifier, NOT when it is lit! The current consumption is of the order of 330 mA and less than 700 mA, respectively. But in the interests of long life, I would opt to operate them at about 320 mA and 630-640 mA, and also mount them on substantial heatsinks for cooling. It is easy to 'spec' the heatsink -- after running for a few hours, one should be able to touch it firmly and keep the finger on the heatsink; anything hotter would be suicidal in the long run.
You need another cheap component-- the LM317 adjustaable voltage regulator from National Semi-- which we will configure as a constant current driver for the LEDs. Be warned that LEDs die an early death if they are operated at currents above their design specs. A simple series resistor might be all right if the driving voltage for the LED is steady as a rock. But any other situation would call for a constant current driver. A couple of small resistors complete the BoM or the Bill of Materials for your spotlight.
Kindly note that here I opted for the 2W LEDs, the holder accommodating three; connected as a series string, they were driven with 620 mA. For a smaller lamp, you could opt for series string of 3 x 1W LEDs (about 320 mA) or if you wanted a 'spread out' light source instead of a spot, for two parallel strings of 3 x 1W LEDs (approx 640 mA).
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A CAVEAT:
I was lucky to get a '12V' SMPS whose output was way above and as a result, the LM317 constant current driver worked all right even with a series string of three LEDs.
Please note that the series string of three LEDs will need about 10V to drive them and the LM317 needs about 2.5 to 3V more than that at its input to work reliably.
If your SMPS outputs just 12 V, it would be best to put two LEDs in series, which need a drive voltage of about 6.6 V and so the 12 V smps should be more than adequate to ensure correct operation.
Do measure voltages and currents with a reliable and accurate muti-meter.
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It is easy to set the constant drive current by selecting just one resistor (R) as per your requirement. The LM317 while operating produces a constant 1.25 V between its output terminal and the adjustment terminal; that means a constant current flows through the current setting resistor and the load connected. Divide 1.25 by the current in Amperes (320 mA is 0.32 A and 630 mA is 0.63 A) to get the value of the current 'programming' resistor in Ohms. Go for a series or parallel combination of a couple of half-watt resistors so that you get as near the theoretical value as possible. For example, I used two 1 Ohm resistors in series that set the current at 625 mA.
Now onto construction. The three LEDs were wired as a series string with short bits of wire so as to fit the lenses in the holder. Pressing a piece of paper onto the lenses provides you with a template for this.
Cut a 2 mm thick piece of aluminimum sheet to serve as the heatsink backplane for the LEDs.
A disc with holes to clear the backs of the LEDs is cut from say an OHP film or plastic to serve as an insurance against short circuits.
Refer to the photos and make the assembly after spreading blobs of heatsink compound on the backs of the LEDs. The alu disc should now be a bit proud of the edge of the holder.
Now smear heatsink compound onto the disc and mount the heatsink with three screws onto the holder. Please note that the heatsink has to be drilled beforehand with the help of pressed-paper templates.
The LM317 also needs some cooling and has to be mounted onto the heatsink (using the third screw). Now wire up the current setting resistor/s and the connecting wires as per the following wiring diagram.
# Warning #
Please note that the metal heatsink slugs on the backs of the LEDs are fully isolated. But the heatsink tab of the LM317 is connected to its output pin. You don't need an insulator here so long as you ensure that all connections are well insulated and that nothing is connected to the metal holder or the heatsink.
Mount the SMPS pcb within a short piece of pvc pipe screwed to the top of the heatsink. The other end of the pvc pipe carries a mains lamp base salvaged from a fused CFL. Connect up the wires paying attention to insulation and safety.
Switch on and you should be rewarded with a really bright beam.
# # # Be careful not to look directly at the LEDs # # # as that could cause some amount of damage to your eyes. It is better to put the lamp in a diffuser shade unless you want to use it as a spot light.
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Warning:
If you want your lamp to be reliable in the long run, pay particular attention to heatsinking. A heatsink that might look and 'feel' adequate (remember the 'touch test' ?!) on the workbench might prove to overheat when the lamp is assembled into a holder-- if my experience is anything to go by! Use a substantial heatsink I did!) to keep the assembly really cool and drill adequate ventilation holes in the holder assembly. Usually the LED current would rise with rising temperature, but the constant current driver does offer some insurance here.
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Another alternative that a friend tried out was to mount the LEDs onto a 1-foot length of aluminium channel used by fabricators of doors/windows etc. Drill small holes to fit the lens of the LEDs in a suitable strip of transparent plastic. Mount the LEDs smeared with heatsink compound onto the alu channel with a few screws that 'sandwiches' the LEDs between it and the plastic strip. The SMPS pcb, insulated well in a tube of plastic, maybe slipped into the square tubular alu channel and the LM317 too may be mounted onto the channel. Mount two plastic end-caps (drilled for ventilation) on both ends of the channel and also screw on a piece of diffuser material in front, and you have a good, bright short "tube light" of sorts!
Happy 'spot lighting' !!
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