Questions about using LED to light my room - Page 1
Forum section: electronics / beginners

author: SH@RK
author_rank: Contributor
timestamp: November 27, 2012, 01:19:38 pm
content: HiI want to light my room using LED lightsAnd I want to build them myself (first useful thing I will make)I got white LED (2000pcs) of thesehttp://www.ebay.com/itm/1000-pcs-5mm-Round-White-Superbright-LED-Light-white-/120644259920?pt=LH_DefaultDomain_0&hash=item1c16f55050And got 1/4W metal film Resistors (1000pcs 50 value)http://www.ebay.com/itm/1000pcs-50-value-1-4W-metal-film-Resistor-1R-10MR-1-/120643705527?pt=LH_DefaultDomain_0&hash=item1c16ecdab7And got DIY Prototype Paper PCB Universal Board 5 x 7 cm (7pcs)So I started with drawing the diagram for one small prototype (usingFritzing)with specs16 LED --- 16 (130 om) resistor --- running on 6Vand this is the final resultyes the soldering is very bad but this is what I can do now with this boardSo the questions are :-!:- is what i did correct until now ?2:- Is there a better way to wire the LED (me doing 1 resistor for 1 LED) ?3:- How many I will need of these to light a (5 X 5 meter) room to the normal life level ?4:- Is it safe ?5:- I am thinking of making it usable for when power is of so I will use 6 (AA or AAA) is it gonna work (and making it portable) ?AA or AAA , can they provide enough current (for this design around 0.32 A ) ?6:- Do u think it will cost more than it worth ?7:- how many years (or months) normal LED can last ?8:- I am thinking of making A PCB for the project so it will be easier to build , so what do u suggest ?I found locally (150 X 150 mm) one side copper board will cost around 4.3$9:- Do I need I box or only cover so it looks nice ?10:- Did not think of the power supply yet but do you have any suggestion ?11:- thinking of using SMD LED because the look nicer and thinking it will be easier to use and organize on the board ?Thanks

author: Skimask
author_rank: Super Contributor
timestamp: November 27, 2012, 01:29:31 pm
content: Read up on your diodes, LEDs, voltage drops, resistors, and just basic electronics in general.You've got one resistor per LED. Assuming 3.5v drop per LED, 130 ohm is a decent starting point with 6v input, gives about 20mA per LED, and 6 parts.Take that same 6v, change it to 12v, put 3 LEDs in series, with a 75 ohm resistor... 3 LEDs, 1 resistor, 12v in, 10.5v drop across 3 LEDs, 1 75 ohm resistor, 4 parts.Double it again to 24v, 6 LEDs in series with a 150 resistor, 6 LEDs, 1 resistor, 24v input, 21v drop across 6 LEDs, 1 150 ohm resistor, 7 parts...And so on, and so on...

author: Simon
author_rank: Unknown
timestamp: November 27, 2012, 01:41:49 pm
content: yes you are "pissing away" a lot of power in those resistors, I did a 225 (15x15) uv led array with just 15 resistors

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 27, 2012, 01:56:56 pm
content: To make assembly easier and cheaper, i'd use a series parallel arrangement, but at such a low voltage (7.2v ( i assume 6x 1.2v nimh AAs)) it doesn't save much, personally, i'd use 18650 lithium ion batteries, 4 in series, that gives you a voltage range of 16.8-13.2v, and a fair amount more capacity, and you could run a string of 4 leds in series with one current limiting resistor, wouldn't have to worry about LVCO for the batteries as the leds would turn off when the voltage got low.As for if you used a power supply, i'd get a 13.8v power supply, do strings of 4 with a 10ohm limit resistor, if you go a 12v supply you can't do more than 3 leds in a string and your current limit resistor uses too much currentBased off the ebay page 3.4v drop and 20ma current.

author: Flávio V
author_rank: Regular Contributor
timestamp: November 28, 2012, 04:49:38 pm
content: Huuum...those leds dpes't seem like a good idea for lighting...but huum...they are cheap...but not really effective and ebay->china->dubious quality....There is a less safe choice than is to get a isolation transformer and put near 80 or 90 in series(if 230V)If you are going to change LEDs i can suggest 2 of these for the room:http://www.digikey.com/product-detail/en/BXRA-C0361-00E00/976-1019-ND/2331647They are cheap and good....but a pain to solder wires if you use bad solder...or wires...or both...

author: Jeff1946
author_rank: Regular Contributor
timestamp: November 29, 2012, 02:19:43 am
content: A few comments, for room lighting you want to diffuse the light, thus wide angle leds are your best choice. Think about color preference. Typical white LEDs give a sterile office fluorencent look to things. I prefer "warmer" (2700-3000 K) ones. As others have said a series parallel arrangement works well. A good power supply is a used laptop one. I used an older Dell one that could put out up to 3.5 A at 20 volts. I checked output. It is rock steady and has little switching noise. I believe these supplies are fairly efficient and meet good safety standards.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 29, 2012, 01:21:29 pm
content: Never connect LEDs in parallel in mass using constant current, if one LED gets hotter, it becomes more conductive, draws more current, gets hotter until blam, it pops, then, your CC source tries to push that current into the remaining LEDs and the whole thing goes until no more LEDs, constant current is fine for series strings but no parallel strings.

author: SH@RK
author_rank: Contributor
timestamp: November 29, 2012, 11:07:13 pm
content: Thank you allI think the resistors eating the power as you all saidso I drew the diagram again with LED strings of four16 LED --- 4 (10 om) resistor --- running on 13.8Vnow the power lost in one resistor = 4 mW4 resistors = 14mW onlyvery very big change (that is what I think)But here are my questions :-1:-If one LED died the whole string will turn off because they are in series , is there any way to solve this thing so the other three will stay working ?My first design does not have this problem (bat have other problems like the lost power)2:-I do not think I will find small battry with 13.8V (even if I found it will not be cheap) so what is the best way to make from 6V (or 5V) to 13.8V ?I am thinking of using 3 mobilephones battreys like iphone's battrey (5V each)3:-In the new design if the voltage is dropped (it is a battrey) to 13.7 the LEDs will turn off because the current will became 10mAand if the voltage is more like 14V the current will be 40mA which is double the max current 20mA so LEDs will burnmy point is if the voltage chnge only by +0.1V or -0.1V it will be a problem ,, and in the real live it will be more than 0.1V chngeSo I have to make sure the voltage will stay 13.8V for all time or else my product is not workingSo what should I do ?Change the design ??that is what I have in mind now , I will be back later to give detailed replyThanks everyone

author: hlavac
author_rank: Frequent Contributor
timestamp: November 30, 2012, 07:23:27 pm
content: Another way:All LEDs in series, high efficiency boost DC/DC converter with constant current output, powered from battery of your choice.Pros:- Constant light intensity over the whole battery discharge cycle- All leds with identical brightness, uniform look- Possibility of brightness control adjusting the current- Wide range of possible input voltages / batteries- Variable number of LEDsCons:- Higher voltage (~50V), have to be more careful- If one LED fails, all go off until you short out the bad one (but then works well again)- A bit more complex and more expensive- Not suitable for a noob

author: SH@RK
author_rank: Contributor
timestamp: December 06, 2012, 06:47:46 pm
content: Quote from: hlavac on November 30, 2012, 07:23:27 pmThanksas you said "Not suitable for a noob" and "If one LED fails, all go off"I think I can not use it .now for my problem"3:- In the new design if the voltage is dropped (it is a battrey) to 13.7 the LEDs will turn off because the current will became 10mAand if the voltage is more like 14V the current will be 40mA which is double the max current 20mA so LEDs will burnmy point is if the voltage chnge only by +0.1V or -0.1V it will be a problem ,, and in the real live it will be more than 0.1V chngeSo I have to make sure the voltage will stay 13.8V for all time or else my product is not working"I am thinking in voltage regulator , then there will be power lost as heat and must supply with more than 13.8 du to voltage dropso it is not efficient (from what I understand)How to make this voltage (13.8 ) stable no matter what happens ?If we solve this , the main problem will be gone .Thanks

author: hlavac
author_rank: Frequent Contributor
timestamp: December 06, 2012, 08:34:22 pm
content: You could use a simple JFET current limiter.Battery voltage to drain, resistor between source and gate, other end is from gate thru LEDs to ground.For example BF862, with 39 ohm resistor it would give roughly 20mA current limit and a minimum of 0,8V voltage drop. Maximum drop would be ~15V with the 300mW maximum power dissipation at the JFET at 20mA.With this, you need a battery that has minimum voltage of Vleds @ 20mA +0.8V minimum jfet drop, and no more than Vleds + 15V max. The lower the less losses.(Vleds is a total voltage drop across the LED string at 20mA, i.e. number of leds * voltage drop at one led).