Posted: 12/21/2007 9:26:12 AM EDT
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During my recent foray into a battery backup system, ar15.com/forums/topic.html?b=10&f=17&t=607803 someone suggested 12 V DC LED Lights. I checked them out, but they were very expensive. However, the idea was just so damn good I kept checking into it. I think I might build a few of my own to see if that is a reasonably cheap, effective way to do it. I think I will work in strings of 3 5mm LEDs in series with a resistor. Those will be plug and play at 12 V DC. I can just put a bunch of them in parallel to get more illumination. Here is the problem - What size resistor? This site www.besthongkong.com/information.php?info_id=12&osCsid=dc924bbab46e61ab0a8edfbe2c1d9ef7 suggests a 100 ohm .25 W resistor. This site led.linear1.org/led.wiz suggests a 120 ohm .25 W resistor. Which is correct and how much does it matter?? Thanks in advance. PS As a sign of gratitude for the answers I know I'll receive, I post a DIY tutorial on making these 3 LED 12 V DC plug and play lights. |
FINALLY -- we have some people thinking! ar-jedi's brief but definitive guide to sizing resistors for driving LED's. in a strictly analog, linear circuit (*)... an LED makes light from CURRENT flowing through it. too little current, it's dim. too much current, and POOF, no more light. the resistor in the circuit limits the amount of current that can flow through the LED. it is therefore known as the current limiting resistor. a given type of LED has two important parameters associated with it: Ifwdmax = maximum forward current, usually specified in mA (milliamps). do not exceed this current in your circuit; otherwise, you need a new LED. Vfwd = foward (junction) voltage drop, specified in Volts. this varies somewhat with applied current. the battery or other source voltage must be above the Vfwd, since it most overcome that for current to flow and therefore to illuminate the LED. luckily, typical's Vfwd is in the 0.7 to 4V range, so one or more batteries can do the job nicely. the current in the circuit MUST be below the Ifwdmax rating of the LED. how do we control the current? with the current limiting resistor. if there is no current limiting resistor, the LED will appear as a dead short to the battery, lots of current will flow, and POOF. in this respect, an LED is unlike an incandescent lamp (say a flashlight bulb). no current limiting resistor is needed because the bulb has an internal self-resistance based on the material used for the filament. a LED does not operate in any way similar to an incandescent bulb. an LED is basically a solar cell in reverse. how to calculate the value of the current limiting resistor? Ohm's law. the basics: V = I x R in the case of our LED circuit, we have a battery (say a 12V auto battery), and LED, and a resistor -- all connected in a nice loop. (note that the (+) side of the battery must go to the LED anode, or no current will flow. it is like a diode in that respect.) the equation for figuring out the value is as follow: (Vbattery-Vfwd) = (Ifwdmax) x R rearranging, R = (Vbattery-Vfwd) / (Ifwdmax) with a 12V battery, 1V Vfwd, and Ifwdmax of 100mA, R = (12V-1V)/0.100A) R = 110 ohms. BUT WAIT! we are not done yet. all of the voltage that "wasn't needed" by the LED is now be dissipated by the current limiting resistor. the basic equation for how much power is turned into heat there is as follows: P = V x I , in watts however, combining this with Ohm's law yields a more useful representation: P = I^2 x R ; power = current squared multiplied by resistance. so P = (0.100A)^2 x 110ohms = 0.01 x 110ohms P = 1.1 Watts. so, the resistor MUST BE RATED to dissipate at least 1.1 Watts OR it will turn to charcoal. for this application a 3W rated resistor would be specified. it will get hot.
awaiting.  ar-jedi (*) there is another way of applying current to an LED, and it involves driving it with a square wave or other non-linear source. for example, it is possible, using more expensive electronics, to make the LED more power efficient by only applying current for 50% of the time. if the rate is fast enough, even though the LED is off half the time, your eye does not notice. driving LED's using digital PWM or similar circuits is outside the scope of my little tutorial. |
given the choice, and assuming you have enough voltage to drive the chain, always put LED's in series. this decreases the amount of power dissipated in the current limiting resistor. given the choice, always use the lowest possible voltage to drive the LED. again, this decreases the amount of power dissipated in the current limiting resistor. ar-jedi |
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I think I might have figured it out, but my math was bad in the last thread so here goes...... I think the guy suggesting 100 ohms was using 12 volts. I think the guy suggesting 120 ohms was using 12.7 volts. I checked out a chart www.otherpower.com/otherpower_battery_metering.html I remembed reading and can see the rational for using 12.7 volts. Mathmatically, it looks like you should use 140, but that voltage will drop. So I guess he split the difference to get 120. Also he said something about using ~60% of the rated power. So, I guess I'll go with 120. My next post might be about homemade fire extinguishers. |
ooops...should've read better....sorry EDIT: aren't most alternator systems recharging at 13.8Vdc? |
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arjedi - you made it clear that the purpose of the resistor is to limit the current However, what voltage do you use for calculations? To size the resistor you would figure the highest wattage - got that. I can see using anywhere from 12.7 to 12.36 (according to that chart). But what about figuring out how many ohms on the resistor? |
what source are you using? a car battery is nominally 12Vdc. whilst the charging system is doing it's job, you may see voltages up to 14.0Vdc. this is just a 16% difference, not really enough to worry about. use 14Vdc in the equations i provided, and you'll have plenty of safety margin.
i thought i explained that clearly. what part did i miss? for multiple LED applications, where they are all in series... example, 3 LEDs: (Vsource - Vfwd1 - Vfwd2 - Vfwd3) = Ifwdmax x R ergo, R = (Vsource - Vfwd1 - Vfwd2 - Vfwd3) / Ifwdmax --- if you tell me the characteristics (Vfwd and Ifwdmax) of the LED you are using i can plug in the numbers and get you an good solution. ar-jedi |
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side note: you also have to remember that resistors come in defined values, and you are not going to be able to get the exact value you want -- but nor do you need to. the resistor value doesn't have to be exact. it just needs to be "enough" such the Ifwdmax of the LED is not exceeded. otherwise, you are on thin ice. so, always side with using a larger Ohmic value resistor -- doing so has the effect of reducing the current in the loop. ar-jedi |
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Here is a chart I made: % Volts resistor ohm 100 12.70 140 95 12.64 137 90 12.58 134 85 12.52 131 80 12.46 128 75 12.40 125 70 12.36 123 65 12.32 121 60 12.28 119 55 12.24 117 50 12.20 115 The resistor amount changes with the voltage (as you know). I plan on using 5mm LEDs at 3.3 V DC pulling 20 mA. + 1 resistor. Am I making a big deal out of nothing? Would either a 100, 120, or 140 work okay? |
okay okay, so I should go with 140, to be sure I don't burn out any LEDS. RIght? |
cool thanks. It's just weird that one calculator says one thing, the ONLY other one I can find says something else, and when I do the math I come out with another answer. I guess I owe you the tutorial!!! (As if I'll be teaching you anything) But I'll be sure to post for everyone. Thanks for all the help. |
So how do I know the toleranced on these things? If the math says 140 and I am able to use a 120 why can't I go down to a 100, 80, etc. I understand I'm getting a bit ridiculous, but you see my point. How do I know when I'm out of tolerance? PS I used 3 LEDs because the hongkong site said 4 wasn't recommended. What do you think about that? |
as the value of the resistor goes down, the current in the circuit (and therefore through the LED) goes up. if the resistor value is too low, Imaxfwd (the maximum rated forward current) of the LED will be exceeded. the manufacturer of the LED specifies that the LED will work up to Imaxfwd -- no more. if you exceed that specified maximum current through the LED, the lifespan of the LED will definitely be shortened. it could be shortened to 2 years or 2 seconds.
no more light will come out of the LED -- and that is a permanent, non-reversible condition.
you still have not provided the specs on the LEDs, namely the Ifwdmax and Vfwd. my best guess is that the Vfwd of your LED is about 3.5V; hence, with a 12V source, you can only put about 3 of them in series before you don't have enough head voltage. backing up to explain that... if the left side of the following equation is negative, your series connection circuit will not work: (Vsource - Vfwd1 - Vfwd2 - Vfwd3) = Ifwdmax x R if that is the case, you have to make a "hybrid" parallel/series circuit. for an example of that, go back to led.linear1.org/led.wiz and create the situation where the Vfwd drop of N LEDs exceeds the source voltage. it will draw you a diagram of the hybrid circuit you need. ar-jedi |
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Since LED (Light Emitting Diodes) are silicon semiconductors, they are prone to something called thermal runaway. Basically, it means that the LED's will get hot especially if used near their maximum rating. As they get hotter, the internal resistance gets lower, and the current draw will increase again in a continuous loop until the LED's self destruct. A series resistor will set the maximum current that can be drawn. Three 3.3 volt LEDs and a 140 ohm resistor is a safe circuit. Four 3.3 volt LEDS and no resistor could lead to problems. Thus the recomendation of using only 3 LEDs in series as being safe (no smoke). RS |
| ok, have to ask, why not just buy a couple of 1156 clear LED tail light bulbs and sockets and make a custom string of them or maybe even "pods" for them? that way you dont have to worry about finding the correct resistor or even soldering the leds to a board, its just plug and play. you could even use red to save your night vision. |
that would work, but defeat the purpose. I want to provide illumination at a low wattage. I already have 13 W flourescent bulbs. Even if the inverter is only 80% effecient, it would only cost me 16 watts to run one of those. I would rather use one of those than the 24 W 1156 - but they would work if attached to my 12 V battery. Also, I went to Radio Shack to pick up stuff to build the 3 LED array. Each white LED was almost $6.00!!! NFW am I going to pay that. So, I'm going to just bite the bullet and order from the hong kong guy. I'll get the stuff in a week or so and then I can start playing around. This should be interesting. Just for fun I did pick up a LED from Radio Shack that was ready for 12 V, already had a resistor. Red and dim. I have higher hopes for my white bulb cluster. |
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Check out the Custom & Homebuilt sections Candle Power Forums For Parts Sandwich Shoppe |
He's right, candlepowerforums is an excellent resource. Read for an hour or two and you will be amazed what you learn. Also, pick up a cheap digital VOM like the harbor freight one for $3.99. Buy 2. Then you can measure the current into your LEDs. Also, on the CP forums, pay close attention to the new CREE and the brand new NICHIA 5mm LEDs. They will give you the best efficiency by far. Lowe's [new] TaskForce 2C cell flashlight uses the new CREE LED and altho it's mosdestly expensive at $29 it is a quick way to get your hands on a CREE. That's what I did to convery my headlamp to a CREE. Will order a replacement to the flashlight when I'm off this trip. To make sure you get the TF with the CREE, don't buy the TaskForce until you read the topic here: candlepowerforums.com/vb/showthread.php?t=182082 I think stringing 4 Crees together in series and running them at ~12VDC and .5 amps for a 6 watt total power consumption would make an awesome emergency lighting backup. Another thing, LED flashlights are undergoing a revolutionary change given the new hi-effiency LEDs available for the last 4 or 5 months now. So you can get the older FLs cheap and gut them for LEDs [not the best effiency but good to experiment with. Harbor freight would be my starting point for FLs to gut. |
Don't forget these kinds. Have seen them at WM, and the box stores. www.ledflashlight-hk.com/admin/product/productlist/60LED_work_Light_92104.htm Cheap quick way to get soom fairly good LEDs.  www.ledflashlight-hk.com/productnew/LED_Ceiling_Light_3.htm You can probably order samples with no problem. |
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A quick warning to those reading this thread... LEDs are very very addicting. I can't begin to tell you how much money and time you can burn playing with them... I recently started working on a 12 volt 20 Cree LED rack lighting system for my Suzuki Sidekick diesel conversion bug out/in vehicle project. I have boxes full of those little LEDs in all flavors including IR and some UV ones that I'm going to use for a portable UV water purification system. Tons of fun..... Visit candlepowerforums.com KDX |
...and a few sources for pre-built LED bulbs and "bare" LED components: Autolumination BestHongKong Deal Extreme SuperBrightLEDs.com |
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Well, my stuff left hong kong on the 26th, but apparently that is where their tracking stops. It will be interesting to see how long this takes. Any of you guys have any experience with http://www.besthongkong.com shipping? Also, now I need to get a soldering iron. Any suggestions for a decent inexpensive one ( I didn't say cheap - don't want crap) ? |
i not only work at my place of employment with a soldering iron in my hand, but i do many projects at home with one as well. more appropriately, you should be aware that i actually have more than one at home. so my $0.02: for general purpose electronics work, i recommend a Weller station, preferably a model with adjustable tip temperature such as the WES51. a variety of tip sizes will allow you to do everything including SMT work. www.hmcelectronics.com/cgi-bin/scripts/product/1980-0430/ HOWEVER for the past two years i have extensively used a Weller Portasol P2KC cordless (butane) iron. i love it. no cord, which means it can be used out in the truck, or down in the basement, or on the boat. a single charge of butane will last 30-45 minutes. i am still working on the original large can of butane that i bought at the dollar store. and by changing tips, this iron also can be used for collapsing heat-shrink tubing -- super handy. the P2KC kit comes with everything you need to get started. there are two limitations to the Weller cordless iron. the first is easily rectified -- the supplied tips are too large for delicate SMT work. you definitely need the PPT1 and PPT5 accessory tips for fine work. the other issue is that with all of the supplied soldering tips, hot air is exhausted downstream of the catalyst at a right angle to the tip. so: when working on ANY project, keep the exhaust port pointed UP and AWAY from the work, otherwise you will inadvertently expose something on your project to a 1000'F stream of hot air. www.hmcelectronics.com/cgi-bin/scripts/product/1980-0352/ losdos.dyndns.org:8080/public/weller-portasol/ ar-jedi   ---    |
Maybe I do want something cheap. lol I don't know how deep I'm going to get into this stuff, so I don't want to spend a ton on an iron right up front. Hell, I might use it once and then never again. I'll probably go to Harbor Freight, Lowes, etc to check out what they have. Oh, I bet Radio Shack will have something too. |
you'll be soorrrrrryyyy.  get the Weller P2KC butane iron kit. you can use it for multitudes of projects. for $44 it is a steal. it is worth *at least* 10X whatever you are going to be able to pick up at Radio Shit. ar-jedi |
cool, thanks for the good advice! |
I was able to find a Weller iron, but not the one mentioned. Here are my new toys!!! Sperry DM-210A and Weller 25 Watt link to bigger picture |
WHISKEY TANGO FOXTROT ?!?!?!? ar-jedi |
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My LEDs and resistors arrived today!! I went out for dinner and a movie, and HAD to try my hand at a bit of soldering before I went to bed. Success!! I don't have my digital camera here, but I was able to put together 2 quick and dirty 3 LED strings that worked. It is dark out and I turned off all my lights and walked around with just the LEDs on. It was plenty bright to walk around without issue. After I sat for a bit, I could read okay (I do have good eyes) from reflected light. The LEDs are directional and I just pointed them at the ceiling and let the light bounce off of it. I only ever had 3 LEDs on at a time. I think this is a viable way to get some decent illumination at a low wattage. I'll do the math later, but they don't pull a lot of power. I'll make some bigger lights that use mutiple strings too. Thanks to everyone for their input! |
Here are my pics of my finished 3 LED 'bulb'. Sorry for the delay, I was having trouble getting my camera to take the close-ups. Don't make too much fun of my soldering, first time!  These three LED were bright enough to walk around with in pitch dark without bumping into anything just using the reflected light (NOT the spotlight). This test was the reason I wanted to build my own. It was hard getting good information on how bright they would be so, I just bit the bullet and got some. The power consumption is really low for the 3 in series with the resistor:  The black figures are known values and the red are calculated values. I think I got the math right... I'll have to figure out how to build bigger arrays to really be able to get some really good light out of them. |