18 Led's powered by 12vdc

[myp71]

I have 18 3mm led's I would like to switch my high mount stop light on my car into these led's.What resistior could I use?<p>Thank you <p>Ryan

[dacflyer]

i have a lantern i made with white leds..
3 leds and a 120 ohm 1/2 watt resistor powers them at about 25ma.
i was averaging 3v for each led..i didn;t get technical..its working great..
so..3volts at 3 leds..= 9 volts..i keep it like that..because using 4 leds on 12 volts wasn't quite bright like they should have been...but it might work for you using red leds..you might get away with 4 leds in series..but still use a dropping resistor..so that the brightness will match the other rows of leds.. perhaps a 100 ohm resistor is sufficient.. try it,,if the resistor gets hot, then increase the value.. i know i am babbling..its almost 3am here..anyway..if i can help more , let me know..g'night ZZZZZzzzzzzzzzzz

[Mike6158]

Somehow I double posted. Sorry about that...<p>[ January 08, 2005: Message edited by: NE5U ]</p>

[Mike6158]

I just built a 12 LED (10,000 MCD each) light that comes on at dusk and turns off at dawn. I used a 1W 30Ω resistor. 3 LED's in series, 4 parallel groups. LED If is 30mA. Vf is 3.4V. We would need to know both of those values and what your configuration is going to be to be able to give you a value for your resistor. Configuration is the most important. Series? Parallel? Series Parallel?<p>There is a great spreadsheet on the N&V FTP site for calculating the resistor required for an LED circuit as well as a great article in the Jan issue of N&V. The author of both are the same.<p>Make sure that you check the wattage requirements of the resistor. Mine was .4W so I bumped the resistor to 1W just to CYA.<p><EDIT> Forget series. You'll never get 18 to light. Well, never is a bit strong. You can do some gee whiz stuff and power them but to keep it simple series won't work. 3 in series then parallel 6 of those groups of three. 20Ω dropping resistor. Used V=13.8V, If=30ma, and Vf=3.4 to calc the result. .48W I would use a 1W resistor but that's me...<p>[ January 08, 2005: Message edited by: NE5U ]</p>

[philba]

I agree with almost every thing there. Though given the LEDs are red, I'd expect 3 parallel of 6 LEDs in series (Vf is about 2 for reds). One difference - I would use 3 dropping resistors of 1/4 W or smaller. One in each serial circuit. This is because individual LEDs of the same type can have a wide range of Vf. This will cause different If and thus different brightness. Maybe 6 (or even 3) in series is enough to cancel out this statistial variation (and cause my concern to be incorrect).<p>If you don't have excel, here's a web page that does LED calcs. http://ourworld.compuserve.com/homepage ... en/led.htm<p>Now, the next thing to consider is can you get higher bightness by running more current through the LEDs in a PWM-like fashion (i.e. pulsed). You could use a 555 with a, say, 75% duty cycle to pulse the voltage to the serial chains of LEDs with a lower dropping resistor (or shorten the chain). You'd have to work out the details but I think it could be made to work pretty well. You could get the same amount of perceptual brightness from fewer LEDs at the cost of <$1 worth of parts.<p>[ January 08, 2005: Message edited by: philba ]<p>[ January 08, 2005: Message edited by: philba ]</p>

[myp71]

Hey Thanks ! <p>The led's I have are way too small for me to work with and the leads on the led's are way too small also. So I was thinking I would get six of these at radioshack.<p>http://www.radioshack.com/product.asp?catalog%5Fn ame=CTLG&category%5Fname=CTLG%5F011%5F006%5F002%5F000&product%5Fid=276%2D086<p>So what resistior value would I need to power these led's off of 12vdc? I would like if I could just use one resistior becuase the brake light housing is kinda small and not enough room for six seperate resistors.<p>Thanks again for your help! <p>Ryan<p>[ January 08, 2005: Message edited by: My-P71 ]</p>

[Mike6158]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by My-P71:
Hey Thanks ! <p>The led's I have are way too small for me to work with and the leads on the led's are way too small also. So I was thinking I would get six of these at radioshack.<p>http://www.radioshack.com/product.asp?catalog%5Fn ame=CTLG&category%5Fname=CTLG%5F011%5F006%5F002%5F000&product%5Fid=276%2D086<p>So what resistior value would I need to power these led's off of 12vdc? I would like if I could just use one resistior becuase the brake light housing is kinda small and not enough room for six seperate resistors.<p>Thanks again for your help! <p>Ryan<p>[ January 08, 2005: Message edited by: My-P71 ]<hr></blockquote><p>OK... First thing... If you are trying to build a brake light bulb replacement make sure you know what your state law requirements for luminosity are. Standard LED's like the 5,000 MCD LED you listed are not all that bright. At least not as it relates to safety equipment like a stop lamp.<p>Also- Aside from being plain ugly and expensive, the 10mm LED's take up a lot of real estate. You can buy 5mm LED's with over twice the MCD rating (12,000) for around 1/2 the price. Your board would be significantly smaller and with the same number of LED's you would emit a lot more light. <p>I'll go through the resistor calcs for your 10mm LED selection so you can see how to do it. Sticking with Philba's recomendation of 6 series / 3 parallel.<p>Calc the series resistance:<p>Vsource = 13.8 VDC
Vf = 2.0V
If = 50mA ( The spec sheet was kind of vague. They only listed the max VF@ 20mA. I'm making the assumption that Vf=2.0V is for If = 50mA based on the power dissipation of 100mW.)<p>R = (Vsource - (n)Vf) / If where n is the number of LED's in series.
R = (13.8 - (6)2) / 0.050
R = 13.8 - 12 / 0.050
R = 1.8 / 0.050
R = 36Ω<p>As it works out, 36Ω is a standard resistor value. You might think that you could get by with an 1/8W resistor based on .1W of power dissipation but since this is a tail light application and heat would be a factor I would jump it up to 1/4W.<p>That was 36Ω for each string of 6 in series by the way.<p>Resistor color code for 36Ω- Orange - Blue - Black - Gold. Gold is the tolerance band. I see no point in using a lesser tolerance but that's just me.<p>There are aftermarket suppliers that have DOT approval for this application. For instance:<p>Superbright LED's

[josmith]

The theory of using led's in series is sound but in practice it is less dependable. One problem is that the low value resistor is more susceptable to voltage fluctuatons,a big problem in automotive applications. Beyond that you still have the problem that each component affects the other. From what i've seen it's better to use one resistor per led(they're cheap enough).<p>I have 1157 replacements from Superbright Leds.com they don't show up as well as regular bulbs and they came with a notice that hinted that they might not be legal.

[Mike6158]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>I have 1157 replacements from Superbright Leds.com they don't show up as well as regular bulbs and they came with a notice that hinted that they might not be legal.<hr></blockquote><p>I wondered about that. If they are hinting about it then I would back slowly away from them and move on to something like a Luxeon or Lamina ceramic, both of which bring entirely new design elements to the equation.<p>I like Superbright's LED's for the simple applications that I use them for. MCD/$ is pretty good in quantities. <p>I have to admit that I cringed when I saw that the application was for a tail light. Maybe it's the time that I've spent on my motorcycle? Motorcycles are apparently invisible.

[peter-f]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by josmith:
The theory of using led's in series is sound but in practice it is less dependable. One problem is that the low value resistor is more susceptable to voltage fluctuatons,a big problem in automotive applications. Beyond that you still have the problem that each component affects the other. From what i've seen it's better to use one resistor per led(they're cheap enough).
<hr></blockquote><p>
I'm not coming from the design end of this- but my experience with Automotive power is that the car will function from 4V to 18V...
(I must say, just Barely at 4... that's a long story in itself!)<p>The design parameters given for your taillight are for optimum voltage at 12V -- but nominal may in fact be higher! <p>I'd consult a shop manual for late-model Cadillacs, as a start- they have LED taillights, and probably reverse-engineering their setup would give a better idea of potential pitfalls... <p>(then again, my experience with GM is another long story, which has its own caveats!)

[philba]

lots to say on this topic but I'll try to be mercifully brief.<p>Dont waste your money at radioshack. Check out mouser or digikey for LEDs. There are speciality site as mentioned above. Mouser usually has very good prices for small quantities and you can get complete datasheets. Buying LEDs at places that have minimal to no info is a bad idea unless you really know what you are doing.<p>Here's an LED brake assembly ready to go. I have no idea if its any good but its not too pricey. Notice that it runs at 12.5V, 100 mA. http://www.allelectronics.com/cgi-bin/c ... type=store<p>A question - if the auto voltage can wander all over the place, how do manufacturers deal with that? My guess is they optimize for the most likely condition (when the alternator is running). I can't imagine them doing anything else - who needs brake lights when stopped with the engine off?

[peter-f]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
A question - if the auto voltage can wander all over the place, how do manufacturers deal with that? My guess is they optimize for the most likely condition (when the alternator is running). I can't imagine them doing anything else - who needs brake lights when stopped with the engine off?<hr></blockquote><p>
(agreed) which is why I offered the advice of reverse-engineering from Cadillac... or Volvo, too.
Automotive electrical systems must be designed for optimal condition... but if the charging system is in failure, the alternator may be still be perfect, and the brake lights still need to work... on alarmingly low voltage (my experience was with a loosened wiring harness, and low voltage due to resistance caused by subsequent corrosion).
The dealer explained to me that the airbag warning light came on because the system voltage was about 4V... the instrument cluster was intermittently useless!

[jimandy]

Wouldn't it be fun to have a small programmable message board mounted as a rear center brake light. When you tap your brakes you would flash a message to the driver behind you. Imagine the possibilities...
and the trouble you might get in.

[myp71]

<p>The led's are for a simple 3rd brake light Not for the whole brake light system.I'm making my own because I can't find any to replace aftermarket.<p>NE5U- I think you came on a little strong! Its not your car so what do you care what it looks like?<p>The led's Cannot be placed in series.<p>I have a six light 3rd brake light I'm replacing one led per bulb and one wire feeds power to all 6 lights so if I could, I would like to use one resistior <p>Thanks <p>Ryan<p>[ January 09, 2005: Message edited by: My-P71 ]</p>

[Mike6158]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>NE5U- I think you came on a little strong! Its not your car so what do you care what it looks like?<hr></blockquote>
First- I don't... Second- Or you misunderstood. Did I say anything about your car? Nooo. You asked for an opinion. You got one. Maybe more than you wanted but nonetheless the numbers and formula are there for you to be able to calculate whatever you need to for whatever you choose to use. Again, 10mm LED's take up a lot of space and offer less light than their 5mm counterparts. However if that's what blows your skirt up them by all means go for it.<p>I can't tell if you followed the Superbright link or not. They make bulbs for various automotive services. 1142, 1156, 1157, BA9's. It's possible that this has been done for you. If you just want to do it yourself that's fine too.<p>I don't get excited about web forum posters. It's not worth it.<p>Using one resistor is easy enough to do. Calc the load as 3 series (per lamp) in 6 parallel groups (per each lamp you are replacing). It comes out to 27Ω Red-Purple-Black-Gold. The resistor is going to dissipate around 2.5W so make sure that you use the appropriate size for the resistor.<p>Looking back... you did in fact say that this was for your "high mount stop light". I missed that part...<p><Edit> If you could see your way clear to use a resistor per LED then you would need (18) 240Ω 1W resistors. <p>Personally I think the voltage regulator on most modern automobiles does a pretty fair job of regulating the voltage so going with the first iteration should work.<p>[ January 09, 2005: Message edited by: NE5U ]</p>