18 Led's powered by 12vdc

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peter-f
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Re: 18 Led's powered by 12vdc

Post by peter-f » Sun Jan 09, 2005 2:33 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by My-P71:
... NE5U- I think you came on a little strong! Its not your car so what do you care what it looks like?<p>[ January 09, 2005: Message edited by: My-P71 ]<hr></blockquote><p>
Actually, I think it's appropriate to be cautioned... you can have your car impounded for illegal equipment in this state... altho I only heard of that once, myself.
What state... what's it matter?

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Re: 18 Led's powered by 12vdc

Post by dacflyer » Mon Jan 10, 2005 8:23 am

JimDandy >> (wish granted) look at..
www.streettags.com<p>http://www.streettags.com/catalog/index ... a0e1d7bfe1<p>http://www.streettags.com/catalog/index.php?cPath=22<p>they got a whole lot of cool stuff.. i actually have the name tag.. great for getting dates..lol
" Hi my name is Ray, I'm Available 555-1212"...lol
you know all holiwood numbers beguin with 555 right ;)

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Re: 18 Led's powered by 12vdc

Post by Tparker » Mon Jan 10, 2005 8:35 am

Here is a technical paper from Lumileds on designing automotive lighting using LED's. Has some very interesting info. and DOT requirements Using Superflux LED's in Automotive lighting

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Re: 18 Led's powered by 12vdc

Post by Tparker » Mon Jan 10, 2005 8:45 am

Here is a $20.00 device that will drive 12-70 5mm leds at constant current. Power Puck

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Re: 18 Led's powered by 12vdc

Post by Bernius1 » Tue Jan 18, 2005 3:25 am

I saw this while perusing some older posts. It seems to me that beyond calculating and wiring for proper current draw, space is the issue. Why not mount everything on one board , with SMT resistors ? This SMT kit has (25) SMT 390-ohm resistors, which give 30mA @ about 3V. It also has 3V zeners and LM317T's , all for $9.99. NO, I DON'T work for them. So, buyer beware, of course. But what a project; a PCB cut in the shape of the lens, with SMT control circuitry, super bright LED's, and a round center hole to allow the original bulb to shine through. Unless you guys have a method of puttng traces of conductive epoxy on plexiglas, in which case the PCB is clear.
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Re: 18 Led's powered by 12vdc

Post by jollyrgr » Thu Jan 20, 2005 9:59 am

As far as adding LED lights to your car.<p>Do so ONLY if the light is DOT (Department Of Transportation) approved. While I could care less what someone elses car looks like I don't want to come up behind someone with malfunctioning or overly bright/dim turn signals and/or brake lights. While it may look perfectly fine on a clear night it won't in blowing rain, fog, and snow.<p>I know I have seen DOT approved after market tail lights for trucks, trailers, or "universal" usage. If your car is 1985 or older you can use anything you want (within reason) for a third tail light as they are not required equipment.<p>Normally you would not want to put the LEDs in series. You would use one LED and one resistor in series. Calculations are shown in other posts and I won't duplicate them.
(Read clarification below.)<p>There are also after market LED third tail lights that are DOT approved.<p>Another option. What is the original bulb type number? If you are going to throw caution to the wind (and screw DOT regulations) you can buy LED bulbs for most automotive lighting (except headlights) in the popular 194, 168, 1157, 1156, and so on type bulbs. So far I have only seen these being sold as "Pacific Rim" items and have not seen any that are DOT approved. If someone knows of a source, please post it as I would like to know about it.<p>[ January 21, 2005: Message edited by: Jolly Roger ]</p>
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philba
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Re: 18 Led's powered by 12vdc

Post by philba » Thu Jan 20, 2005 6:36 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Jolly Roger:

...
Normally you would not want to put the LEDs in series. You would use one LED and one resistor in series. Calculations are shown in other posts and I won't duplicate them.
...
<hr></blockquote><p>I'm curious as to your reasons for for not putting LEDs in series. One sees this in an awful lot of designs. I agree that each parallel leg should have its own dropping resistor.

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CeaSaR
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Re: 18 Led's powered by 12vdc

Post by CeaSaR » Thu Jan 20, 2005 7:53 pm

I'm not sure about Jolly Roger's reason, but I would rather have them in parallel simply for reliability. If one component on the series string fails, then the whole string fails. If one component fails in the parallel configuration, only that particular combination fails leaving all others working. This is paramount when something is being used in a critical situation such as a third brake light. Of course, the decision between cost and safety is one that all designers must consider. In this case, I'd side with safety - the cost really isn't that much more.<p>CeaSaR
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Dave Dixon
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Re: 18 Led's powered by 12vdc

Post by Dave Dixon » Fri Jan 21, 2005 4:45 am

CeaSaR, When using LED's in series strings, I always put a diode (parallel-back to back) with each LED. If a LED opens up, the current flows through the diode, allowing the others in the series string to operate. The other advantage is it seems to provide some ESD protection for the static sensitive white LEDs we use. I made some proto boards without the diodes, walked across the carpet, and blew 'em up real good. It hasn't ever happened with the diodes installed! Regards,
Dave

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Re: 18 Led's powered by 12vdc

Post by Engineer1138 » Fri Jan 21, 2005 7:16 am

Dave, I'm having trouble visualizing this. If the diodes are paralleled anode to anode, then the second diode would need to have a higher voltage drop than the LED or most of the current intended for the LED would flow through it instead. If you put them anode-cathode, then the diode is reverse biased, and nothing flows.<p>What am I missing here? :confused:

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CeaSaR
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Re: 18 Led's powered by 12vdc

Post by CeaSaR » Fri Jan 21, 2005 7:18 am

Another excellent idea, all in the name of safety/reliability.<p>CeaSaR
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Dave Dixon
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Re: 18 Led's powered by 12vdc

Post by Dave Dixon » Fri Jan 21, 2005 9:32 am

Whoops, I should have been more specific. I'm not a morning person!! I meant to say a ZENER diode (5.6V) Parallel anode to cathode/cathode to anode. In normal operation, the diode doesn't conduct, just the LED does. If the LED becomes an "open" then the diode (less the diode drop) lets current flow keeping the others in the series working. Sorry for the confusion. Dave

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philba
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Re: 18 Led's powered by 12vdc

Post by philba » Fri Jan 21, 2005 3:16 pm

anyone know the failure rate of LEDs?

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Re: 18 Led's powered by 12vdc

Post by Mike6158 » Fri Jan 21, 2005 6:08 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
anyone know the failure rate of LEDs?<hr></blockquote><p>I've done some pretty dumb stuff with LED's and my failure rate is 0% :D Today's LED's, purchased from a reputable company aren't your grandmother's LED's... <p>I'm not sure that there is a "best" way to "fire" LED's. I know for a fact that series, parallel, or series parallel all have applications that they work well in. Most of my circuits consists of 3 series LED's in parallel strings of up to 6 groups. One is currently being tested outdoors. It runs all night long. A photocell trips it off and on. So far it's working flawlessly... Once I get a couple of hundred hours on it in varying temperatures I'll stick a fork in that one.<p>In the future, thanks to prior discussions and a lot of web research I am going to design for max efficiency and try to do away with the dropping resistor(s).<p>Link to a PDF that will answer your question- Failure Rate of LED's The short version is 100,000 hours is typical. Over 11 years. The MTBF (mean time before failure) can be as high as 10,000,000 - 31,000,000 hours. You need to understand the definition of MTBF though...<p>[ January 21, 2005: Message edited by: NE5U ]</p>
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jollyrgr
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Re: 18 Led's powered by 12vdc

Post by jollyrgr » Fri Jan 21, 2005 9:35 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
<p>I'm curious as to your reasons for for not putting LEDs in series. One sees this in an awful lot of designs. I agree that each parallel leg should have its own dropping resistor.<hr></blockquote><p>OOOPS!!!! That is NOT how I meant to say it. My thought was there when I was posting but it didn't make it to the keyboard correctly. Thanks for pointing it out. What I was trying to say is what you stated (and combine it with another thought). I was trying to say don't parallel all the LEDs and then put them in series with one resistor. I HOPE I'm saying it right this time...... <p>Normally you do not want to put all of the LEDs in parallel then put the paralleled LEDs in series with one resistor. You want each LED in series with its own resistor. Then you can parallel each of these series circuits.<p>Now my reasoning for TRYING to say put each LED in series with its own resistor then parallel each of these setups instead of putting all the LEDs in series. Yes, you can put LEDs in series and drop the voltage across each one. But like a string of Christmas lights if one LED fails in an OPEN failure mode, all LEDs go out. You can get resistors cheap (about a nickel each for 1/2 watt units in quantities of 10) so cost should not be a factor. For a toy, flashlight, nightlight, Holiday lights, or non critical device a failure is a non-event. Having inoperable automotive lighting could get you a ticket or at least pulled over.<p>Another point is the number of LEDs being used. A red LED typically drops 1.5 volts. Thus putting 18 in series would drop 27 volts; this won't work correctly in an automotive 12 volt system. Having two strings of 9 LEDs would give a 13.5 volt drop; somewhat better. In fact you could consider the worst case of the alternator at high output of 14.5 volts. Then take the one volt "overhead" and the 20mA typical current flow and use a 50 Ohm resistor. But this is just a bunch of "assumptions".<p>[ January 21, 2005: Message edited by: Jolly Roger ]</p>
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