Any electronics geeks?

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MikeSantor
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Any electronics geeks?

Post by MikeSantor » 02 Jul 2011, 02:01

Im looking to make an LED bar for my gun case. I have never done anything with LEDs that have been plugged into an outlet. I read that its a good idea to use a power plug from electronics that are 9v like a cellphone charger. I just want to double check my work since the last time I took an electronics class was like 9 years ago...

using this calculator:

http://www3.telus.net/chemelec/Calculators/LED.htm" onclick="window.open(this.href);return false;

LED data:
DC Forward Voltage: 3.4v
DC Forward Current:300mA
9volt power supply.

I guess my big question is in the calculator where I have to plug in Diode forward volts, Do I enter just 3.4 no matter how many diodes I have or do I need to multiply 3.4 by how many LEDs im using. Im planning on using 7 LEDs in series.

so if my bar is in series, and there is 7 LEDs, would it be:

(1) 22ohm resistor with 7 LEDS (calculated using 3.4v)


Because if I have to multiply the 3.4 x the amount of LEDs im using (7), then I cant use the 9v power supply, i would just plug it right in to the 110v wall and use a 330ohm resistor.

come to think of it, is there any reason to use a 9v power supply in the first place or should I just plug it right into the wall?

should have payed more attention in HS...


BTW, these are the exact LEDs I purchased
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 2828wt_905" onclick="window.open(this.href);return false;

They have a pretty big reputation on the net for having good LEDs at great prices.

N4TAB-Tom
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Re: Any electronics geeks?

Post by N4TAB-Tom » 02 Jul 2011, 11:32

Mike,

For a series configuration, the forward voltage Vf for each LED has to be added. That's 23.8 VDC total. Don't put those across an AC line source. The maximum reverse voltage for each is around 5 VDC and on each half-cycle, the array will see the instantaneous line voltage (nominal 117 VAC). One LED will fail, increasing the current for the others and then all will fail, possibly in a catastrophic fashion. Not to mention the safety issues of having lethal line voltage in the circuit. You need a regulated DC supply of at least 28 VDC. Using the calculator that you mentioned, you'll see that it will require a 15 Ohm 2W resistor for a 7 LED series array of the specs given for your particular LED type. Use a larger power type resistor (metal case) and mount it on a larger metal plate to dissipate the heat. I'd also think about mounting the LEDs in a configuration that will conduct heat away from the LED body.

You should be able to find a surplus power supply 28V, 2-3A at allectronics.com or mpja.com.

HTH,

Tom

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 02 Jul 2011, 11:39

Tom, you had me at "for a series configuration"...



So my curcuit will be the 7 LEDs, regulated DC supply of at least 28 VDC and a larger metal cased resistor of 15ohm. Just out of curiousity, there is no way to make this array so that the LEDs wont get hot? I have see tons of videos of home made light bars on line and no one ever mentioned over heating issues. I guess this project is a little more entailed then I thought!!! thanks Tom!

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Re: Any electronics geeks?

Post by N4TAB-Tom » 02 Jul 2011, 19:28

Hi, Mike.

**Mods if this should go PM or private, pls advise.**

The information given at the eBay site is very sparse. Typically, I would expect to see a manufacturer's datasheet of many detailed pages with all sorts of luminosity charts and, certainly, an applications section dealing with heat transfer. Like all solid state light emitters, whether visible or otherwise, none are 100% efficient. Most of these high-output visible LED products are not particularly efficient . That means, for every luminous milli-Watt radiated, another non-luminous milli-Watt or more is generated as heat. When we're dealing with Watts, generated in tiny dies and connected with a group of tiny wires, that leaves a lot of heat to he dealt-with. There are only three mechanisms of heat transport: conduction, convection and radiation. This isn't the place for the physics (unless approved), but a small body, generating lots of heat, operated without a specific mechanism to remove that energy from the die, will fail. The only practical mechanisms available are die-bonding to a substrate that can remove the heat by conduction or a big source of airflow to remove the heat by convection. Sometimes the die is bonded to one of the device "power connections", but that's usually not the case with through-hole parts like this, as opposed to surface mount components.

Ask for a fully detailed datasheet from the supplier and all should be revealed.

Happy to help as desired and permitted. Also hpleased to respond to a PM.

Tom

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f3rr37
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Re: Any electronics geeks?

Post by f3rr37 » 02 Jul 2011, 22:44

You're fine Tom. :)

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 03 Jul 2011, 07:30

Tom, in this case, where is the excessive heat coming from? I have put many LEDs in friends cars and PCs and never had them even get warm. Is this due to the fact we are working with 110vac compared to 12v?

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Re: Any electronics geeks?

Post by Visceral_Malice » 03 Jul 2011, 10:41

Mike, I have personally rigged up a lot of small led setups like what you are talking about. What kind of leds are these? Most typical leds are between 20 and 50 ma. That power supply would likely die very quickly if they are drawing 300 ma each. As for configuration definitely can't do a single series. There's not enough voltage. You'd need 3 parallel series of 2 and one in parallel alone. You can just use one resistor but I would probably use a smaller o e on each series. Its more of a failsafe.

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Re: Any electronics geeks?

Post by fooschnickens » 03 Jul 2011, 12:22

Wouldn't it be a lot easier to buy an LED bar? They're pretty darn cheap nowadays.
Image
http://www.f00tography.com" onclick="window.open(this.href);return false;
Corporation: n. An ingenious device for obtaining individual profit without individual responsibility.
-Ambrose Bierce

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 03 Jul 2011, 14:19

chicken, can you find an LED bar for 110vac? I couldn't... if I can find one im all about it. Im just very perticular on what im looking for size wise.

Malice, So are these LEDs just strange for drawing 300ma? This is the first time I purchased from these guys. The LEDs seemed fairly bright so I went with them. I could always go for a lower drawing LED if it will make things easier. My gun case is a small POS anyways, its just REALLY dark...

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Re: Any electronics geeks?

Post by Visceral_Malice » 03 Jul 2011, 15:26

Thats a pretty hefty draw for an led. Are the some kind of ultra high brightness? I usually go with 5mm leds and they are between 2-4V and draw 20ma usually. I need some more info on your project. Are these leds you plan to solder in yourself or do they just plug in to a socket or some other kind of mechanism? Do you have a link where I could check them out?

I have made several lighting setups and what I found to be easiest is getting a univeral AC adapter from somewhere like radio shack. I get the 1000ma adjustable voltage adapters and I cut off the end and splice on my setup. These adapters serve to regulate the voltage to your needs and work as a rectifier to change the AC to DC. I made a 3 tier bar with 30 leds of various colors using a setup like that and it works pretty nicely.

If you plan on soldering all this together yourself I can help you with a diagram.

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 03 Jul 2011, 15:33

Malice, Did you catch the link to the LEDs in the first post? its towards the bottom. As I mentioned, that seller is pretty popular with LED modders.

Im planning on soldering them myself. What I have seen done before is people take those wire hiders you can get from home improvement stores. Its 2 pieces. One side is mounted where ever you want. the other side has the holes drilled in it for the LEDs to sit in. all the wireing is done, etc. then that piece is slid in to the track on the other piece that is mounted.


This: just not as wide.
http://www.amazon.com/Wiremold-CMK30-Sc ... _e_title_1" onclick="window.open(this.href);return false;

Thanks for the help guys!

N4TAB-Tom
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Re: Any electronics geeks?

Post by N4TAB-Tom » 03 Jul 2011, 19:25

SRI, been away from the keyboard.

For a typical LED lamp of the ilk that which you indicated, at least 80% of the energy consumed by the device has to be dissipated as heat. In a tiny die, 800 mW is a LOT of heat. Nothing to do with the power source, but how does that tiny die/pkg dissipate that heat? If you look at typical LED specs, you will see that the If (forward current) is dependent upon the junction temperature. Here entrains that which is often missed. It's a race. More temp=more current=more temp=more current until something fails.

This isn't intended to be any form of an insult or an EE 101 or physics 101 course, but there are some seemingly subtle issues that aren't subtle, as it turns out.

If you attend to some basic premises -get the heat off the device[s]), regulate the DC supply and protect against transients that might violate safe-area conditions - you can enjoy this arena with impunity. Don't be discouraged. but don't put $ in the trash can.

This is an emerging technology (emitters and device drivers) but not outside the domain of experimenters.

Happy to help where I can do so.

Best wishes for the 4th to all,

Tom

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Re: Any electronics geeks?

Post by Visceral_Malice » 05 Jul 2011, 13:59

Yeah Mike, I've seen those setups and they are nice. If that is the correct information for the leds then I'm going to echo Tom on this. Normal leds put off negligible heat, but something like that is going to require a heat sink or other means of dissipation. That goes a bit beyond my field of experience with leds. I've only used the lower power leds, so I wouldn't know where to start as for the size and makeup of a heat sink for those.

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 05 Jul 2011, 14:24

I have been looking into this and im just going to buy some normal LEDs. not sure what I was thinking what I bought these. didnt pay close enough attention to the specs.

So you guys still consider a parallel circuit with "normal" LEDs?

romer522
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Re: Any electronics geeks?

Post by romer522 » 05 Jul 2011, 14:31

I'd just go with 12v automotive LEDs and a 12v power supply with adequate amps.

MikeSantor
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Re: Any electronics geeks?

Post by MikeSantor » 05 Jul 2011, 15:07

This is also an option i will look at.

Visceral_Malice
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Re: Any electronics geeks?

Post by Visceral_Malice » 05 Jul 2011, 18:14

It all depends on your power supply. Running is series is best if you can, but usually if you want more than 4 then you need to get some parallel circuits going. If you still want 7 or even more I would set up parallel circuits of 3 in series with a 12v supply provided they aren't over 4v leds. Let me know your specs and I can give you more of a hand.

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