LED's in parallel?

[GaryRC]

I'd like to wire up to 20 LED'S for illumination on a larger plane. I don't have any need for 'extras' ie: no blinking, different colors, trx switching, etc. I visited RC-CAM's website, which was very helpful for my smaller planes. However, using the LED calculator, I quickly realized that 20 will require a very high voltage. What I'd like to do ideally is use a 9v battery as opposed to running off the bec. Can these be run in parallel? Or a combination of a number of paralleled in series?

- Gary

[Miami Mike]

Yes, you can do that. First come up with a working circuit using five LEDs in series, then wire four of those circuits in parallel.

[GaryRC]

Thanks Mike,

This should get me going. I'll let you know how it works out.

- Gary

[Mr.RC-CAM]

I suggest you use a 4-cell NiCD or NIMH pack instead. The AAA size is small and you can recharge them on your R/C charger.

If you are using a traditional 9V battery, then it will probably only support one Hi-Lumen LED per LED string. That is because a 9V Battery is considered "dead" at about 5.5V and the Vf's of most Hi-lumen LED's is at least 3.7V. So, along with your 20 LED's, you will need 20 resistors. Just plug the numbers into the web page and wire each LED/Resistor pair to the battery (all in parallel).

RC-CAM

[GaryRC]

AH HA! And guess what I just found? A 4-cell AAA NiCD pack that was shipped to me by mistake and never returned! When I read your post it jogged my memory and I scrambled through my 'shop' and found it! (still relatively new)
On to the LED calculator portion of RC-CAM's website! >>>>

Thanks again - Gary

[vintage1]

Each series string will need a resistor to set the current.

Haveing done that, you can wire the strings in parallel.

[GaryRC]

vintage1,

I'm still a bit confused about the wiring. I took RC-CAM's post to mean all in parallel, with a resistor for each LED. Do you mean I can also wire let's say: four strings, each consisting of 5 LED's in series using one resistor for each string, then parallel the four strings together?
In any case I really appreciate the help you guy's are giving me. I know barely anything about electronics but I'm learning with your help.

- Gary

[Mr.RC-CAM]

Look at the rc-cam web page again. There is an example of two LED strings in parallel. You will do EXACTLY the same thing. But in your case, there will be twenty of these strings.

BTW, what is the Vf of your LEDs? You can install two or more in series as long as the sum of the series connected LED's is lower than the lowest battery voltage that will be used. I assumed that your Vf's are the typical 3.7V or higher spec, but if they are under 2V then you can get more creative.

RC-CAM

[Miami Mike]

Do you mean I can also wire let's say: four strings, each consisting of 5 LED's in series using one resistor for each string, then parallel the four strings together?No, I guess that won't work after all. Even with 9 volts and LEDs that have less than 2 volts drop each, that's probably cutting it too close. I meant to say wire 4 LEDs in series with a resistor and then wire 5 of those circuits in parallel, but forget about that, just use Mr. RC-CAM's scheme.

If you change your mind about blinking LEDs, there's a Radio Shack red LED, part #276-036, that can run directly off of a 4 cell pack (5 volts) without any extra circuitry.

[GaryRC]

RC-CAM,

Got it. And yes, the Vf's are the typical 3.7V.

Miami Mike - Thanks for the heads up on the RS part. May need one for another project. Also picking up an electronics book for a good background on this stuff.

Thanks again guys.

- Gary

[mhmitchell]

Just one dropping resister would be required to accomplish your project see drawing

[Mr.RC-CAM]

One resistor is not recommended. The LED's have to be closely matched, throughout their temperature range. Basically, it may work, or it may not.

RC-CAM

[Andy W]

What mhmitchell posted will not work reliably, as RC-CAM stated..
..a

[mhmitchell]

You may be right but these devices a pretty forgiving.

Putting a reistor for every LED certainly would be better. Just calculate the value using the current for one LED then 1/4 Watt resisters would be fine.

A really better way would be to use a 3.6 voltage regulator (less than 5 dollars) would be best. Then until battery fell below 3.6 volts the LEDs would remain at the same brightness. You could use a zener diode but that involves more calculations.

[Miami Mike]

A really better way would be to use a 3.6 voltage regulator...I don't think so!

[Warske]

Seems like your best bet would be to go with RC-CAM & others suggestion, but let me toss in a couple more ideas for the fun of it: :)

1) Wire all 20 LEDs in series and use a boost switching inverter current source to supply the required current at 74 volts.

This would be the most efficient solution because you can eliminate all the power-wasting series resistors.

Also, if this is an electric plane, it could be run off the main battery, eliminating the need for an extra battery.

Radio interference would be a concern, but probably not a problem, depending on the switching frequency and the design.

I'm not prepared to design it for you, but someone might and it could be interesting. There are probably switching inverter IC's that could handle that small current. You might even be able to buy such a thing complete, depending on your battery voltage. A google search on "switching inverter" might be a start.

Edit: For example
Efficiently Powering Nine White LEDs with the MCP1650
http://ww1.microchip.com/downloads/en/AppNotes/00948a.pdf
http://www.analogzone.com/pwrp0405.htm

2) Instead of LEDs you could try the cold cathode fluorescent technology. Somewhat less rugged. The efficiency might be comparable, but I haven't looked into it (does anyone else know?). I have seen stuff like that available in the automobile section of department stores. Also you can often get the parts at All Electronics http://www.allelectronics.com/ search for CCFT.

Edit: This looks interesting, but I haven't viewed it:
Efficiency Analysis of Various Approaches
http://www.national.com/onlineseminar/2002/wled/wled.html

Good luck. :)
Warske

[mhmitchell]

Not wanting to anger anyone but the company I work for used the voltage regulator method to drive 5 LEDs. Would you like to explain it to me so I can tell our EE's they need to go back to school?

[Mr.RC-CAM]

LED's can be driven in parallel when they are closely matched. Otherwise, the intensity will vary widely (some might not even illuminate at all). Cell Phones and PDA's usually use this method using a charge-pump voltage regulator AND independant current regulation for EACH LED. Some of this is discussed in this app note: http://www.maxim-ic.com/appnotes.cfm/appnote_number/1037/ln/en

Most hobbyist will use a random collection of LED's. If they all came from the same mfg, are the same part number, and have similar lot-varied characteristics, then the simple parallel wiring can work. Otherwise, the intensities will not be even.


...but the company I work for used the voltage regulator method to drive 5 LEDs.
Sounds like a portable instrument app. But, are you absolutely sure there wasn't some sort of current limiting for each LED? Besides brainless resistors, there are IC's that do this very efficiently. Otherwise, the LED's have to be matched fairly well (but that can be handled with careful purchasing and/or device selection via testing).

RC-CAM

[mhmitchell]

Well we probably had pretty closely matched LEDs I am not arguing just trying to give a simple answer to what has turned out to be a more complex issue than I realized.

By the way I like your web page

By the way I called the R&D dept head Phd EE he just pooh poohed around and basically said the same thing I did voltage reg or dropping resistors he sees no difference between individual resisters for each LED and one resister for all. If they are in parrallel they both will or may have the problems you described. Hey what does he know he thinks current flows from positive to negative.!