L.E.D. in parallel question

I have 9 white led's hooked up in parallel using 6 volts power (4 AAA). Looking at the ol' resistance calculator, I came up with a 13.3 ohm resistor needed. I hooked it up with a single 10 ohm resistor, the light is nice and bright. Am I supposed to use a resistor for each led or is 1 for the whole lot suffice? Bear with me here guys, I know I'm a newbie. /ubbthreads/images/graemlins/blush.gif

Vasdef,

You will get better results if you use individual resistors. That is, each LED has its own resistor and it is the LED/resistor pairs that are connected in parallel. Remember to calculate the individual resistor values based on the current of a single LED - 13.3 ohms times 9 equals 119.1 so use a standard value of 118 or 121 ohms. I am assuming you chose a current around 20mA and a nominal LED Vf of 3.5V so you are dropping 2.5V (at 6.0V) across each resister. 9 1/8W resistors will do the trick. The current will be around 25mA with fresh batteries (6.6V).

The reason for using separate resistor/LED pairs is that the Vf of your LEDs cannot be depended on to be real close. Therefore, connecting all the LEDs in parallel and using a single resistor will not result in the good current sharing - some LEDs will be much brighter - and pulling a lot more current - than other LEDs. A parallel array and single resistor only works well if you match the Vf in your LEDs.

Of course, there are lots of assumptions and limitations not mentioned. But this should get you up and running.

Henry.

LED resistance calculator
I looks like you are overdiving them a little but not bad considering when the battery voltage drops you will have longer run time starting out with a slight overdrive.
many flashlights start out with much higher overdrive than what you are using.

Also my calculations find you using a 270ma of current with the resistor trying to drop about 2.5 volts which adds up to over 1/2 watt of power dissipation it would probably be cheaper to use individual resistors because otherwise you will have to go with a 1 watt resistor which probably costs near as much I think.

[ QUOTE ]
Lynx_Arc said:
LED resistance calculator
I looks like you are overdiving them a little but not bad considering when the battery voltage drops you will have longer run time starting out with a slight overdrive.
many flashlights start out with much higher overdrive than what you are using.

Also my calculations find you using a 270ma of current with the resistor trying to drop about 2.5 volts which adds up to over 1/2 watt of power dissipation it would probably be cheaper to use individual resistors because otherwise you will have to go with a 1 watt resistor which probably costs near as much I think.

[/ QUOTE ]

Thank you /ubbthreads/images/graemlins/tongue.gif very much.

If you have enough space, consider using an incandescent bulb as a series resistor. They act a little bit as a constant current source.
You may try a #222 bulb or a standard MiniMag bulb for that reason.

Thanks for the info guys, one more thing. Is there any recommended reading to give me a jump start on led's?


Vas

I was looking at an led light that's 3.6 v, 80 mA. If I hook up 10 of them with 6 volts, the resistance calc. suggested 3.3 ohm resistors, 3.2 watts. Do they make them that big? is it worth it, or will it suck my batteries up too fast?

Thanks

Vas

I have seen 5 and 10 watt power resistors I am sure they make them a lot BIGGER than that. is that single led's running at that current or several? When it comes to resistors you can always use the parallel resistance formula and wire 4, 1 watt resistors in parallel. If you want 3.3ohms that would work out to about 4x13 ohm (1watt) should get you close to it.

A note of caution: unless you are using an LED designed for 80ma current load a *normal* 5mm LED would be severely overdriven at that rate possibly to a level it wont last very long from what I have heard.

Don't worry too much, the ARC AA/AAA push them at 65mA anyhow, and folks here claim they last forever...

I don't know very much about overdriving LEDs, I am still learning although I have read arc uses hichias which are able to handle overdriving well and I dont have any idea how well other brands compare in that way, perhaps vasdef can give us an idea what he is using for LEDs and someone else can tell him if that particular LED will take driving at that current well with/without some heatsinking if useful.

I was joking, the lifetime with even Nichia's is greatly shortened when overdriving.

I have been interested in overdriving specs myself because when you use resistors in LED lights when the voltage approaches 3.6 volts the current drops dramatically and if you start out overdriving some you still have useful light around that area. I have found 4ma on 4 LEDs can product enough light to see in the dark to walk when you get your night vision working and 10ma across 4 LEDs you can get enough light to put out a beam to walk by.

I need to just get about 100 cheap LEDs and start making resistored 4.5v-6v lights and get an idea of brightness and
useful light/drive currents/qty of LEDs.

the LED's I'm currently using are 5mm white, view angle 20, forward voltage 3.6 and 25 mA. Intensity is 12,000 mcd. These are .90 a piece, way better than the Radio Shack $5.29 per.

The specs of the last led's I wrote about are:
(they're called THC3)
5mm white
view angle 30
F. voltage 3.6
F. current 80 mA, 100mA max
intensity 24,000 mcd
These are 2.54 a piece. Both led's I mention here I saw at www.lsdiodes.com

If its feasible, I'd like to take 10 of those THC3's and hook them up to 6 volts.

I don't see any reason you cannot do them. at 80-100ma you would need to make sure and use batteries that could put out about 800ma-1.1A at however long of run time you want. I would consider using nothing less than 1800ma nimh AA batteries. that should give you a run time in excess of 2 hours I would think. If you are going to drop 2.4v at close to 1amp I would make sure you use 3watts of rated resistors and also heatsink the lamps however you can to protect them should your batteries overdrive them from the start till the voltage drops.

Yea, forget radidio shack. I only buy stuff there I have to, they tend to charge a premium for some items while others the price is acceptable for the convenience of getting them quick (if they decide to have them in stock that is).

I want to save up about 20-30.00 and pick up 100 LEDs from either chiwing or someone that sells in quantity some reasonably nice ones. I don't see paying a fortune for nichias for the stuff I want to do.

I finally got the THC 3 led's the other day, I put together a lightweight headlamp, boy is it bright. I have 10 led's in parallel with 3.3 ohm resistors on each bulb, powered by 4 AAA's. I'd like to attach a picture of the finished product, any idea how I would do that?

resize your picture to 800 pixels or less in width (I usually use 640 or less myself, loads faster) save as JPG file and go to http://www.imageshack.us/ and upload the pic using the browser function, using the top link copy the url and past in a posting here, using *Preview Reply* to check to see if you got it right and save the URL in a txt file so later you can have it to use on another post if needed.

I would also check on the current draw of the whole lot to see how you did on your resistor calculation keeping in mind a lower current equates into longer runtime, higher current brighter light. Divide by total number of LEDs for average LED current.

thanks for the advice. Here's the link.
http://img99.exs.cx/img99/3663/10ledheadlamp.jpg

Some notes regarding ArcAAA, I went through some ArcAAA Nichia BSx on populated boards(around 26 of them) recently. The LEDs are driven at around ~30ma, none of them reach 40ma. Power consumption of the circuits from a fresh NiMH AA battery is around 60ma to 70ma.

vasdef I think you copied the wrong link for instant view,
the one with [image] in front will show it here without clicking on it I believe it is the top one on that page.

I'm getting 3.0 volt reading using my multimeter on the headlamp, am I doing this right to get the proper reading?