i want to use a 9v batter for a power supply to a LED array. what is the right resistance to use. the LED's are going to normal red and green. they operate around 2 to 3 volts.

 

Do you have a more exact forward/operating voltage? Or the normal current that the can handle?

To find your resistor, just do (9-(forward voltage))/.020 (most leds can handle 20 mA continous.
There are online calculators that will give you a standard resistor value too. Just google for LED series calculator.

__________________
Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

ok, here is my general idea. any pointers

Attached Images

led_711.gif (1.4 KB, 972 views)

 

pick a current and use R=V/I.

if each LED needs 15mA of current, and has a forward operating voltage of 1 volt, and you have 4 in parallel, then you need 60mA of current at 1 volt. with an 9v battery, you would have R=(9v-1v)/(.06A) = 133 ohms.

for quickly estimating resistors, as a general rule of thumb, i use at least 330 ohms for one LED at 5 volts. doubling the voltage means doubling the resistance, and adding more LEDs in parallel means halving the resistance. so for 9v you'd about double the resistance to 660, then divide by 4 for the number of LEDs and get about 165 ohms.

either way, most LEDs will light just fine on 10mA or less, so you should be fine using a resistor somewhere around 200 ohms.

 

Just remember to pair only the same colored led's, as the voltage drop is not the same for all colors.

 

if the forward voltage on the LEDs is 2-3 volts, y not just use some 2 AA or AAA batteries? they'll last waaaay longer 2.

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I'm no electronics god, i just talk too much.

 

It isn't good to put leds in parallel with one current limiting resistor.
Due to the variance while manufacturing, some properties of the leds are different, so one LED could get more current than the rest and burn out quicker.
I'm not saying that it wont work, but if your leds burn out kinda often, this is probably why.

__________________
Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

i was going to use a 9v for the convienace of it's size and no need for a larg battery clip. what i'm trying to make is a flashing light device for wearing when i run at night. i woould rather build one then pay $20 - $35 for one. of course my version of such a device will probably be twice as large and more combersome than the store bought version.

so i should put a resistor on each LED insted of having just one?

 

You should put LED's in series when you can, with a 9V battery you could probably manage four in series with a single resistor (this will minimize waste in the resistors). You shouldn't put LED's in parallel, unless they each have their own current limiting resistor, it's extremely poor practice.

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Yes, and like Nigel said, you should put them in series, then you can get away with only using 1 resistor.

I found out the hard way with needing a seperate resistor for each led in parallel. And unfortunitly my leds were mounted in the headliner of my car, so changeing them would be a pain. I have redesigned it since then and I've been running the leds for a couple months now with no problems.
I have some pics posted on page 2 of the www below this post. but it appears to be down at the moment.[/url]

__________________
Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

Hi Diroga,
Zach is right. AA cells last much longer than a 9V battery. Have you seen the tiny size of the 6 cells that are inside an expensive 9V battery?

I have a chaser project that blinks bright LEDs in a circular sequence. It uses 1.7V to 1.9V red LEDs and only 2 AA alkaline battery cells that last "forever". It uses a 74HC14 IC that works down to a voltage lower than the LEDs need and the LEDs are still pretty bright when the battery needs replacing (2V).
I use a diode/resistor to make a short (30ms) "on" duty-cycle for the LEDs to conserve battery power.
You could make a simple oscillator from 1 inverter of a 74HC14 like I did but dispose of its speed control. The rest of the inverters can be paralleled like mine and used to drive a transistor more powerful than mine, to drive all your LEDs. You can also dispose of the inverter that I use for PWM brightness control, and dispose of the 74HC4017 sequencer.
Using 3V to 3.5V ultra-bright LEDs and a 4-AA cells battery (the max supply voltage for the 74HCxx series is 7V), it can be seen for miles and is awesome! My schematic is here:

Attached Images

3v_led_chaser_schematic.jpg (73.1 KB, 906 views)

 

audioguru-

where did you get your LED's? the radio shack LED's aren't vary bright. i also found a site where i can buy UV LED's! i was going to use a transitor capasitor self trigoring thingy to cycle 180 so one let of LED's on the other off. yeah i dont know the techincal term for it.

 

I get mine from ebay in 100+ quantities, most of the people that put them in their cars get them from LSDiodes.com.
I have 62 4000MCD blue leds in my roof. I had a fog machine going for that pic...

I wouldnt go with the UV leds, unless you wear white clothing or something, the UV's are more reflective than bright (white stuff glows purple).

__________________
Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

I have been using HLMP CB15 for narrow angle and HLMP CB30 for wide angle, blue LEDs originally made by HP but now Agilent. They project a perfect pattern, their brightness keeps increasing and their prices keep dropping. I get them from Newarkinone (owned by worldwide Farnell now), they have them selected for brightness and charge less if you buy more.
I tried some cheap Indian and Chinese blue LEDs but their pattern is different for each one and brightness varies too. Frequently their colours are mixed-up. I had to test them one-by-one in the store, what a nuisance. The next guy got my rejects, or they went to Ebay.

I looked at LSDiodes site. Their LEDs' brightness are rated at max, not a guaranteed min like everyone else, and some have their brightness rated at a melting 80mA instead of 20mA like everyone else. Instead of a discount for quantity, they give freebies (stickers and stuff that I don't need).

 

what color would be best? blue or red? how about green or white? white is more expesive.