Powering LEDs?

**ScatterPlot** · 03-12-2005, 03:36 PM

Use a resistor. To figure out a good one to use, use Ohm's Law:
Amps=Volts/Resistance

You said the LED's were 20mA (or .020 A) at 3.6V.
.020=3.6/R
Solving for R gets...
3.6/.02=R
180=R

So you should get a 180 ohm resistor, or somewhere thereabouts, rated at 1/2 watt should do the trick.

Now I'm not exactly all-knowing on this subject, but I think that's right. Somebody check my work, lemme know if that's right or not. Good luck!
Bert

Oh, and one thing- your doohickey will last longer if you use, say, two AA batteries (I think). That little camera battery is really pretty weak, but it might work- I just don't know. You should be able to use the same resistor with either setup, but again I don't know. I'm just kinda gettign started in this myself

**White_Noise** · 03-12-2005, 06:34 PM

hey matt,

yeah, im not sure why those led's were able to take the 12 volts straight from the battery. every other led ive seen uses only 3.6 volts.

sounds like a resistor is going to be the way to go. not sure if radio shack has stuff like that, but i know that if you get down to K-town, you can get them at Chesters. thats actually where i got my LED's from originaly as well.

as for energy draws, and how long you can get the led to run off of a specific battery, i think if you do a google search you can find a chart or something like that. i think i came across one when i was looking for led's.

if you do happen to burn out the led and need a new one, let me know, i have 7 blue ones here(almost white) if you want one.

**Number13** · 03-12-2005, 07:51 PM

Thanks guys for your help so far. I've been cruising the net in my spare time today, and have a little better understanding of LEDs and batteries. I should emphasise "little." Scatter's quite right about the camera bats versus AAs. And I'll use yellow LEDs instead of white to save on money and cut down on the required volts from the power source.

Now I need help from someone who knows the formulas.

If a AA Energizer battery has 3135mAh and 1.5V, and if I'll need a 39 Ohm resistor that uses about .019 Watts, and if the LED uses 22mA of current and 2.15 Volts, what's the formula for figuring out how long the battery will last on that circuit? If someone could write out the formula for me so that I can calculate how changes in the set up will affect the battery life as well, I'd be very appreciative.

I didn't find the charts you were talking about, Mikey. Any suggestions on keywords?

Much thanks,
Matt

**White_Noise** · 03-12-2005, 08:56 PM

hmmm...im not sure on the chart stuff....ill have to try to remember what exactly i was looking for at the time. if you want some cheap led's the blue ones i have would cost you $.45 each so im sure theyre cheaper than what youre looking at. if you want 1/some of them, let me know. ill look at some physics stuff later to see what i can do about those conversions.

**ScatterPlot** · 03-13-2005, 12:00 AM

Radio shack definately has all the stuff you'll need, albeit much more expensive than online places. But for like 5 bucks worth of stuff it's hardly worth the extra wait.

**swanster** · 03-24-2005, 08:14 AM

Just use a 256 ohm resistor, thats what we use in my electronics course and we've never had a LED blow up, and we use 9volt batterys, so 3.6volt won't do anything to it.

Oh and the 20mA is how many amps the LED will use, in this case 20 milli amps

Carl

**undescriptive** · 03-24-2005, 09:32 AM

just rigged an ultrabrite blue LED in my gripframe (pics soon!) to pimp it a little bit...

I used a CR2032 battery and rigged it directly (via a battery housing) to the switch and LED

it should be good enough for about 30 hours or so (it's only a little button cell like in a calculator)

2AA's should last a good time...
if space isn't an issue, go for the big D cells!

**bjjb99** · 03-25-2005, 07:53 AM

Originally posted by Number13

What I would like to do is power a white LED that's listed as "20mA (max). Typical Voltage is 3.6V." I want the light to run continuously on a battery for a minimum of 9 months. (It's not related to paintball--I'm trying to make a gift for my gf.) I can find 3.6V batteries easily enough, it's the 20mA that confuses me. The batteries I find are 10-100x that current, and I don't know if that discrepancy in amperage will make a difference. I don't want to fry the LED or something, since I plan on having it run for so long. Also, anyone know of a power consumption chart or something that will tell me what size/how many batteries I'll need to run the LED for that long?

To approximate battery consumption, just multiply the current drain of your circuit by the amount of time it's running. I'll use your original post as an example...

At its maximum rating, the LED will consume 20mA. You want it to run for 9 months. I'm going to assume 30 days per month, so you're looking at 270 days of continuous operation. That's 6480 hours of runtime. To get the power consumption of the LED alone, just multiply the current by the time it's running (20 mA * 6480 hours = 129600 mAh). You will need batteries that can supply 129600 mAh of energy, assuming there are no other sources of current drain in your circuit. That's a lot of batteries. The AA battery you mentioned delivers about 3000 mAh of energy, so you would need 44 of them to run for 9 months... and that's assuming that you were running the circuit at 1.5V!

As for a circuit, you will probably want a switch, a capacitor going to ground to absorb transients from the switch, a current limiting resistor, and a large variable resistor to set the brightness. I (hopefully) have attached a schematic that should work, based on your original post. I used a 6V input instead of 3.6V.

BJJB

**ScatterPlot** · 03-26-2005, 03:39 AM

On a semi-side note, I can get where you get the values of resistors to use (Ohm's law) but what do you use to determine the size capacitor? Sorta trial and error and then get a feel for whats good, or is there a set in stone method?

**bjjb99** · 03-26-2005, 07:32 AM

Originally posted by ScatterPlot

On a semi-side note, I can get where you get the values of resistors to use (Ohm's law) but what do you use to determine the size capacitor? Sorta trial and error and then get a feel for whats good, or is there a set in stone method?

I wanted something that would absorb transients from the switch (switch bounce, etc.) to help protect the LED. I also wanted something that still appeared to turn on instantaneously when the switch was thrown. I figured a circuit with a time constant of at least 0.01 seconds would be sufficient.

For an RC circuit, the time constant is simply the product of the resistance (in ohms) and the capacitance (in farads). The circuit's current limiting resistor is 120 ohms, so I needed a capacitor that would work well with that value. 100 microfarads was about right, giving a time constant of 0.012 seconds. If the variable resistor is set to anything other than zero, the time constant becomes longer. If the variable resistor is set to maximum (1 Meg), then you're looking at around 100 seconds for a time constant... but the LED probably won't turn on with that little current flowing anyway.

In hindsight, that variable resistor could probably be 100 K instead of 1 Meg... either should work nicely.

BJJB

[edit: Man I need a new keyboard... the letter 's' kept failing to appear on request]

**ColdFuzion** · 03-26-2005, 10:01 PM

They've said how to make it work, but now you mind telling what this gift is going to be? I'm nosy...

-Cold

**Number13** · 03-28-2005, 10:47 PM

haha, np ColdFuzion.

I'm making a change jar and lamp/nightlight for my gf. She works part time at a drive in and she wanted a change jar to put tips in. I'm painting the outside and putting LEDs on the inside to illuminate a budding flower pattern. I've decided it would be better to just put it on a switch than try to make it run all the time. And the jar was to be sealed so that she'd have to break it open to retrieve the change. I felt it would aesthetically improve the jar if it was built to be destroyed. By having the light on all the time, I wanted to metaphorically represent the life of the flower, but it will work fine on a switch (since I'd apparantly have to plug it into a wall socket to feasibly get it to run long enough anyways).

Much thanks to all of the help! I might not get it all completed until I finish the semester, but I'll try to remember to let you know how it turns out.

Big thank to bjjb99 for his help!

**bjjb99** · 03-29-2005, 07:01 AM

Heh, no problem, Number 13.

If you're going the route of plugging it into the wall, you will probably want to go with a 5 or 6 volt power supply that has a decent voltage regulator in it. Otherwise you might see a bit of flicker in your LEDs resulting from the 60 Hz line voltage frequency sneaking its way into your circuit. Some folks are more sensitive to that sort of thing than others.

BJJB

**Bad_Dog** · 03-29-2005, 09:59 AM

well this is a little off topic but I'm working on running LED's on my new truck...

I'm splicing them into the brake lights... which means they are getting somewhere around 12vts of juice... what size resistor do I need to throw on my LED's???

any help would be greatly appreciated!

Powering LEDs?

Powering LEDs?

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment