IR Led's

[things]

hi all, i have bought 25 IR led's, and wired them 13 on one row and 12 on the other row. the led's in the rows are wired in parallel, and the rows are wired in series. i am trying to run them off a usb port, but the led's get kind of warm directly connected to the usb port, but every resistor i try gets hot, or the led's are really dim?? anyone have any suggestions on what resistor to use?
thanks!!

 

[Hero999]

Use a separate resistor for each LED.

What's their foward voltage drop? About 1.3V?

What output voltage does the USB give? About 5V?

Connet them in eitght series groups of three and calculate the appropriet resistor for the desired forward current and wire the last one up on its own.

 

[HarveyH42]

What are the specs on the LEDs? IR is usually 1.4 volts, the current varies widely, 20 - 100 mA continous. Don't know much about the USB port, but think its 5 volts and under 500 mA. So, only 2 in series. The current limitting resistor depends on what current you need for the LEDs.

**broken link removed**

The link takes you to an online wizard for building LED arrays.

 

[things]

that link was very helpful!!!
thanks guys!

 

[things]

i have trashed the usb idea, as i have allready soldered the led's, so i can't put resistors in. so i just found a ac to dc plug pack, its rated 3 volts dc out, but its pulsed dc, so it messes with the camera. is there anyway to "smooth" out the pulsed dc, like with a capacitor?

 

[things]

anyone know if that will work?

 

[bananasiong]

Do you know that the squarewave is form by the sum of a fundamental frequency sine wave and many harmonic frequency sine wave? Use a LPF to filter out the harmonics.

 

[Hero999]

Unsolder the LEDs if you've soldered them.

Start from scratch, you've obviously made a bit of a mess of it.

How much current does your 3V supply provide?

Save yourself the trouble anbd buy a 9V 300mA adaptor and connect the LEDs in five chains of five LEDs.

 

[things]

ok i think i'll just unsolder them, but 25 is going to take a while!!!

 

[Stellarcore]

I've recently bought 150 IR leds from digikey for my LED coffee table project. Mine are rated at about 50mA but I'm running them at around 38mA through two 220 ohm resistors in parallel (I had one 100 ohm resistor but it was getting quite warm so I put two 220ohm resistors in parallel).

But consider the total current you need, if you want to power all 25 LEDs at 38mA (as in my case) you would need a current source capable of nearly 1A. This amount of current cannot be provided by a single USB port. If you are trying to power this from a wall-wart transformer as you mentioned earlier, make sure that its rated to provide enough current for your needs.

I was curious how you are measuring the brightness of the IR leds. I have been using a digital camera to try and see the IR through the tiny LCD on the back, but I dont think its a very good solution.

Another problem that I was wondering about is what sort of materials are IR transparent, I'd like to shine my IR through a sheet of glass and then have the reflection of solid items on the other side of the glass reflect back to be detected by my IR transistors, but several glass or plastics that I try seem to reflect the IR on their own and set off the photo transistors.

Id be interested to find out what you're doing with your IR LEDs. Care to share?

 

[justDIY]

@things:

i recommend as high a forward voltage as possible, so you have big series strings ... lets say 1.3v for each IR led, a 24v power supply (cheap from surplus sites) will let you run 18 leds in series, and starting from there you can connect as many 18 led strings in parallel as you need. You only need one resistor for each string.

The advantage to a long series string is the light output of your leds will be very consistent since minor variations in Vf for each led is offset by the constant current they are forced to share, by being wired in series.

 

[things]

it's for a night vision type thing. the problem is, they still get slightly warm running on 3 volts at about 200ma, but they aren't getting too hot, so i just wired it up with that, but now i found out that the transformer has a pulsed dc output, so trying to view that with a camera, there is lots of lines. i tried to put a capacitor in, and it smoothed it alittle, but now they are alot dimmer?

oh i'm viewing them through a modified (ir filter removed) eyetoy

 

[justDIY]

you want higher voltage not lower. Like Hero said already, unsolder them, or cut the leads and solder to the stubs.

To put it bluntly, you have it wrong! Several low power leds in parallel sharing one resistor will not work, they will all fail in a short amount of time. There's several right ways to do it, and none of them involve a big gob of leds without proper resistors.

 

[audioguru]

Since the LEDs are now a lot dimmer then you have destroyed them with too much current.