FreeTrack Forum

Hi,

I'm trying to build my first 3 LED tracking system. On first attempt everything worked for a short while but then i think my 3v 300ma power supply burnt out.

I've now bought a 3v 400ma power supply and again things seemed to work fine for a few minutes but then nothing. The power supply is red hot again.

I'm clearly doing something wrong and i think it's because i don't have any resistors in my parallel circuit. I must be entering the wrong info in the LED wizard because i just can't seem to make it work.

Below are the specs of the LED's i'm using.

OSRAM - SFH485P - IR EMITTER, 5MM, 880NM

IR EMITTER, 5MM, 880NM
Wavelength, Typ:880nm
Forward Current:100mA
Voltage, Vf Max:3V
Viewing Angle:160°
Case Style:Radial
Operating Temperature Range:-40°C to +100°C
Current, If AV:100mA
Full Angle:160°
Half Angle:80°
LED / Lamp Size:5mm / T1.3/4
Max Radiant Intensity:6.3mW/Sr
Min Radiant Intensity:3.15mW/Sr
Pitch:2.54mm
Radiant Intensity:48mW/Sr
Turn Off Time, t Off:0.5µs
Turn On Time, t On:0.6µs
Voltage, Vf Typ:3V
Wavelength, Peak:880nm
LED Colour:Infrared

Can someone please tell me where i'm going wrong. I should add i'm using a webcam with a natural light filter.

Thanks,

Rich.

Use 3 x 30 ohm rersistors !

Voltage = 3V
Vf=1.5V
Current=50 mA

Thanks for this benmeijer. I'll give those a go.

One other thing, i notice in your post regarding a simple system that anyone can build

http://forum.free-track.net/index.php?showtopic=1856

that the resistors are placed on the positive wires, i don't understand why this is? A battery is a DC circuit which therefore means electrons are flowing from negative to positive so why does your diagram show the resistors seemingly placed after the LED's in the circuit?

Rich.

Because this diagram (from ledcalc on this site)  has also the resistors on the positive side of the leds:


I think it doesn't matter where you place them, but I'm not sure about that.

read this site about leds and resistors:
http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/

Thanks again benmeijer. Very useful link to that LED site.

I've since discovered by checking with an electrician friend that the reason why the resistors are before the LED's is because in a DC circuit, electrons flow from negative to postive but the actual electrical charge flows from positive to negative. Therefore the resistors have to be placed before the LED's to reduce the current, if placed afterwards the current won't be reduced before it gets to the LED and you'll have burn out.

Cheers,

Rich.