FreeTrack Forum

Hi,

first of all i wanna say hello because thats my first thread

I am thinking about buying a trackir for quite some time, but making it myself is actually much better

Since I have nearly no clue about electronics I wanted you to present my "plan" and ask if this is ok.

From reading a bit I understand that a good current for the Osram SFH485P  is 30mA and that the resistance of these things is about 43 Ohm => Voltage at LED ca. 1.3V.
So if i use a 9V block battery I actaully need a resisistor (in series with the LEDs) of 170 Ohm. Is that correct?

Iam looking forward to your replies

hello gonzo

good current for the Osram SFH485P  is 30mA and that the resistance of these things is about 43 Ohm => Voltage at LED ca. 1.3V.

Well... There are several errors in this sentence.....

1)
The SFH485P has a forward voltage of about 1.5v not 1.3v

2)
The driving current depends of your setup.
25mA to 50mA is OK if you're using an IR modded webcam.
50mA to 75mA fit better if you're using a stock webcam (not IR modded)
75mA is the max safe value to drive a SFH485P.

3)
30mA current / resistance 43ohms / voltage 1.3v

ABSOLUTELY NOT !!!!

You assume a led IS a resistor , and use the Ohm law to calculate the led characteristics.

That's not the case. A led is a diode (a semiconductor), so it's characteristics can't be found using the Ohm law. This law applies only for resistances.

To get a led U/I chart (what we need to know), you have to rely on the manufacturer's datasheet, or build a test setup allowing you to measure the characteristics.

So just stick on these values :
forward voltage : 1.5v
forward current : 0 to 75mA , depending on the brighness you need.

So if i use a 9V block battery I actaully need a resisistor (in series with the LEDs) of 170 Ohm. Is that correct?

Resistors for your setup can be :

Power supply : 9v battery
Setup type : 3 point

cabling type : serial mounting

resistor  vs desired current in your leds :

25mA => 180 ohms
30 mA => 150 ohms
50 mA => 91 ohms
75 mA => 62 ohms
Pick your preferred setting.

If you need other values, the led wizard on the website frontpage might help you.


cya