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FreeTrack Forum > FreeTrack : English Forum > Support : Tracking System > LED's needs power...

RSS >  LED's needs power..., sigh... help
BrokenArrow #1 25/10/2008 - 11h46

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Hi guys!

So, i finnaly got 5mm LED's.

So, now, i built my cap, very neat job and everything...

So, the thing is USB powered.

I worked out: the LED's needs 6V of power. USB only gives of 5V.

So, the LED's arent bright enuff for my cam.

So, is it in any way possible to give more power to a usb port?

If not, can i put a battery into the stream? (it's a serie stream)
benmeijer #2 25/10/2008 - 13h13

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Use only 3 leds (it's enough) to build a 3 point clip or cap and you have no problem. Use a resistor 10 ohm!
Writech #3 29/10/2008 - 14h23

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First you have to think which tracking model you want to use.
For example: I´m using 3 led cap.
I want my system to be USB powered. USB gives 5V and current may not exeed 500mA. One SFH485P LED needs 3V and 100mA or less to work properly.
So you must use parallel connection. I, personally, used only one resistor for all of the 3 LED's. Together all the three leds need current 0,3A.
Now to the calculations:
[5V (USB) - 3V (LED)] / 0,3A (3 LED's current) = 6,7 Ohm resistor
2V * 0,3A = 0,6W * 2 = ~1W resistor

So you need 6,7 Ohm 1W resistor to power up all the 3 Leds in parallel with proper voltage and current.

If you want to use one resistor for one led, then...

[5V (USB) - 3V (LED)] / 0,1A (1 LED's current) = 20 Ohm resistor
2V * 0,3A = 0,2W * 2 = 0,4W resistor

...you need 20 Ohm 0,4W resistor.

And I suggest you to put a 0,5mA fuse to the circuit to protect your motherboard from overloads.
tristan68 #4 29/10/2008 - 16h22

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@ Writech :

One SFH485P LED needs 3V and 100mA or less to work properly.




THATS A NO-NO !!!!

This has already been stated and explained several times in this forum :

SFH485P DON'T WORK AT 3V (or they at least they really don't last very long...)

SFH485 specs are : 1.5V (not 3V) and 100mA max

if you found this 3v value at the RS / Farnell website : they have a well know error in their spec.

if you found this value in the SFH485P datasheet : please read the datasheet again : the SFH485P can be driven at 3V when using PWM drive. Thats NOT the case in freetrack setups.


@benmeijer :

You can't give a resistor value , if you don't know the leds specs, the setup type, and the current you plan to use......


@ Brokenarrow :

Can you please provide the specs of 1 led, or better the led model ?

thx.
cadcoke4 #5 29/10/2008 - 16h24

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Nouvel, I've read in several places that going with a very low resistor, in the kind of range you are using, it not normally advised.  I imagine this is because a fairly low variation on the value of the resistor (from normal manufacturing variations) will cause a lot of difference in the current.

I also suspect that your choice of LEDs has a lot more light output than needed, if you are sitting in front of a computer monitor.  If you are in front of a large screen TV perhaps 5 or 10 feet, perhaps it may be of benefit to be that bright.   But, it still seems overkill.

Do your LED's get warm?

Joe Dunfee
Edited by cadcoke4 on 29/10/2008 at 16h24.

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