Math for led calculations

Discussion in 'LEDs and Optoelectronics' started by conntaxman, Mar 12, 2013.

1. conntaxman

89
0
Jun 17, 2011
Im tring to figure out how to do the math for using it in a cir.here's what it is.
http://www.mouser.com/ds/2/389/CD00156614-115578.pdf
The Current setting. ----FOR A ----stcs1phr
The current is set with an external sensing resistor connected to the FB pin. The feedback
voltage is 100 mV, then a low resistor value can be chosen reducing power dissipation. A
value between 1 mA and 1.5 A can be set according to the resistor value, the resulting
output current has a tolerance of ± 10 %.
For instance, should one need a 700 mA LEDs current, R
F
should be selected according to
the following equation:
RF=VFB/ILEDS=100MV/700MA=142MΩ
------------------
here is the spec for a 10watt led that im going to use.
and power it by a 12 car battery.[VF]9-12 [IF]1000MA
.
To figure the amps out is it 12 divied by 1000 =.012
what dose RF AND VFB,I think vfb = forward voltage ,
don't know what RF stands for.
tks
John

2. BobK

7,682
1,688
Jan 5, 2010
Feedback resistor. You need to choose a resistor that drops 100mV at the desired current. At 1000mA that would be

V = I R
0.1 = 1 R or R = .1 Ohm

Bob

3. duke37

5,364
771
Jan 9, 2011
100MV at 700MA is quite enormous. Do you have your own generating station!

I do not follow your calculations. I do not see what current your leds take, perhaps it is 1A, The feedback resistor has to give 100mV. R = V/I = 0.1/1 = 0.1ohm.

Rfb = Feedback resistance
Vfb = Feedback voltage

4. BobK

7,682
1,688
Jan 5, 2010
MV = Megavolts mV = millivolts
MA = Megamps mA = milliamps

What's 9 orders of magnitude between friends?

Bob