Figure out current res.

Discussion in 'The Projects Forum' started by conntaxman, Nov 9, 2012.

1. conntaxman Thread Starter New Member

Oct 30, 2009
12
0
can someone explain this in different terms. I always have problems reading how to figure the current out.Im planing on use these for different leds.But here is one led i will be using off of a 12 deep cycle battery.spec for led
10 watt led.
The last line for the math is the one i mean.
tks
John
FV. 9-12
FC 1000
and spec for
""STCS2 is a BiCMOS constant current source
Current setting
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 2 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, RF should be selected according to
the following equation:
RF = VFB / ILEDs = 100 mV / 700 mA = 142 mΩ

2. wayneh Expert

Sep 9, 2010
15,001
5,454
An LED needs a specific current level to provide brightness over a long life. You usually want to set the current level well below the rated maximum current, for instance less than 20mA for an LED rated at 30mA. High power LEDs have an additional concern of heat. Without a heat sink, you cannot get anywhere near the rated maximum.

Your driver is designed to provide more or less current - whatever it takes - to hold the FB pin at 100mV. So you use Ohm's law V = I*R to calculate the required R in ohms, given I = your LED current in amps and V = 0.1 volt.

I think your LED will draw about 1A at 10V to produce 10W, but it's not clear. You should verify and again, don't expect it to stay cool without a proper heat sink.