# Help in math

Discussion in 'General Electronics Chat' started by conntaxman, Nov 16, 2012.

1. ### conntaxman Thread Starter New Member

Oct 30, 2009
12
0
I need some help in math, ha ha ha . could some one explain the last line of this.Guess the " Letters" screwing me up as to what they mean.like RF = VFB and so on. i know the ma,stands for mill amps,and the ohms sign,
This is so i can use a STCS1 for a led driver.
tks
John
.
"""" quote"""
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, RF should be selected according to
the following equation:
RF = VFB / ILEDs = 100 mV / 700 mA = 142 mΩ

2. ### John P AAC Fanatic!

Oct 14, 2008
1,632
224
Always a pleasure to meet a guy named John.

Rf means "resistor, feedback". It's a small resistor used to produce a voltage that's proportional to current.

Vfb is the voltage across that resistor, which for this component is apparently always 100mV. The device will control the current to achieve this voltage.

ILEDs is the current that's flowing through your LEDs and also through the resistor. So the basic equation is
Rf = Vfb / ILEDs (resistance = voltage / current)

Then they put in some real numbers and use them to calculate the resistor value you should use
100 mV / 700 mA = 142 mΩ
Or you could say
0.1/0.7 = .142 Ω

3. ### conntaxman Thread Starter New Member

Oct 30, 2009
12
0
John, im back at this again.I got discusted and put this off.
so with this part stcs2spr led driver, to get the value for FB it would be100/700 = 0.1 ohm . Dose that 100 stay the same if you wanted different amps at FB.
http://elcodis.com/parts/691022/STCS2SPR.html
Then i have to remember/find out the value for Rin. Im running 12vdc.But if im running off a car battery that is 12volts, so I wouldn't need a restor for Rin. would I.
tks
Jonn

4. ### kubeek AAC Fanatic!

Sep 20, 2005
4,669
804
well there is a subtle difference between 0.1ohm and 0.14ohm, and the difference is that one gives you 700mA and the other gives you 1A through the leds. It is up to you to decide whether the diodes can take the higher current.