# ele Math

Discussion in 'General Electronics Discussion' started by conntaxman, Mar 1, 2012.

1. ### conntaxman

89
0
Jun 17, 2011
Need some help understanding the math out for this chip.Im looking for Current /voltage for a 10watt led.Forward voltage: 10-12V DC-Forward current: 1000mA. Here is the chip that im using.http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00156614.pdf
This is how they say to do the math for it, But could some one do it our for me so that I can see were you div/mul/add/or sub. tks
-----
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 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
---
what dose vfb and the 1leds mean. I dont know if its "one"leds or two leds,
tks
John
I could see the 100mv is div by the 700ma
so i guess all i have to understand is the first part. Lol

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,411
2,779
Jan 21, 2010
Ileds means the LED current. If you have more than 1 in series it is still the LED current. It is also the current that you're regulating.

RF = VFB / ILEDs = 100 mV / 700 mA = 142 mΩ

RF = 0.1 / 1 = 0.1 ohms

3. ### conntaxman

89
0
Jun 17, 2011
math

Tks ,.I also see were they got the 100,its at the top of ele spec." Electrical characteristics
(VCC = 12 V; IO = 100 mA; TJ = -40 °C to 125 °C; VDRAIN = 1 V; CDRAIN = 1 μF; CBYP = 100
nF typical values are at TA = 25 °C, unless otherwise specified)
.
Thank alot.
John

4. ### conntaxman

89
0
Jun 17, 2011
led math

Hello steve.Could you tell me if this is correct using this chip,it is the same one but can handle 2 amp's. http://pdf1.alldatasheet.com/datasheet-pdf/view/230545/STMICROELECTRONICS/STCS2.html
it will be running off a car battery. the led will be a 20watt draw will be 1200ma it say 12vdc.it dosen't show the high voltage. [12-14] so 100divied by 1200= .083 so is that going to be an 8 ohm resistor? or round it off and use a 1 ohm
Tks John

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,411
2,779
Jan 21, 2010
no, 0.083 ohms

6. ### conntaxman

89
0
Jun 17, 2011
Hello/ again. I tried to find the right place to post this ,but couldn't.so here it is. I want to make a light using white leds .The batteries are 3.2vdc-1000ma. I will be putting some of these in parallel to get more amp's. the led's are the small white one, 3.5 vdc FV. and i think around 25 ma's.Could I run about 51 in parallel ? or would it be better just to connect each to the 3.2 battery.
tks
John

25,411
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Jan 21, 2010
8. ### BobK

7,682
1,686
Jan 5, 2010
What is the difference between putting them in parallel and connecting each of them to the battery?

Youi won't get much light out of an LED with a forward voltage of 3.5V by applying 3.2V to it.

Bob

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