Discussion in 'The Projects Forum' started by adreano_borgir, Oct 23, 2010.

Oct 19, 2010
3
0
hi all
i just want to know if this calculation is right or not
i'm starting new project, and need some help

*led light calculation*
[source voltage-(No.of led * led voltage)]/resistant= *1*
*1*/resistant =continuous forward current

*brightness degree*
continuous forward current/max.continuous forward current

*resistant limit*
[source voltage-(No.of led * led voltage)]/continuous forward current=*2*
*2*/[continuous forward current/1000]

*resistant reliability*
[(source voltage-led voltage)*(continuous forward current/1000)]*2

is this right or i need any equations????

2. ### AlexR Well-Known Member

Jan 16, 2008
735
54
It might help if we knew what you are trying to do.
A brief description of your circuit would help, a circuit diagram would help even more.

I don't know where you got those equations from but to me they look like pseudo-science mumbo-jumbo. Terms such as *brightness degree*, *resistant limit* and *resistant reliability* don't have any meaning to me.

If you explain in plain English (without the pseudo-science jargon) just what it is that you are trying to calculate then we will probably be able to help.

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
I'm not quite sure what you're trying to calculate here.

However, the first thing you need to do is determine how many LEDs you can operate in a series string at the recommended current.
LED_maxcount = INT(Vsource*0.9/ LED_Typical_Vf)
where:
LED_maxcount = the maximum number of LEDs that can be wired in a series string.
INT() = a truncate function; it returns the integer portion of a floating point number.
0.9 = returns 90% of the voltage supply; use this if your voltage supply is regulated. If it is an unregulated voltage supply, you will need to take that into account by reducing this factor.
LED_Typical_Vf = specified on the datasheet for the recommended continuous current.

Once you have calculated LED_maxcount and it is greater than 0, then:
Rlimit >= (Vsource - (LED_Typical_Vf * LED_maxcount) / LED_Typical_I)
where:
LED_Typical_I = the current specified in the datasheet for the typical Vf.
Rlimit = the current limiting resistor value to use.
A decade table of standard resistance values is here:
http://www.logwell.com/tech/components/resistor_values.html
Use the E24 column values.

Then calculate the wattage requirement for Rlimit:
Rwatts >= (Vsource - (LED_Typical_Vf * LED_maxcount))^2 * 2 / Rlimit