urgent help needed please

Thread Starter

adreano_borgir

Joined Oct 19, 2010
3
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????
 

AlexR

Joined Jan 16, 2008
732
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.
 

SgtWookie

Joined Jul 17, 2007
22,230
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
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.
Vsource = your voltage supply.
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
 
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