Blue L.E.D voltage issue

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rajaram04

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Hello sir

As we know that a typical forward voltage for blue LED is 4.5 volts & maximum forward voltage is 5.5 volts so if i apply 4.5 volts directly then is it safe for the device ?

Too what if i apply a regulated 5.5 volts directly to it ?

please comment
 

Generally spoken, I should not supply the led from a low impedance voltage source, as due to spread in Vf the current can be too low, or too high (and blow your LED).

You should incorporate some mechanisme to handle Vf variations. A curent source would be a good option (if efficiency is of importance, use a switching current source). when efficiency is not of importance, you may feed from a higher voltage source and add a series resistor.

The min/max Vf are given for a certain LED current. So in fact they feed with a prescribed current, and measure Vf.
 

An ordinary 3mm or 5mm blue LED has exactly the same range of forward voltage as a white 3mm or 5mm LED, somewhere between 3.2V and 3.6V.
Their lifetime and brightness are rated at 20mA.
 










sir in the last link it is much easier to calculate

but only if we are using same type of LED (example : white)

but what to do when we have diffrent type of LEDs for combination ???

i mean red green blue white
 

For each type, model and LED specification provided by manufacturer and described in datasheet you need to use adequate resistor, if you want to limit current with resistor.

In parallel combinations of various LED each LED array or each individual LED, should have separate and adequate resistor value.
 











hmm sir but in all our discussions we have only & only an LED with no specification no labellings & no datasheets etc so we ll have to work on it complety thats why i started so many threads

let us suppose that we got some 4 LEDS (green red yellow blue) & 10 white LEDs on roadside while taking a walk morning time

all are working correctly when we checked it with a 3 volts coin cell

now we want to make a decorative lighting item with coloured LEDs

& simple emergency light with white LEDs , too we wnat to connect this emergency light module to a remote sensor unit operated with a battery

we have an 8 volts 2 amp battery & a simple 9 volts toy battery

now we ll have to think in that way actually
 



Each LED have their spec and you should respect that if you want to have LED long life.

In case if you dont have LED spec, there is no reason to mess and mix multiple LED together in parallel connection. First I will try to identify each LED separately, I already describe how you can identify LED current in one of your threads. For forward voltage you can apply general rules for each LED.

When you identify LED safe voltage and current range then you can make circuit and some device with one or more LEDs.



Fundamentals of LED Drivers
http://www.lightingassociates.org/i/u/2127806/f/tech_sheets/Fundamentals_of_LED_Drivers.pdf

Tutorial
http://ariel-today.blogspot.com/2008/09/hooking-up-led-to-case-more-on-this-in.html









Ballast Resistor Calculation – Current Matching in Parallel LEDs
Texas Instruments Application Report SLVA325 – April 2009
**broken link removed**


Powering Light Emitting Diodes (LEDs)
http://www.horrorseek.com/home/halloween/wolfstone/Lighting/litlpo_PoweringLEDs.html






I hope this will help for better understanding.



Best regards,
Peter


;-)
 
Reactions: WimRFP

    WimRFP

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yo yo yo sir thats a very clear explanation i ever read before
thanksss & i am proceeding as per theory , doing a lot of experiments
 

rajaram04 said:
yo yo yo sir thats a very clear explanation i ever read before
thanksss & i am proceeding as per theory , doing a lot of experiments
Click to expand...

Its good to have few digital instruments, at least two, one for voltmeter and second for ampermeter, and practice.



Best regards,
Peter

:wink:
 

ohh kk sure , i ve single digital one , but now ill take one more for all that , thanks for all ur support . .

mankind thumbs up
 

A 3V "coin" battery is 3V only when it is brand new and has no load. It might not be able to produce enough current to burn out a 2V red LED.
 

Audioguru said:
A 3V "coin" battery is 3V only when it is brand new and has no load. It might not be able to produce enough current to burn out a 2V red LED.
Click to expand...





ya i ve one for general testings but when i connect any other l.e.d. like blue white or green in parallel with red one , then only red glows & no effect on rest , thats the issue
 

rajaram04 said:
ya i ve one for general testings but when i connect any other l.e.d. like blue white or green in parallel with red one , then only red glows & no effect on rest , thats the issue
Click to expand...
You must NEVER connect LEDs with different colors directly parallel to each other because the red one clamps the voltage to 1.8V to 2.2V then the 3.5V blue and white LEDs do not have enough voltage to work. When each LED has its own series current-limiting resistor then all of them will work if the battery has enough voltage and current for them.
 







hm i see , it means a lot of led designs could not be driven by single 3 volt coin cell ? & we need some magnifiers ? right ?
 

Most ordinary LEDs are specified with a current of 20mA. But a little 3V coin cell can provide only a few mA for a few seconds.

Look at the website of a battery manufacturer like Energizer. A CR2016 3V Lithium coin cell can provide 97uA (a little less than 0.1mA) continuously or provide 6.8mA for 2 seconds 12 times per day.

I have a white LED that is bright at 21mA and its voltage measures 3.26V. But when it is connected directly to a fairly dead CR2032 3V Lithium coin cell it is dim at 2.8V. The fairly dead cell has an internal resistance that limits the current.
 





okk so why this cellls are having high internal resistance ?
 

The internal resistance isn't a physical component, when you load a cell it's voltage drops and it can therefore be modeled as a perfect voltage source (one that NEVER drops) with a resistor in series with it. The drop from a real cell can be understood as a fixed voltage with the internal resistance in series with it. For example, a cell that drops 1V under a 10mA load would be said to have an internal resistance of (R=V/I) 1/.01 = 100 Ohms. All cells drop voltage under load, the coin cells in question have a resistance high enough that they cannot normally provide enough current to damage the LED. The internal resitance of a cell isn't a fixed value, it gradually increases with the cells age and state of discharge. That's why measuring the voltage across a cell while it isn't under load is not a good indication of it's condition, you can have a good and a bad cell showing the same voltage out of circuit but when current is drawn, the good one keeps going while the higher internal resistance of the bad one makes it's voltage drop lower.

Brian.
 

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