White LED parameters

[rajaram04]

Hello sir

For a white LED what are the values of maximum forward voltage , typical forward voltage , maximum forward current & maximum reverse voltage value ?

Are they differ with size & type ?


i mean

 

[tpetar]

Answer is in manufacturer datasheet for particular LED product.

Voltage for white LED is usually in range 3,4V-4V but this means nothing in comparison with manufacturer datasheet.

Each LED have product ID with lots of parameters including max forward voltage, max current, angle,....

 

[rajaram04]

Answer is in manufacturer datasheet for particular LED product.

Voltage for white LED is usually in range 3,4V-4V but this means nothing in comparison with manufacturer datasheet.

Each LED have product ID with lots of parameters including max forward voltage, max current, angle,....

ya sir but i don't have any product ID cause i brought it from the dealers who don't have such accessories with them but have packs of LEDs so i don't have such a way to solve it so please let me have a link

 

[WimRFP]

Because of the spread in Vf (and in your case unknown components), make your circuit so that the current through the LED doesn't depend on Vf strongly. It is not the LED forward voltage that destroys a LED, but the current.

If you don't know the maximum safe current for your LEDs, then you are in real guessing mode. Check some datasheets for similar components (I mean LED's in same package and with same metallic construction appearance).

 

[tpetar]

Make tests. Try to test diode on 3,4V at two currents 25mA and 35mA. If you notice that led heating then apply lower current ratings for that LED.

 

[rajaram04]

Make tests. Try to test diode on 3,4V at two currents 25mA and 35mA. If you notice that led heating then apply lower current ratings for that LED.

kk sir would yoy please explain it a lil.bit in brief . . how the procedure should be excecuted ? :grin:

 

[tpetar]

LED test explanation :

1. Set power source to 3,4V
2. Connect LED
3. Measure current with ampermeter


1. Set power source to 5V
2. Use resistor to limit current for LED forward voltage 3,4V first try 25mA then 35mA
3. Monitor heating of LED body and legs. If LED heats up then use lower current ratings for that LED.

 

[pranam77]

Every manufacturer specifies related technical details on the pack or pouch of every LED. In case you have an unknown LED, you may test it with the tester posted in other thread of yours. Hope you are opening multiple threads with same topics..

 

[rajaram04]

Every manufacturer specifies related technical details on the pack or pouch of every LED. In case you have an unknown LED, you may test it with the tester posted in other thread of yours. Hope you are opening multiple threads with same topics..

no its not same check again , every thread depends on diffrent issue
see here you just explained about pouch but here some electronic component dealer just asked whole sale shop for some 100 or 200 LEDs & they distribute in that way so its a new issue here & too people get puzzled replying in same thread for a very long time i watched so i don't want to irritate

also my work is to make full proof solutions to the end

 

[rajaram04]

LED test explanation :

1. Set power source to 3,4V
2. Connect LED
3. Measure current with ampermeter


1. Set power source to 5V
2. Use resistor to limit current for LED forward voltage 3,4V first try 25mA then 35mA
3. Monitor heating of LED body and legs. If LED heats up then use lower current ratings for that LED.

sir in second set of procedure you adviced with 5v to connect a resistor to limit the current
so whats the value i should choose . . please comment

 

[adi0803199]

The value of resistance that is chosen to limit the current supplied to the LED is dependent on the resistance of the LED that you are using which is determined by using the Ohms Law(V=I*R).You know the values of V(drop in the LED ) and the current drawn (from measurement).Apply the law, and you can arrive at the value of the resistance of the LED.Now to limit the current to say 25 mA,the resistor R(for denoting this unknown resistance) is added along with the resistance of the LED(R_led) as they are connected in series and the formula now is,

1)25 mA=5 V/(R_led+R)
2)35 mA=5 V/(R_led+R)

Then proceed with the steps 2 and 3 to determine the correct rating suitable for the LED.

 

[Audioguru]

You should NEVER apply a voltage source to an LED (3.4V and 5V were said) since nothing will limit the current.

An LED does not have a resistance since it is a diode. The value of the resistor is determined by (V-power supply - VLED (that you do not know) / the current that you also do not know.

 

[rajaram04]

The value of resistance that is chosen to limit the current supplied to the LED is dependent on the resistance of the LED that you are using which is determined by using the Ohms Law(V=I*R).You know the values of V(drop in the LED ) and the current drawn (from measurement).Apply the law, and you can arrive at the value of the resistance of the LED.Now to limit the current to say 25 mA,the resistor R(for denoting this unknown resistance) is added along with the resistance of the LED(R_led) as they are connected in series and the formula now is,

1)25 mA=5 V/(R_led+R)
2)35 mA=5 V/(R_led+R)

Then proceed with the steps 2 and 3 to determine the correct rating suitable for the LED.

ok got it but why we are assuming 25mA ? It could be more or less depending on type i guess ?

Too sir would you please tell me the very practical step say where to connect & to measure quantities by meter etc etc ??

- - - Updated - - -

You should NEVER apply a voltage source to an LED (3.4V and 5V were said) since nothing will limit the current.

An LED does not have a resistance since it is a diode. The value of the resistor is determined by (V-power supply - VLED (that you do not know) / the current that you also do not know.

okk hmm then how to know that unknown quantities as per your idea ?

 

[Audioguru]

ok got it but why we are assuming 25mA ? It could be more or less depending on type i guess ?

Most 3mm and 5mm diameter LEDs are spec'd with a current of 20mA. Many LEDs have a maximum allowed current of 30mA.

Too sir would you please tell me the very practical step say where to connect & to measure quantities by meter etc etc ??

To measure the forward voltage connect a 330 ohm resistor in series with an LED then measure the voltage across the LED. Red LEDs are about 2.0V, dim old green LEDs are about 2.2V, new very bright green LEDs, blue and white LEDs are about 3.3V.

then how to know that unknown quantities as per your idea ?

Simply buy name-brand parts that have a detailed datasheet.

 

[rajaram04]

Most 3mm and 5mm diameter LEDs are spec'd with a current of 20mA. Many LEDs have a maximum allowed current of 30mA.


To measure the forward voltage connect a 330 ohm resistor in series with an LED then measure the voltage across the LED. Red LEDs are about 2.0V, dim old green LEDs are about 2.2V, new very bright green LEDs, blue and white LEDs are about 3.3V.


Simply buy name-brand parts that have a detailed datasheet.

hmm actually i can buy that but problem is , i ve to buy it in whole sale , say a pack of 1000 to 2000 LEDs of same colour then only the provide a sealed packet on which everything is mentioned (i listned about it)
Too here all are not allowed to purchase that , only specified shopkeepers are allowed . . thats a big deal & irritating matter . .

Well lets come to the topic , you said 330 ohms resistor in series but with what voltage level ?????????

 

[Audioguru]

you said 330 ohms resistor in series but with what voltage level ?????????

Sorry, I forgot to say to use a new 9V alkaline battery. The 330 ohm resistor in series with an LED will limit the current to about 21mA for a red one and to about 17mA for a bright green, blue or white one.

A red LED could have a voltage from 1.7V to about 2.3V and the other colors can be from 3V to 4V.
When you know the actual voltage of your LEDs then you can calculate a certain current and resistor value.

 

[rajaram04]

kk let me proceed with this one

 

[rajaram04]

Sorry, I forgot to say to use a new 9V alkaline battery. The 330 ohm resistor in series with an LED will limit the current to about 21mA for a red one and to about 17mA for a bright green, blue or white one.

A red LED could have a voltage from 1.7V to about 2.3V and the other colors can be from 3V to 4V.
When you know the actual voltage of your LEDs then you can calculate a certain current and resistor value.

okk well why you said alkaline battery ?

 

[Audioguru]

okk well why you said alkaline battery ?

Because a 9V alkaline battery is modern, has a long shelf life and has fairly good capacity.
Maybe you want to use an old carbon-zinc or "Super Heavy Duty" 9V battery designed 60 years ago with a short shelf life and very low capacity?

 

[rajaram04]

Because a 9V alkaline battery is modern, has a long shelf life and has fairly good capacity.
Maybe you want to use an old carbon-zinc or "Super Heavy Duty" 9V battery designed 60 years ago with a short shelf life and very low capacity?

naa naa no compromise with power supply atleast . . surely i turn the way of practice in that manner