L.e.d. combination problem with red one

[rajaram04]

Hello sir

i have a problem with l.e.ds combination

if i connect white green & blue l.e.d. parallely to any cell say a coin , a 3.6v , a AAA size 3 volts supply it is ok as all glow without effecting negligbly each other but when i introduce a red l.e.d. with this combination in same parallel way then only red glows & all fail :-(

why so happen how to over come ??

red l.e.d. with any other single l.e.d do not allow to glow up that

how to glow wach of them with same power with any cell source ?

 

[seeker13]

Hi rajaram,

if I understood correctly, you connected 3 LEDs in parallel directly to the voltage source, without any balast resistors. If this is true, you just need to connect 1 resistor in series with each LED, with the approximate value R=(V-Vd)/I where V is the supply voltage, Vd is LED knee voltage and I is the desired LED current.

 

[upand_at_them]

Red LEDs have the lowest forward voltage.

 

[rajaram04]

Hi rajaram,

if I understood correctly, you connected 3 LEDs in parallel directly to the voltage source, without any balast resistors. If this is true, you just need to connect 1 resistor in series with each LED, with the approximate value R=(V-Vd)/I where V is the supply voltage, Vd is LED knee voltage and I is the desired LED current.

ohh okk i see , hmm then how to determine the knee voltage & desired l.e.d. current ??
whats the theoritical procedure

 

[betwixt]

Start with a voltage higher than the anticipated Vf, I suggest 5V for a normal LED. Connect a resistor in series with the LED, I suggest 1K. The LED should light, then measure the voltage across it. This is Vf at that current. Note that LEDs are not perfect constant voltage devices, they have a dynamic resistance which varies with current so at a higher current you will find Vf is slightly higher. The figure you are trying to find is Vf measured at the current you intend to use. In a data sheet you will note that it tells you what current was used for the measurement.

Brian.

 

[rajaram04]

Start with a voltage higher than the anticipated Vf, I suggest 5V for a normal LED. Connect a resistor in series with the LED, I suggest 1K. The LED should light, then measure the voltage across it. This is Vf at that current. Note that LEDs are not perfect constant voltage devices, they have a dynamic resistance which varies with current so at a higher current you will find Vf is slightly higher. The figure you are trying to find is Vf measured at the current you intend to use. In a data sheet you will note that it tells you what current was used for the measurement.

Brian.

hmm kk but is the data sheet available on internet are all same for red l.e.ds available in market ?

 

[betwixt]

No, almost every data sheet will tell you a different value but for each color they are usually close together. The chemical difference in the LED to produce each color gives a different Vf, red is generally lower than other colors but it varies from one type of LED to another and there is some cross-over between the ranges of color and Vf. Even LEDs of the same type have slight differences, that's why you can sometimes buy them 'binned' according to brightness or Vf.

Brian.

 

[rajaram04]

No, almost every data sheet will tell you a different value but for each color they are usually close together. The chemical difference in the LED to produce each color gives a different Vf, red is generally lower than other colors but it varies from one type of LED to another and there is some cross-over between the ranges of color and Vf. Even LEDs of the same type have slight differences, that's why you can sometimes buy them 'binned' according to brightness or Vf.

Brian.

ya okk in post 3 too i got an info about red LED forward voltage & i agree with past days practicals

so i found a link on LEDs while googling a few hours ago

https://electronicsclub.info/leds.htm

please check about the chart detail showing downside in the page

what about it ? should i follow ?

 

[seeker13]

Sure, the table data seems reasonable. As betwixt told you, the most reliable way is to measure it yourself (quite easy, as you can see) because there are always variations in V-I characteristics between individual diodes. However, those are slight, and the resistance calculation is almost always approximate. So you can use only estimates such as those given in the table.
If you don't want to bother with the measurements and want just a quick fix, you can simply connect resistor R=80..160 ohm in series with the red LED, and then connect this series connection in parallel with LEDs that you say work ok. I used this value assuming battery V=3.6 V, red LED forward (or knee) voltage approximately Vf=2V and current between 10..20mA.

I underline that if you use larger battery voltage (higher then 4V), you should connect resistors in series with other LEDs as well, otherwise they'll probably burn up. The reason why it works now without resistor is that battery voltage is actually slightly below their forward voltage, so no significant current flows. However, if you add another 1.2V battery (therefore, total bat voltage of 4.8V), the current through LEDs will rapidly increase, they will get hot and probably burn up.

You can find details on this for example here: http://letsmakerobots.com/node/4948 and here: http://www.evilmadscientist.com/2012/resistors-for-leds/

 

[Audioguru]

The chart is wrong.
A dim OLD green LED was 2.2V but a modern very bright green LED is made from the same material as a blue or white one. Their voltage is from 3.2V to 3.6V.