# [Résolu] How to place resistor with multiple leds

Hi,

When i use multiple leds, is it possible to place one only resistor on the source (VVC) (see attachement) ?
I know i have normally to place a resistor for each led.

Yvan

Yes, on the condition that ALL the leds are IN SERIES.

LEDs in parallel is sort of ok. Assuming one LED can take most or all the current.
If all LEDs are the same colour and from the same batch, then individual current/brightness is about the same.

But...
Different coloured LEDs have different working voltages, so if you put a red LED and a blue LED in parallel,
then only the red one with a lower Vf (working voltage) will be on.

Another problem, but mostly noticed with power LEDs, is that Vf is temp dependent.
A 'hotter' LED has a lower Vf, and draws more current than it's neighbour, making it hotter, drawing more current, ending up using most of the current that could be meant for two or more LEDs.

Resistors are not expensive. It's usually better to use one for each LED.
Leo..

The illustration shows the LEDs in parallel.

So the answer is NO, unless ALL LEDs are IN SERIES.

And of course you need to do the correct calculations to find the right value of the resistor.

MAS3:
So the answer is NO, unless ALL LEDs are IN SERIES.

The question was if it is possible. The answer to that is YES.

I think I have covered all the pitfalls if you do so.
Leo..

Well… literally and technically speaking… it is possible to do anything. But let’s assume the idea is to achieve good results. In that case … one method is a series arrangement… provided that the voltage source can deliver the required voltage and current. Otherwise … as somebody mentioned… 1 resistor for each led… with resistor and led forming a branch… with other branches in parallel.

BTW: OP, if you want to learn how to embed an image, like this, check out: How to Embed An Image

The short answer is, yes this will “work”, assuming all of those LEDs are the same type [i.e. part number]. But, it’s not advised. Why? The concise answer is: "Because of the exponential relationship between current and voltage, in the forward bias characteristics of an LED. Small variations in the voltage will cause large variations in the current. Thus, you have the case where one of the LEDs, with a forward voltage even slightly smaller than the others, will cause much more of the current to flow through it, and as a result, less current in the others. And, in fact, it can be the case that one of the LEDs can hog enough current to exceed its MAX and fry!

So, the usual solution is to but a series resistor on each LED. That way, each LED has its own “current regulator” [the world’s worst current regulator! But, it does the trick–with the least efficiency–but not a big deal when dealing with a typical 20mA-or-less LED–on high power LEDs this definitely becomes an issue, and a Switch Mode Current Regulator is a much better solution].

For more in depth coverage of this subject, check out my Blog: http://www.sinistercircuits.com/Blog/LEDs_LightemWithoutSmokinem/index.php

It's perfectly clear...
See you.

Southpark:
Well... literally and technically speaking.... it is possible to do anything.

Reminds me of the George Carlin joke:
"If God is all powerful, can he create a rock so big that he, himself, can't lift it?"

ReverseEMF:
Reminds me of the George Carlin joke:
"If God is all powerful, can he create a rock so big that he, himself, can't lift it?"

Haha! True. When I meant 'possible'.... yes.... sure it is 'possible' to physically put a resistor there. Whether the goal or aim is achieved or not is another matter. Hence the assumption of putting a suitable resistor there is supposed to fulfil the aim of having the led system functioning.

LEDs in parallel is more commonly used than you might think.
Those ultra-flat downlights you see on ebay all have five 0.2watt LEDs in parallel.
And use more of less of those "groups of five" (3-12), depending on downlight size.
All COB (ChipOnBoard) LEDs (spotlights) have LED strings in parallel e.g. in a 33, 66, or 10*10 array.
Leo..

Wawa:
All COB (ChipOnBoard) LEDs (spotlights) have LED strings in parallel e.g. in a 33, 66, or 10*10 array.

But of course, these always use chips from the same wafer/ batch so are essentially precisely matched, and mounted close together on a common heatsink. They essentially function as a single large uniform LED.

It's "possible" to jump off a 30m cliff...

Is it a good idea? Probably not.

Its always good to know why something is a "bad idea" before you commit to it.

pwillard:
It’s “possible” to jump off a 30m cliff…

Is it a good idea? Probably not.

Its always good to know why something is a “bad idea” before you commit to it.

You right !

Thank you to all of you for all your answers. Appreciate !

Regards.

Krayon