LEDs in parallel ..is it acceptable?

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treez

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Companies such as Forge Europa are producing LED Lights comprising LEDs in parallel without any series limiting resistors being used.
Is this ok?
Do you believe the contents of the attached report evaluates the situation correctly?
 

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  • Parallel LEDs evaluation.doc
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Are you sure that those LEDs don't have series resistors inbuilt in them?
 
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For some funny applications LEDs are used without a series resistor, like in keychain fed from a lithium 3V cell.
Do not attempt it for a serious application! LEDs have slightly different forward voltages, and connecting them in parallel even with a common drop resistor results in an overload of one while the other stays cool.
LEDs require a current limiting device be it a resistor or a PWM electronic current source. Follow good advice by manufacturers.
 
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I won't be surprised knowing someday that, for specific big buyers, a series resistor (or alike) is integrated in each of their LEDs (or in some of them) to ease the production of their products. Obviously, the resistance of the internal series resistor is made suitable for specific applications only.
 
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if the current source is used for every series led string then indeed it isn't a problem.

If a single current source is used to supply an entire series/parallel array with no limiting resistors or circuitry then it is a problem.

Adding series resistance increases inefficiency, and LEDs are already struggling to overtake fluorescents in terms of efficiency.
 
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For current limiting purpose only resistor is used... In your docs all the LED's are powered by current sources... So no problem at all...
 
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in the doc, not all the leds are individually powered by a current source.

Hi every LED will share the current unless mixing type of LEDs.....
Click to expand...
..do you have literature from a LED foundry actually supporting that statement?
 

treez said:
in the doc, not all the leds are individually powered by a current source.
Click to expand...

Yes... you are right... For parallel connected LEDs you need current limiting resisters...
For that you can use some special LED which are expensive...
All that they have given in the doc itself...
 
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treez said:
do you have literature from a LED foundry actually supporting that statement?
Click to expand...

Hi assuming they are Identical electronic components, you can connect them parallel because the knee voltage of LEDs will be nearly same for a same kind of LEDs so there wont be much difference in the current dividing... I have seen many places where the LEDs are connected parallel without resistors..
But its still good considering about it, I don know how many designers considered about it...
 
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The problem is the positive feedback effect of temperature on the current distribution among the LEDs connected in parallel.
It is the same in case of parallel diodes and bipolar transistors.
Fortunately, the ON resistance of MOSFET increases with temperature and this gives a negative feedback effect instead.
 
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Obviously, some LED lighting products are using LEDs in parallel circuit. The disadvantages and possible prerequisites (e.g. Vf binning) have been discussed in various previous threads. At the end of the day, it's a pure technical decision if the solution is acceptable for a specific design.
 
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jiripolivka said:
For some funny applications LEDs are used without a series resistor, like in keychain fed from a lithium 3V cell.
Click to expand...

You'll find that these cells have series resistances in the several ohms range. So, in a way, they act as current limiter in that way
 
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If I have to design a product that has several LEDs of a similar type to be lite together, I drive them (connected in series) with a suitable small boost inverter (SMPS) that regulates the LED current as 20mA, 75mA or 150mA... etc.
 
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well, I wrote to Philips and asked if they had leds that could be paralleled without limiting resistors.
For some reason, I was answered by someone from futureelectronics.com, and he said that he really did not recommend the paralleling-without series -resistance of hi power leds of philips and LGIT. He also said that mid power leds (50-150mA) of Philips and LGIT did get paralleled by some people, but he recommended against doing this.
 
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Thank you for your valid info @treez...
 
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