Leds In Parallel Are Better?, cheaper driver stages?

Hello,

Recent advances in LED technology mean its now easier to get the forward voltages of batches of LEDs very similar. Thus the LEDs can be placed in parallel, as long as thermal coupling is good.
Since the switch mode drivers for LEDs in parallel can be made much cheaper then those drivers for long series chains of LEDs, you generally get more value for money with parallel LEDs.

So do you know of off-the-shelf LED luminaires which contain parallel LEDs? I wish to buy them.

(-I am referring to LEDs in parallel but with no series resistors.....these now not being needed with modern, equal Vf LEDs)

Not again...

Actually, I'm gonna reply to this. I'd very much like to avoid that this thread shows up on Google results, folks reading this and go "oh, hey, cool, I can place LEDs in parallel without series resistors". So I'll add a disclamer.

A majority of this board tends to agree this is not a good design practice and ussualy will result in a very bad design or a lot of smoke. Some arguments have been made that under certain conditions and if certain parameters are well understood, characterised and measured you *might* get away with it. Seasoned engineers and hobbyist on this board have indicated they do not place enough confidence in this method or their understanding of it to implement it in any of their designs. As such, it would be wise not to attempt this in any of yours.

For the record: Treez, I object to your statement these now not being needed with modern, equal Vf LEDs

I agree with Ice-Tea, this topic has been beaten to death.

Agreed...

I Problem with this type of circuit design is that you get a cascading failure effect, If one LED fail's, the rest pickup the excess current, and excess current causes LED failure...

The first LED failure may not set off this cascade, but after a few the entire lot of LEDS will fry....

A good example is bipolar technology.... it is so mature, it qualifies for retirement benefits ( ), but there is still enough device-to-device tolerances that you can't parallel them without letting the smoke out eventually.

Agreed...

I Problem with this type of circuit design is that you get a cascading failure effect, If one LED fail's, the rest pickup the excess current, and excess current causes LED failure...

The first LED failure may not set off this cascade, but after a few the entire lot of LEDS will fry....

Not if you're driving them with constant voltage.
I don't think I've ever fried a 5mm led that didn't deserve it...

what MG said ...
"not if you are driving them with constant voltage"

but again , the want of deeper understanding rears it's head .
and that is the gist of the clarification that ice-tea wrote .

..if you look at the SD24 flasher LED product, you can see that this has only 1206 size series resistors in each paralleled string of leds................1206 would not normally have enough power dissipation, but because we now have " modern, equal Vf LEDs ", 1206 size is fine.

So thats what i mean here.

The SD24 is a parallel led product with enormous sales volumes, and close to zero failure rate...............i think this qualifies my point about "modern, equal Vf LEDs"

...by the way, the SD24 has a miniature switch mode driver, with a cheap OTS inductor, -NOW just try putting the 24 leds of SD24 in series and doing a switch mode driver anywhere near as small and cheap as whats in the SD24.


As discussed, SD24 "does" have series resistors in each LED string (3 strings of eight leds in series, in prallel) , but due to the superb Vf matching of the LEDs, these resistors are OK being just 1206 size......in days gone by, these resistors would have needed to be 2 Watt axials to allow the paralleling of the LEDs.

The SD24 prooves my point, and qualifies everything i have said.

SD24 epitomises academic piousness versus entreprenurial initiative, and the academic piousness comes out second.

Series chains of LEDs.............not any more......now do paralleled LED strings, with conveniently small equalisation resistors (eg 1206 size)

So, the product you bring forward to proove series resitors are not required has series resistors?

*claps hands*

wut

You don't say anything of the value of the 1206 resistors. Obviously they are not normal current regulating resistors, but only very small valued resistors put in to regulate exactly the small differences in Vf for each string of LEDs. This is a very good design practice, and is in no way against what's been said in the thread. We are talking about mV values here and the resistors will normally be in the sub 1ohm area, with little power loss.

The controller is putting out a current regulated voltage, and the value of the resistors is only big enough to compensate for the variation in voltages in the batch of LEDs, to distribute the current evenly between the 3 strings of LEDs, and not as a normal voltage drop resistance to generate the correct current.

TOK

...the resistors are 10R, 1206.

With 150mA in each res, thats 225mW in each 1206......Hmmmmmm

Why are you dropping 1.5V over the resistor? Makes little sense if it's only to allign the Vf differences.

TOK

aligning the vf differences is precisely why ....at least, i presume this.