LEDs in parallel offer better value for money?

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.

Some people get ebay LEDs and they didn't share

There is another thread about 20 watt LEDs.
The factory selected 9 dies of the same voltage and put them in the same pack.

OK thanks, also i forgot to say that i mean putting LEDs in parallel with no series resistors which usually get used in this situation to equalise the parallel currents....these resistors not needed nowadays because of modern, equal Vf LEDs

Where did you read this?

If you use careful selection of LEDs and use 3 or more in parallel along with a constant current driver you can often use them in parallel without additional resistors, but to just try to use them in parallel with a constant voltage is still a problem.

At the factory we sort LEDs into 5 types then parallel like parts. (parallel 3)

I have used a 1A current source to drive 4 strings of LEDs in parallel. BUT I added 4 resistors with only 0.2V across them. This helped equal out the current with out burning much wattage.

Where did you read this?

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...i assume it, from seeing the products of companies, which use parallel LEDs....take the SD24 product....this uses parallel LEDs, and is a superb product....it does have series resistors though, but only tiny 1206 ones.

If the LEDs are well matched to begin with, it won't take much resistance to accommodate any small variation that might remain. Also, the big OEM users can order LEDs pregrouped that are made from chips from the same wafer.

As far as whether power drive circuits for parallel vs series arrays are more economical, I think you are trying to over generalize the question. Every application needs to be evaluated on it's own terms. While your assumption may be true in some cases, it won't be true in all cases.

Flyback said:

...i assume it, from seeing the products of companies, which use parallel LEDs....take the SD24 product....this uses parallel LEDs, and is a superb product....it does have series resistors though, but only tiny 1206 ones.

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The "tiny 1206" resistors can have a large resistance value. The physical size of a resistor is unrelated to its resistance.

It's generally OK to parallel series strings of LED's - usually a minimum of four in series is enough to equalise the voltages out when the strings are put in parallel. Obviously the entire parallel/series block is fed from a constant current or similar.

I don't see that the OP's claim that switch-mode drivers would be cheaper if all LED's were just in parallel is true?, if there's any difference, I would imagine the opposite to be the case?.

Incidentally, I've just had to repair a ceiling panel light - it had 96 LED's, 48 on each opposite edge, shining sideways into a plastic diffuser. They were wired as four series strings of 12 in parallel on each side, then the two sides were wired in parallel again - so a total of 8 series strings of 12, wired in parallel.

The original fault was that one of the input wires had pulled the PCB pad off the tiny PCB, I cured this by running an extra wire down to the start of the next string of 12 (hence why I know the configuration). After putting it back together I noticed one corner kept going dark, so took it to pieces again and found a dry joint on one of the LED's.

Chinese quality - got to love it!

We also had three of the PSU's 'blow up' in the first few days as well - out of a total of 36 or so lights.

I own two LED flashlights. All white LEDs are wired in parallel, use no resistor, and are powered directly by 3 - 1.5V batteries in series. One has 21 LEDs driven by 3-AAcells. The other has 90 LEDs driven by 3-D cells. It's awesome.

And the LEDs were matched to work together. Seeing LEDs working in parallel does not indicate what it took to make that happen.

ChrisP58

As far as whether power drive circuits for parallel vs series arrays are more economical, I think you are trying to over generalize the question.

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and
Nigel Goodwin

I don't see that the OP's claim that switch-mode drivers would be cheaper if all LED's were just in parallel is true?, if there's any difference, I would imagine the opposite to be the case?.

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-sorry but i can assure all that when you give yourself the option of putting the LEDs in parallel, then you can allow yourself to arrange for the output voltage to be lower than the input voltage of the switch mode led driver....this in turn means that you can use a "current regulated hysteretic buck converter" to power the parallel strings, and i assure all, that this is a much cheaper and simpler way of driving LEDs than any other switch mode way.

crutschow

The "tiny 1206" resistors can have a large resistance value. The physical size of a resistor is unrelated to its resistance.

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..very true, though by "tiny" i was referring to the power level , which is 200mW for a 1206, but realistically, you would want to limit max 1206 power to ~110mW maximum.


Bob Scott

I own two LED flashlights. All white LEDs are wired in parallel, use no resistor, and are powered directly by 3 - 1.5V batteries in series. One has 21 LEDs driven by 3-AAcells. The other has 90 LEDs driven by 3-D cells.

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I would be very grateful if you could give me the brand and part number please?
...were there any series current equalisation resistors used?

Nigel Goodwin

Incidentally, I've just had to repair a ceiling panel light - it had 96 LED's, 48 on each opposite edge, shining sideways into a plastic diffuser. They were wired as four series strings of 12 in parallel on each side, then the two sides were wired in parallel again - so a total of 8 series strings of 12, wired in parallel.

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...I'd be really grateful if you could provide the brand and part number for this parallel LED light?
...were there any series current equalisation resistors used?

Flyback said:

-sorry but i can assure all that when you give yourself the option of putting the LEDs in parallel, then you can allow yourself to arrange for the output voltage to be lower than the input voltage of the switch mode led driver....this in turn means that you can use a "current regulated hysteretic buck converter" to power the parallel strings, and i assure all, that this is a much cheaper and simpler way of driving LEDs than any other switch mode way.

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You've now introduced an entirely new restriction - that the supply is too low to feed a series chain - you never mentioned that previously.

...I'd be really grateful if you could provide the brand and part number for this parallel LED light?

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They were similar to these:

...were there any series current equalisation resistors used?

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No resistors at all, none needed with the paralleled series chains as current sharing isn't a problem, and the lights are fed from a constant current PSU (1A at around 42V).

The "trick" is in the constant current supply. That way the LEDs don't run away due to a nominal mismatch.

ronv said:

The "trick" is in the constant current supply. That way the LEDs don't run away due to a nominal mismatch.

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No, the 'trick' is in the series chains, which balance out any variations. For single LED's in parallel the 'trick' is in very closely matched LED's. The constant current source has nothing to do with it, it's simply a superior method to a current limiting resistor.

the 'trick' is in very closely matched LED's.

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..do you think that these closely matched leds are much more expensive than just standard off-the-shelf LEDs?

the 'trick' is in the series chains, which balance out any variations.

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..so you are saying that by the laws of probability, the total forward voltage of each series string is likely to be the same?

If so, then do you think that relying on laws of probability in this way is acceptable?

Flyback said:

..do you think that these closely matched leds are much more expensive than just standard off-the-shelf LEDs?

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No, but unlikely to be available apart from to manufacturers in large quantities - but unless you're making LED torches you don't need them anyway.

..so you are saying that by the laws of probability, the total forward voltage of each series string is likely to be the same?

If so, then do you think that relying on laws of probability in this way is acceptable?

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No, I'm asserting that the LED manufacturers produced application notes showing how it's perfectly OK to use them in that way - as far as I can remember (it's been a few years since I read one) putting three in series was enough to allow paralleling of the series chains.

LED manufacturers produced application notes showing how it's perfectly OK to use them in that way - as far as I can remember (it's been a few years since I read one) putting three in series was enough to allow paralleling of the series chains.

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...and you are saying that its only the LEDs that are available to manufacturers in large quantities that can be used in this way? (3 in series chains, paralleled up)

Also, do you have one of these app notes we could look at?

I wasnt sure if you meant its ok to parallel chains of 3-in-series of any led, and whether or not you meant that paralleling the "3-chains" can be done without using a series resistor in each "3-chain"?

hi,
If you manufacture enough 'lamps' the supplier of the LED's will supply you with batches of matched forward voltage LED's.

In that way you can use lamps made from paralleled matched LED's.