LEDs in parallel offer better value for money?

[ronv]

It's not hard. Try it with the LEDs you want to use.

 

[ChrisP58]

Thanks ronv

near the end of the article you pointed out...it says

You should include a current regulator in every parallel branch.

....this is ambiguous, do they mean a current regulator for each series string which make up the parallel branches, or do they mean a current regulator for a "bank" of four parallel strings?

Interesting that the advice to use multiple current regulators, comes from National semi, who make and sell....yes, you've guessed it, current regulators for LEDs.

You really should quote the rest of the paragraph.

You should include a current regulator in every parallel branch. A ballast resistor may suffice for some applications, and you can use a linear-regulator-current sink/source for others, but a switching regulator is the best choice for greatest power efficiency and flexibility.

Please note that the author first mentions the use of a resistor as a current control element, before talking about active methods. And even then, does not say that National (now TI) makes such devices. He leaves the decision of how to do it, and from whom to buy the parts, up to the project engineer.

 

[Flyback]

true, but i think we all see the merry-go*round that LED lighting is on here.....some saying that parallel chains of leds can be used without individual string current control, and some that individual current regulation is needed in each string.

There is no unified answer and yet nobody is "prooving" why LEDs in parallel with no extra reg in each string is OK.

 

[kinarfi]

Proof, what do you want, I have several sets of 4 3watt LED strings burning in various places, a single set of 4 over my bench for lighting, my lights on my off road vehicle - 2 light with with 6 strings of 4, all operating from 12 to 14 Volt supplies.
Would you like to come see them for your self?
Kinarfi

 

[Mr RB]

...i'm sure you realise that high Vf does not mean "suitability" for paralleling
...

No it means "HIGHER suitability for paralleling". If a voltage regulator has Vin regulation stable within 5mV Vout, white LEDs that have a high Vf and soft knee will go zero to full brightness over about 0.4v, so that's 3.0 to 3.4v and paralleling at say 3.3v will be fine.

With an old red LED the range 0-max might be 1.1v to 1.25v, the Vf is much lower and knee is much harder. Running a heap of these in parallel from a Vreg supply with 5mV is not so good.

You need to realise the high Vf and especially the "soft knee" give a very similar result to having a small series resistor inside each LED. manufacturers can take advantage of this, and they do take advantage of it.

You also argue that "nobody knows" if they match LEDs, I'm saying based on common sense they probably do not, because;
1. You can buy LEDs in large batches that have quite closely matching Vf, I test LEDs when I buy large batches. (You can also buy ebay rejects from the manufacturer that have massively varying Vf, sold off cheap to the hobby market).
2. Manufacturers use parallel white LEDs because they CAN because of the high Vf and soft knee, and it saves parts, PCB space, energy etc, all in line wiht making a cheaper product. They don't parallel LEDs to make a product cheaper, but spend unneccessary $$ on labout to match 100 thousand LEDs up!

As far as the soft knee on white LEDs goes, it would not surprise me if this is a deliberate feature built into the silicon, ie a small series resistor. If you test white LEDs some brands have a slightly harder knee and some (usually newer types) have quite a soft knee. A manufacturer that makes LEDs that can be more safely paralleled for the massive market of white lighting would be a preferred supplier to a lighting manufacturer, so common sense again hints there could be something to that idea of deliberately matching Vfs at LED manufature and deliberately designing a particularly soft knee.

 

[Mr RB]

(edit) Sorry, duplicate post.

 

[Flyback]

Proof, what do you want, I have several sets of 4 3watt LED strings burning in various places, a single set of 4 over my bench for lighting, my lights on my off road vehicle - 2 light with with 6 strings of 4, all operating from 12 to 14 Volt supplies.
Would you like to come see them for your self?

..but they may not be failing because the heatsinking is over-sized and overly expensive?
..or you may just have had "lucky" leds with matching Vfs
..Maybe you were very scrupulous in ensuring close thermal coupling between leds...something thats too expensive to do in a production environment.

..you have tested your leds and shown them to not be suffering the ills common to parallel leds...but in a production environment, its too expensive to extensively soak test all the individual products off the production line.

 

[kinarfi]

..but they may not be failing because the heatsinking is over-sized and overly expensive?
..or you may just have had "lucky" leds with matching Vfs
..Maybe you were very scrupulous in ensuring close thermal coupling between leds...something thats too expensive to do in a production environment.

..you have tested your leds and shown them to not be suffering the ills common to parallel leds...but in a production environment, its too expensive to extensively soak test all the individual products off the production line.

The 3 watt LEDs I'm using are rated at 3.6V a string of 4 would be rated at 14.4, my max voltage is 14.2, I'm just running them slightly under specs, none of that other stuff, I'm messing around at home with these and I like the 3 watt LED because of the available lenses for them.
I guess what I'm saying if that if you take 500 3.6 volt LEDs and run the in parallel at 3.5 volts, they should work fine, just not as bright as they would at 3.6, I'm getting away with running them at 3.55v/LED, maybe it is just luck, but they work!!! , But I think I need a better heat sink, even with a fan inside that shell, there is no way I can get rid of all that heat.
Kinarfi

 

[audioguru]

The 3 watt LEDs I'm using are rated at 3.6V a string of 4 would be rated at 14.4, my max voltage is 14.2, I'm just running them slightly under specs

No.
A zener diode has a voltage spec, an LED has a range of voltage. Your "3.6V" LEDs might be 3.2V (very bright and will burn out soon) or 4.0V and be dim or will not light.

 

[kinarfi]

You say potautoe, I say potato, I still have 12 strings of 4 3 watt LEDs with no resistor on my vehicle that light up when the power is applied, dim slightly when engine is off or idle and brighten up as the engine revs up and one over my bench that take a bit more abuse because it is well heat sinked and fan cooled.

 

[ChrisP58]

You say potautoe, I say potato, I still have 12 strings of 4 3 watt LEDs with no resistor on my vehicle that light up when the power is applied, dim slightly when engine is off or idle and brighten up as the engine revs up and one over my bench that take a bit more abuse because it is well heat sinked and fan cooled.

The fact that the brightness changes with a change in voltage tells me that there is something in the system with resistive characteristics. There may not be an actual 'resistor' but there is parasitic resistance in the PCB traces and other interconnects. And due to the temperature coefficient of copper, the resistance grows quite a bit when the system gets hot.

Even a few tenths of an ohm can be enough to balance strings of LEDs that start out closely, but not perfectly, matched.

 

[Mr RB]

That's what I have been saying using the term "soft knee", white LEDs have quite a large range of voltage where they go from zero current to max current, in a lot of ways they behave like there is a small series resistance in the LED itself.

Some of that might be the little gold leadwires inside the package giving significant resistance, especially with higher current LEDs. Some might be deliberately designed into LEDs designed for lighting use, although that's just speculation.

 

[kinarfi]

What Ever! the reason for the dimming is the alternator puts out 14.1 volts when a little bit above idle and when the engine is off, the battery puts out 13.6 and decays from there until I start the engine again and the alternator is running every thing with the higher voltage.
Because I was concerned about heat, I put the light on my work bench and powered it up with 14.1 volts, measured at the plug with a Fluke, and inserted a thermocouple inside the light and let it run for around an hour with a 4" muffin fan blowing air across the light from about 2 feet away, the heat got up to almost 90° C, so I put it back on the vehicle.
Kinarfi