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Colin Matten

LED Light Interference

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This discussion was about RF interference from LEDs.

The Jayco light is a very bright one which is connected to the caravan 12 V "battery" supply via a bridge rectifier, making the power supply almost identical to one run from the mains.

If you look at the Jayco light it is hardly a torch, its 24 LEDs on a board.

Now it is proven that the LED is not the source but the high speed switching to limit the current.

James you can suggest all you like, but as soon as you use a parallel string, it is the same problem as a pair of LEDs in parallel. Each string will now have different currents. It all depends on the number of LEDs involved, and the voltage of the power supply.

The characteristics of parallel and series circuits will never change. In simple circuits resistors are used to limit the current which will also produce no RFI. You can also use transistors to be analog current limiters which will also produce no RFI. It is only when you use switching current limiters you get interference.

Perhaps a simpler reference might be better http://www.instructables.com/id/LEDs-for-Beginners/step8/Wiring-up-multiple-LEDs-in-parallel/

 

Alanh

 

 

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Dear Reader

It really should be easier than this but I am a half glass full person and so I will treat this as an opportunity to see where it first went awry for the poster above using quotes from his contributions.

" This discussion was about RF interference from LEDs." 

Well actually it is not, the original discussion is about RF interference from LED lighting which of course includes the LED itself and all the associated circuitry for operation from its power source. Unfortunately and quite inexplicably, despite CMatten, Malich and others clearly knowing the interference was a result of the regulation circuitry not the LED itself, the poster above decided to declare that the interference was not from the LED itself and cited that a LED torch operating from DC created no interference as ultimate proof. Since the poster above was apparently unaware that all LED lighting, be it DC or AC has switching regulation it sort of made his explanation and proof just a little silly.

 

"James you can suggest all you like, but as soon as you use a parallel string, it is the same problem as a pair of LEDs in parallel. Each string will now have different currents. It all depends on the number of LEDs involved, and the voltage of the power supply."  

Not choosing to be too critical but several here have attempted the have the poster above understand that these are current driven devices.  By having a current limiter on each string of seriesed LED's you can parallel as many of these strings as you choose, the beauty of this is that the power supply voltage can vary all over the place so long as the minimum voltage is greater than than required by the sum of the breakdown voltages of the LED's in the series plus the voltage drop of the regulator. Of course the power supply should not be permitted to go so high that the voltage rating of the regulator is exceeded.

In essence if we go back to CMattens light which I understand is 12vdc, it should be designed to operate from 10-16vdc quite happily. Regarding the poster above's statement on problem paralleling, I dare venture to say that CMatten is likely to have a second light in his caravan and depending on the level of opulence possibly a third, fourth or fifth. I believe that I can state on CMatten's behalf that he does not cringe in anticipation of switching on the second, third, fourth light in the van in fear of it disturbing the first light. In case the penny still hasn't dropped for the poster above, oh yes, turning more than one light on in the caravan is paralleling seriesed LED lighting.

The poster above then notes this " The characteristics of parallel and series circuits will never change. In simple circuits resistors are used to limit the current which will also produce no RFI. You can also use transistors to be analog current limiters which will also produce no RFI. It is only when you use switching current limiters you get interference."  BINGO !!! Dear reader there is light at the end of the tunnel, the poster above has something right.

 

Finally after the burst of light just above (excuse the pun on LED lighting) this is stated"

" Perhaps a simpler reference might be better http://www.instructables.com/id/LEDs-for-Beginners/step8/Wiring-up-multiple-LEDs-in-parallel/ "

We can only hope so, no doubt the poster above will let us know.

James

 

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Guest Malich

OK...

So CMatten was kind enough to send me the part # & link to the datasheet of the chip on his LED light (he actually did that a couple of weeks ago, but I was on a work trip to Perth & then on holidays & mostly offline). Turns out it is a buck converter / constant current driver - a type I haven't seen before in this sort of lamp (as I said all have been either simple dropper resistor setups, or boost converter / constant current drivers). So Alan was at least partly correct.

I'm stilll not sure how or why Alan came to the conclusion that anyone was claiming the interference came from the LEDs themselves. As far as I can see, no-one - neither myself nor anyone else - has said anything like that. If somehow anyone did get that idea from what I wrote, I apologise - I asumed it was obvious and self-evident that the interference comes from the switching regulator driving the LEDs, not the LEDs themselves.

But on to a couple of remaining specifics:

On 10/1/2017 at 2:21 AM, alanh said:

I don't want one LED failure to stop all the LEDs. This was the problem with fillament Christmas tree lights which were all in series.

Then you'll be absolutely shocked to learn that in the vast majority of cases - battery powered or mains, doesn't matter - they are in fact connected in series strings (with multiple series strings in parallel). There's some obvious reasons for that, which I won't go in to - anyone with a basic understanding of electronics should be able to figure them out.

For safety-critical / high reliability applications (e.g. vehicle brake lamps, emeergency lighting, etc), they often use shunt protection devices - as described here, or simlar devices from Littelfuse, ON Semi, etc. - across individual (or small groups of) LEDs. They ensure the rest of the series string continues to operate if an individual LED fails.

 

On 11/1/2017 at 4:12 PM, alanh said:

I know Instructables are usually pretty terrible, or at least simplistic to the point of facile. That one borders on useless when it comes to explaining LEDs...

(Ah, I've figured it out now. It's the first result when you Google "wiring LEDs"...)

Edited by Malich
Update, in blue.

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Malich,

Apology accepted.

The shunt protection article is 5 years old. Its disadvantage is that it make the other LEDs in the string more vunerable to failure.

The way to reduce the current variation in each LED junction is to make them all on the same substrate, using SMT. So all junctions will be at the same temperature and created with the same doping concentration. If not integrating a constant current source for each LED is also possible. Consider the huge large LED displays at entertainment venues which have around 8 million LEDs.

Alanh

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Guest Malich
31 minutes ago, alanh said:

Malich,

Apology accepted.

Thank you. You still owe everyone quite a few though...

 

32 minutes ago, alanh said:

The shunt protection article is 5 years old. Its disadvantage is that it make the other LEDs in the string more vunerable to failure.

I only posted the article for you as an introduction to the kind of devices used in high-reliability applications. You might like to follow up the manufacturer pointers I gave & read other, more detailed documentation to understand why they don't "make the other LEDs in the string more vunerable to failure" (hint: the point has been made repeatedly in this thread that LEDs are current-driven devices and, in such applications, should be fed by a proper constant-current source...)

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On 14/01/2017 at 11:48 AM, Malich said:

If somehow anyone did get that idea from what I wrote, I apologise - I assumed it was obvious and self-evident that the interference comes from the switching regulator driving the LEDs, not the LEDs themselves

I keep being deleted, puzzling, is it the terminology used? Should I write this a different way? Here goes....

Malich, your apology evoked a single acceptance, you must have been quite surprised.

James

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Guest Malich
13 minutes ago, James T Kirk said:
On 14/1/2017 at 11:48 AM, Malich said:

If somehow anyone did get that idea from what I wrote, I apologise - I asumed it was obvious and self-evident that the interference comes from the switching regulator driving the LEDs, not the LEDs themselves.

I keep being deleted, puzzling, is it the terminology used? Should I write this a different way? Here goes....

Malich, your apology evoked a single acceptance, you must have been quite surprised.

James

Yes, I was ;)

Edited by Malich

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