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Colin Matten

LED Light Interference

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Things are very quiet here these days. I thought I'd post some stuff that is sort of relevant and may be interest.

My Jayco caravan came with LED lights that makes the FM /TV 6M, 2M and 70cm Ham bands (50,144 and 430MHz) unusable. I now a combination of incandescent and LED lights in it.

Purple = Peak / Max Hold
Yellow = LED Turned Off

Jayco LED Mag Probe.jpg

 

I checked out the LED light using my Spectrum Analyser with my poor mans Officeworks probes ;-) Probes M and E.JPG

DIY M and E Probes.
M Probe is as per the bottom Left using semi rigid coax.
E Probe is as per the bottom top right using RG174 (?)
emcprobes.jpg

 

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Wow, DC to daylight!

Don't appear to be too many suppression components on that circuit board. And fairly regularly spaced peaks (harmonics). Maybe there's room for improvement with a bit of simple RFI suppression.

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Guest Malich

The tick system is designed useless. Doubly so for anything that falls under series 6 or 7 (household appliances & lighting; AS/NZS CISPR 14.1 & 15). Those basically amount to:

  • Creates interference <30MHz? Don't care.
  • Creates interference >30MHz? Don't care much...

It's quite possible that these lights actually meet the standards, particularly since they're not mains-powered (so EN61000-3-2 & 3 don't apply).

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The switchmode power supplies are the source of the interference and they are connected to the power mains. If the filtering is inadequate can feed interference back into the mains as well as  feeding it into the wiring to tle LEDs. How much interference comes from an LED torch which is battery powered! None!

Alanh

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Guest Malich
On 22/12/2016 at 3:19 PM, alanh said:

The switchmode power supplies are the source of the interference and they are connected to the power mains. If the filtering is inadequate can feed interference back into the mains as well as  feeding it into the wiring to tle LEDs. How much interference comes from an LED torch which is battery powered! None!

Alanh

Spoken like a man who has never had to chase down & deal with RFI from a battery-powered lamp.

Yup, I'm sure the 12V T10 base Jayco caravan lamps Col has tested above are connected to the power mains, and aren't battery powered at all... </sarcasm>

(p.s. you obviously didn't bother to read your own links & the documents they refer to. If you had, you'd have known that the EMC standards AS/NZS CISPR 14.1 & 15 apply to both mains-powered and battery-powered appliances & lighting...)
 

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Juar shows how little you know of electronics.

LEDs themselves run at around 2 V depending on the colour. The 12V has to be reduced to that voltage and another switch mode power supply is used!

Alanh

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Guest Malich
On 25/12/2016 at 10:11 AM, alanh said:

Juar shows how little you know of electronics.

Hey, I know those things often use switchmode converters. After all, it wasn't me who said ...

On 22/12/2016 at 3:19 PM, alanh said:

How much interference comes from an LED torch which is battery powered! None!

That was you.

You seem to have a problem with remembering who said what - even when it was you who said it!

 

On 25/12/2016 at 10:11 AM, alanh said:

LEDs themselves run at around 2 V depending on the colour. The 12V has to be reduced to that voltage and another switch mode power supply is used!

Back in the real world, those little T10 panels (and many battery-powered LED lamps) fall into 2 types:

  • Ones with 12~15 LEDs, and dropper resistors feeding each parallel string of ~3~4 series-connected LEDs. Many of these also include a bridge rectifier so they work regardless of source polarity. These produce no RFI.
  • Ones with 15~16 LEDs, and a boost converter/constant current LED driver that boosts the 12v to ~18~38v (depending on the current - remember, LED's are current-driven devices, not voltage-driven). The boost converter usually incorporates it's own reverse-polarity handling. These produce anything from minimal to lots of RFI, depending on how cheap and nasty they are.

By both visual inspection of the back of the board shown and the EMI generated, CMatten's lamps are obviously of the latter type.

It's possible I guess, in theory, that someone makes them with a buck converter to reduce the 12v supply to ~2v like you say (implying that the LEDs are all wired in parallel). All I can say is I've never seen any like that in real life, have never seen any like that for sale anywhere, and an extensive search online has found none. They're all either the dropper resistor or boost converter types I outlined above.

Oh, and I think you'll also find most white LEDs - either RGB triplets or phosphor-converted - run around 3v~4v Vf, not 2v. So even LED torches - which are usually single or double cell (i.e. 1.5v or 3v)* or 18560 (i.e. ~3.7v) - also use a boost converter / constant current driver to boost the source voltage enough to overcome the LED Vf.

(* decent ones, that is - not the chinese cheapies that use 3 or 4 AAA or AA cells in series...)

Looking forward to what bit of your technical ignorance you'll expose in your next post or comment...

Edited by Malich
(clarification, in blue)

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Wow, I think he got you there alanh, you did indeed claim no interference comes from a battery powered LED torch

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Not at all,

LEDs themselves do not generate EMI as I stated, after they are a light emitting Diode. The example Malich used uses a switching buck inverter which takes the 12 V DC and chops it up so that only a small percentage of the time the output is switched on, the result is fed in to a capacitor to average it out. It is the high speed switching which causes the interference. The above process is identical for mains powered LEDs as well, but with the addition of a rectifier to convert the AC mains into DC first.

If you are using a battery to power the LED you don't want to use dropping resistors as they waste power, making the above circuit the most efficient.

As for the voltage of LEDs, the voltage as I said depends on the colour. The real world is a LED is a semi-conductor junction whos turn on voltage is controlled by the doping of the semiconductiors. It is upto you or the manufacturer as to whether they are connected in series or paralell.

Alanh

Edited by alanh

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Malich has obviously never soldered in a single LED http://www.altronics.com.au/p/z0700-red-8mcd-3mm-led/ as I said about 2 V, so is 2.2 V close enough! Green is also 2.2 V, orange 2 V, Yellow 2.2 V, blue 3.5 V, White 3.5 V. No where near 12 V.

I was however talking about the ones with AAA cells not car batteries!

By the way the switching in these devices is usually performed by a transistor.

 

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For the reader,

Malich has clearly demonstrated operation of light emitting diodes (LED's) possibly from a designer's perspective and certainly from a perspective of having used them.

For anyone choosing to use LED's in a circuit, always base your design on current controlled/limited source. Despite what some have written here, do not parallel LED's directly, although their breakdown voltage for any particular device will fall into a tight voltage range, they will never share current equally and reliability will be jeopardised. Series your LED's with current limiting on each string, then you can safely parallel these strings without risk.

CMatten's tests are quite telling, I particularly liked your probes and backs up what is known to now be all too common a problem, switching with uncontrolled rise times, poor snubbing and insufficient pre and post switching element filtering whether it be bipolar or MOS based.

James

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Dear Reader

The poster above has kindly included a link that describes an impact of incorrect use of LED's through paralleling non current controlled strings. Most kind, although it's content appears to have been misinterpreted by this poster in this case.

For clarity, it is worth reading for yourself.

James

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It also says how to connect them in parallel so that a single LED failure wil not cause total failure.

 

Alanh

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Dear Reader

When one reads what another writes for any particular endeavour, you can tell if they are simply a reader of that endeavour or actually involved in it, by that I mean some things are understood and others need to be stated.

" It also says how to connect them in parallel so that a single LED failure will not cause total failure" should be an example of the understood.

I note also that the article quoted by the poster above having merit in terms of it describing the need for current limiting/control, I do not advise using the circuit they describe. I suggest equally simple circuits that permit individual string limiting independent of their neighbouring parallel strings is possible and superior and available even at the time of this quite old reference.

James

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