EMI with PWM's led-string

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Plons
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Posted by Plons: Sun. Jan 24, 2010 - 10:14 PM
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Hi guys,

I would appreciate your insights on the following:
For my daughter's new house (dated around 1600 ;) ), I am working on a modern way of illumination: led-strings instead of light bulbs

For that purpose I use an AVR-controller with 3 powerstages. The ledstrings are 1 to 3 meter long, and I wonder if it would be wise to put some filtering on the PWM-outputs. 12V pulses @ 62500Hz, 2A per 3m string.
Normally I would go along with the schematic on the left, but since there are several strings involved I am considering the right approach (pun intended :) ).

1. Is the right approach a good one, from EMI point of view ?
2. Will there be visible interference between two separate led-strings (each having it's own controller/crystal) ?
Note: interference is a problem with sound f.i.

Looking forward to your insights.

Nard

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Image icon PWMdrive_avrfreaks.png

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Learning and Information, General Electronics
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Bronze Markjs
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Posted by js: Sun. Jan 24, 2010 - 10:23 PM
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You will have all sorts of problems, you are using a resistor in series with the leds!!
It will never work. :mrgreen:

Are the leds on PCBs or just wired up? If the later you could try to twist the 12V with the rest of the wiring.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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jayjay1974
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Posted by jayjay1974: Sun. Jan 24, 2010 - 10:27 PM
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Is the PWM to control current,e.g. a switching current source, or just to dim them?

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Plons
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Posted by Plons: Sun. Jan 24, 2010 - 10:56 PM
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@js:

Quote:
It will never work. Mr. Green
LOL
I will post a picture. The leds are on some kind of flex PCB. They are not meant to be twisted.
The top-string is RGB en enclosed in a silicon envelope.
The bottom string, warm white, is a soft transparent plastic, with visible copper-strips in it.

@jayjay: it's for dimming purposes. PWM drives a logic level Nch fet.

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Image icon P1250996.jpg

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Bronze Markjs
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Posted by js: Sun. Jan 24, 2010 - 11:24 PM
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Quote:
The leds are on some kind of flex PCB
I see, so you buy them in? My son in law is doing something similar for his NEW house (not from 1600) but he is using commercial controllers from a central room. May have a look at how they do it when the controllers come in.

YES he has a ~4m x ~4m room JUST for I.T., media and lighting stuff. A lot of wires to pull in.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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jayjay1974
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Posted by jayjay1974: Sun. Jan 24, 2010 - 11:50 PM
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Is 62.5KHz not a bit high for just dimming? Anyway, a small filter that softens the edges is always a good idea.

Maybe you could test by tuning an AM radio to an harmonic of the switching frequency and see if a filter has any effect :)

Are you afraid the RCD will be on her doorstep after a few days? :)

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valusoft
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Posted by valusoft: Mon. Jan 25, 2010 - 12:12 AM
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jayjay1974 wrote:
Are you afraid the RCD will be on her doorstep after a few days? :)
Are they the Dutch "emissions police"?

Ross McKenzie ValuSoft Melbourne Australia

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jayjay1974
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Posted by jayjay1974: Mon. Jan 25, 2010 - 12:40 AM
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Yes :) Though the name has changed into something else.

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oddbudman
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Posted by oddbudman: Mon. Jan 25, 2010 - 05:05 AM
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I agree with jayjay, why such a fast rate?

You can probably get away with 120hz or even a touch lower.

Your circuit on the right has me a little confused. 100uH at 2A at that high frequency may play up (i've got no idea on the PSU you are using for the 12v).

You may even wish to slow the FET down a little with an RC on the gate.

oddbudman

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Plons
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Posted by Plons: Mon. Jan 25, 2010 - 08:35 AM
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@John: yes, I bought the strings as they are in a group-buy action.
The warm-white are € 11 per meter, the RGB € 13.75 per meter.

@Jayjay&oddbudman: The edges of the PWM are my worry: their V/us determine the highest freq component in the PWM spectrum.

I agree that I can (and will) cut back on the PWM frequency. And have to find a compromise between dissipation and amount of EMI (and RFI)

How about the interference matter ? Will the perception be a slow ride between an illuminated room and a dark room ?

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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jayjay1974
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Posted by jayjay1974: Mon. Jan 25, 2010 - 08:55 AM
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You can either slow switch the FET, not good for efficiency, or slow the edges using an LC filter. Filter heavily enough and you have made yourself a switching power supply with only DC feeding the LEDs :) But for dimming you still want to PWM the LEDs, IIRC color depends slightly on current; If you don't want that you end up with switching anyway :)

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Plons
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Posted by Plons: Mon. Jan 25, 2010 - 09:13 AM
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Quote:
Filter heavily enough and you have made yourself a switching power supply with only DC feeding the LEDs
:) Exactly !

About the interference: tested with two DDS's, two leds: 3kHz and 3.001 kHz: no visible interference. With one led between the 2 DDS's: yes, the led slowly meanders between on and off.

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

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krazatchu
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Posted by krazatchu: Mon. Jan 25, 2010 - 07:26 PM
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Check out this ebay sale...
http://cgi.ebay.com/5M-5050-SMD-RGB-Car-LED-Strip-Controller-A-C-12V-D-C_W0QQitemZ270454486541QQcmdZViewItemQQssPageNameZRSS:B:SRCH:US:101

You can't buy the PLCC6 LEDs in bulk for that cheap...
Plus you get a remote, controller & PWR supply...

Michael

Edit: I'm not the seller or have any affiliation...
I did however, order the same thing a few days ago...
If anyone's interested I'll post the results when it arrives...
It really takes the fun out a project when it's already available and for cheap....

Michael
--
http://krazatchu.ca/

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Plons
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Posted by Plons: Tue. Jan 26, 2010 - 11:49 AM
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Michael wrote:

Quote:
It really takes the fun out a project when it's already available and for cheap....
That's exactly what I felt last night, ... when I read your message. The same RGB-strings as I have.

Oh well, the only comfort is that my controller will probably be better than the one in the Ebay-offer.

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Guillem Planisi
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Posted by Guillem Planisi: Tue. Jan 26, 2010 - 09:29 PM
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AFAIK, LED strings are controled directly with square waves. But they are commonly drived with some kind of current controller that accepts 5V PWM signal input. Tomorrow at my job I will check if I have some IC numbers to give you, since we use some of those thingies: LED's in strings that are supplied up to 40V.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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jayjay1974
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Posted by jayjay1974: Tue. Jan 26, 2010 - 09:48 PM
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There are plenty of LED (string) controllers from manufactures like Linear Technology, TI etc.

LTs list

I once used the LT1618, on this one you can set the current with a resistor. It also has differential current sense input, so you can put your Rsense where ever you like.

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Plons
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Posted by Plons: Tue. Jan 26, 2010 - 10:39 PM
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No need for that, Guillem, thanks for offering. The schematic design is finished and so are the PCB's. A larger one with all bells and wistles, and a compact one, with just the basic functions.
Technically not a very challenging job, layout took most of the time.

Nard

Edit: Jayjay, it's a home project, so I don't need the optimum chip for the job. But thanks for sharing.

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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