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    PWM dimming down long wires to LED?

    Hello,
    Has anybody experienced problems when sending "PWM dimming style" current pulses down wires to LEDs (wires of length up to 2 metres long)?....I am referring to sending PWM dimming pulses to a 42V, 2.5A LED module. Presumably the current pulses should not have too steep sides or else radiated emissions will result?

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    Re: PWM dimming down long wires to LED?

    we have had problems during EMC testing with PWM driving LEDs at particular frequencies - the wire lengths were only 20 or 30cm and the problem seemed mainly from the connectors


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    Re: PWM dimming down long wires to LED?

    Hi,

    And the radiatiln depends on switching frequency and it's harmonics.

    Klaus


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    Re: PWM dimming down long wires to LED?

    the step changes in current that occur during pwm dimming, would , I would have thought, bring about significant emc problems.
    I mean, just imagine a square current wave, of frequency 200Hz and 2.5Amps peak, going down a long wire.....the edges would surely cause emc havoc?



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    Re: PWM dimming down long wires to LED?

    Twist the wires together like Ethernet cable. It helps.
    Frank



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    Re: PWM dimming down long wires to LED?

    thanks, that's what I was hoping you wouldn't say, because this is a suspended light, with two suspension wires 30cm apart....and one is the "power", and the other is the return.....so we cannot twist them together.......well we could, but it would be more unsightly doing it that way.



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    Re: PWM dimming down long wires to LED?

    Why not use shielded audio cable? Some of it has stuffing to make it look perfectly round. If one wire feeding the LEDs with PWM is already at earth ground then use a cable having only one shielded wire. Connect the shield to earth ground of the mains wiring.


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    Re: PWM dimming down long wires to LED?

    thanks, BTW the wires feeding the leds are ultimately from the output of an offline smps...they are not earth grounded, they are floating.

    This is a differeent question, but again about these same led supply wires, which run along the suspension wires of the suspended led luminaire.
    We want our mains smps to supply 200vdc, and send this down the suspension wires to the led lamp. (high voltage to reduce i^2.R loss). The Led string is just 42V, but obviously this is produced by a buck led driver with vin=200v, and the top led in the string is referenced to the 200v rail.....but since the voltage across the leds is just 42v, and since the 200vdc is fully isolated, i presume we are ok here in terms of safety regulations, and can consider ourselves in the SELV area concerning the leds?
    Last edited by treez; 16th May 2015 at 14:56.



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    Re: PWM dimming down long wires to LED?

    Since the two wires are floating and not earthed, connect the shield of each shielded audio cable to earth ground.


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    Re: PWM dimming down long wires to LED?

    sorry but we cannot do that as this is all up in the air with the suspended lighting system, there is no access to earth ground



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    Re: PWM dimming down long wires to LED?

    Why isn't the live, neutral and earth ground mains wires up in the ceiling feeding the power supply?


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    Re: PWM dimming down long wires to LED?

    We want our mains smps to supply 200vdc, and send this down the suspension wires to the led lamp. (high voltage to reduce i^2.R loss). The Led string is just 42V, but obviously this is produced by a buck led driver with vin=200v, and the top led in the string is referenced to the 200v rail.....but since the voltage across the leds is just 42v, and since the 200vdc is fully isolated, i presume we are ok here in terms of safety regulations, and can consider ourselves in the SELV area concerning the leds?
    If the open circuit voltage (e.g. in case of a broken wire, defective LED etc.) is > 60 V DC, the power supply can't be rated as SELV. And of course, SELV would also require safe Class II isolation of the SMPS.

    Regarding EMI, are you really talking about 200 Hz PWM? Slowing down the edges quite a bit (as e.g. intelligent power switches do) should be sufficient to keep EMI regulations.


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    Re: PWM dimming down long wires to LED?

    Why isn't the live, neutral and earth ground mains wires up in the ceiling feeding the power supply?
    ..sorry please allow me to explain...the offline psu does have L,E,N to it. However, it produces an output of 200VDC (isolated), and the earth is not connected to the common of the output...so it is entirely floating.



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    Re: PWM dimming down long wires to LED?

    You amaze me with your understanding on basic magnetic fields from current loops.

    If you want to eliminate radiated noise in a loop, convert to DC with MOSFET driving L,C to output.
    ESR of C plus wiring must be 0.1% of ESR of source to get 60dB filtering.

    Floating secondary will be prone to increased radiation from CM fields, so RF cap to Earth is critical.

    Even with this, there will be emissions.

    Can you measure the problem?
    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

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    Re: PWM dimming down long wires to LED?

    sorry there is the Y cap in the offline PSU which indeed connects the secondary common to the primary DC bus common...which of course has a ycap to the earth....so there is capacitive connection to the earth from the secondary common.



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    Re: PWM dimming down long wires to LED?

    If you are getting EMI from SMPS and current loop to LEDs the noise will be radiated proportional to the area of the loop and the current pulse I.p-p and radiation loss according to Friis Equation.

    Obvious not having twisted pairs and feed each end with a single is a worst case design or best case noise generator.

    The only way to avoid it now is with an expensive differential LC filter with a CM choke,

    .... or wire it properly with just a CM choke to balanced twisted pair, 8 twists per ft approx. throughout the entire path, which if daisy chain, will demand most current on the 1st drop.
    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

    ... Tony Stewart EE since 1975
    - slightly north of Toronto


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