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DALI communications setup not working

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Flyback

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Hello,

Please help us with our lamp which sometimes fails to correctly interpret DALI signals that are sent to it.

We have an offline, non isolated 150W LED lamp which is DALI dimmable.

In fact, whilst in the factory, before we ship the product, we send it a DALI signal which tells it what power level to start at, when its installed by the customer.
However, in the factory, when the mains cable running to the lamp was long, the lamp would not correctly interpret the DALI signal. When we shortened the mains cable, the lamp would correctly interpret the DALI signals.

The attached pictures show the setup which does not work, and the setup which does work.

It is inconvenient for us to use the short mains cable. What can we best add to the “long mains cable” setup to make it work?
I was thinking of passing the mains cable through a high permeability ferrite torroid several times just before it enters the lamp. Would this be best?


(incidentally, the lamp has a small 1W Buck converter bias supply, but the LED drivers operate in linear mode. The Buck converter has a differential mode CLC filter before it. There is no common mode filtration. That is, no Y capacitors and no common mode choke in the lamp. In fact, on the lamp PCB, upstream of the mains rectifier bridge, there is no filtration components at all, either diff mode or common mode.)
 

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alec_t

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Most Helpful Member
You show a 15m mains cable extender, but its input and output and output are only 2m apart. If the remaining 13m is coiled up it will act as a choke. This may have some bearing on the problem, though I don't see why that would affect the DALI signals.
 
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Flyback

Well-Known Member
Thanks, we suspect (but don’t know for sure), that the reason for the problem is that when the mains cable is long and coiled up as it is, (and therefore presenting a larger inductance back to the mains), the laptop power supply is drawing noise currents from the actual lamp itself….and thereby upsetting the DALI receive on the lamp PCB. We could potentially mitigate this by putting an X2 capacitor at the mains input to the lamp. However, the lamp PCB is surface mount only, and X2 capacitors are not available in surface mount. We do however, have a 220n, 630V ceramic capacitor downstream of the mains diode bridge in the lamp PCB. There are no filter capacitors or inductors, either diff mode or common mode, upstream of the rectifier bridge on the lamp PCB.

Do you think this is the problem?...ie the laptop power supply drawing significant noise currents from the lamp itself, due to the high impednace "seen" going back to the mains?
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Incidentally, when we tried to send the DALI signal to the lamp at a different location, (in the engineering office) where the mains cable was indeed short ( about 1 metre), the DALI signal was not received correctly if the lamp’s heatsink was not earthed. (the lamp driver PCB sits on an insulation pad on top of the heatsink)….but it was received properly when the heatsink was earthed.
 

alec_t

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Most Helpful Member
If the mains cable is 2-core+earth, then coiling it will introduce a choke on the earth wire, effectively raising the impedance to earth. This seems to tie in with your experience re the heatsink earthing.
Presumably the laptop supply (DC output) is floating, not earthed?
 

Flyback

Well-Known Member
Presumably the laptop supply (DC output) is floating, not earthed?
Thanks, i am not sure if its earthed within the laptop power supply case, i'll check. I doubt its earthed , but i'll check on monday.
If the mains cable is 2-core+earth, then coiling it will introduce a choke on the earth wire, effectively raising the impedance to earth. This seems to tie in with your experience re the heatsink earthing.
Thanks, this appears to imply that we really need to add Y capacitors and a common mode choke at the mains input to the lamp...that is, because we really need to use long cables in this test. (we are currently in a temporary production facility, and there are no wall sockets near the test bench.)
 

alec_t

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Most Helpful Member
I'd also assume that the customer might use a long mains lead, so you need to design for that.
 

Flyback

Well-Known Member
I'd also assume that the customer might use a long mains lead, so you need to design for that.
Thanks, yes indeed, though in this case, the impedance of the mains lead, would , i believe, act in our favour, and prevent the drawing of significant noise pulses from our lamp's input terminals , due to the impedance of the mains cable to the lamp, filtering out such things?
..as you know, we won't have the laptop installed up with the lamp, so we should be OK?
 
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alec_t

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Flyback

Well-Known Member
Then why does the long mains lead now cause a problem?
Thanks
The problem is, we believe, the fact that in our production test facility, we have a laptop with a noisy power supply, which is fed from the same long mains cable that the lamp is fed from….and the laptop PSU, is drawing significant noise currents from the input terminals of the lamp, due to the high impedance “seen” going back to the mains. Do you agree?
 

alec_t

Well-Known Member
Most Helpful Member
I can't really say. I can only guess :). As you've got it set up already, and have the test gear, can't you measure the noise and actually track the source down, rather than 'believing' it to be the culprit? Or use a short mains lead but deliberately add some extra inductance/capacitance here and there to narrow things down?
 
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