Disconnected Fairy Lights on tree inducing DC voltage?

BradsXmasLights

BradsXmasLights

WiFi Interactive
Joined
Dec 23, 2010
Messages
605
Location
Brisbane, Australia
For the past few nights I have been trying to trace the cause of aa annoying flicker that was sending some of my front gutter 2811 strips nuts at mid-high brightness levels. They would be fine until my front garden 30VDC PSU was connected - then they'd start flicking - sometimes occasionally (bearable), sometimes completely wreaking the show. The garden itself has 5 trees of (200 red/200 blue bigW and 300 OO green lights that have been twisted together then spiralled around each tree). These are wired to a 27ch DMX controller, then back to the 30V PSU. All PSU VE's are bridged together.

Now whilst testing this afternoon, I noticed I was getting ~2.08VDC from the 27ch DMX controller's input EVEN when all possible sources of power cabling was disconnected - including the DMX lead in. Then as each tree was unplugged, this voltage dropped back. Measuring the disconnected lights themselves, this voltage was higher - varying initially between 6- 12v, but once it cooled down this afternoon they all seemed to drop to ~3v. I couldn't measure any current from them - my multimeter's smallest scale was mA and it saw nothing.

The strings themselves are your usual 2ch red/blue/green strings that have been lightly twisted together. These strings have then been spiralled around the tree from bottom to top. Each tree is probably 1m wide by 2m tall.

Now I suspect this is something to do with capacitance of the cable? I know power lines have the ability to induce voltage - but i didn't think this sort of thing could happen with a short run of xmas lights.

Any one else seen this before? And more importantly - do you have an idea about how to solve this issue?
 
My guess is that the induced voltage in the DC channels is probably not the cause of the flickering. It's just inductance and capacitance at work leading to phantom voltage reading with your (high impedance) meter.

Maybe it's more of a case of the PWM from the DC controller and / or EMI from the power supply bleeding into the pixel system?
 
The voltage being measured could be stored voltage due to the capacitance of the led rather than the cable. I've also wondered if the leds themselves work as tiny solar panels as I have noticed that during the day i reckon I can see a faint glow from some of my led strings.
As for your problems it is possibly coupling between the 27ch board driven cabling and the 2811 cabling. This could be capacitive, inductive or rf. About the only option is to separate the cabling or maybe to change the clock rate for the 2811 pixels as a different frequency may not suffer as much.
 
Changing the 2811 helps a little, but it just changes how the 2811's flicker. eg: at higher brightness rather than a lower brightness flicker.

The voltage reading too was also opposite to the normal polarity of the strings too.
 
I've had the lights off all night as I've been out, but i just took another reading and the previous ~2V reading from the garden power feed is now only 0.03V

As for stored capacitance - wouldn't shorting the cable cause it all to discharge? Upon unshorting the voltage reading came straight back
 
So the plot thickens. Swapped RayWu 27V@30V PSU for a DSE bench supply @ 29V (to stay under 2.5A PSU limit) and no more flicker!

Will try the Ray PSU again tomorrow at 27V to see what happens too. Otherwise I can try another Ray 36V PSU wound down or my lab supply is going to have a new home for the next month :p
 
All switchmode power supplies can have EMI issues, but some of the cheap Chinese ones seem much worse than others.

I have 6 Mean Well units (3 in each controller enclosure) and no issues with interaction between my DC24 and PIXAD8 running WS2811 strips.
 
So problem solved... ended up shortening the lead in cable for the 2811 strip. It was about 1m long. Now it's only 30cm, and can handle a higher clock rate now too.
 
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