DALI Levels not tracking

I have a DALI system working using a DALI gateway and several Sylvania ballasts. I am using Broadcast DALI (easier to maintain) and the ballasts are giving me a range of dimming from 5% to 100%(measured by a light meter under the lamp). The problem I have is one of dead travel. As I ramp the lights down they hit the bottom of their dimming curve (the 5%) at a CBUS level of 50%.


Has anyone experienced a similar effect with DALI ballasts? Is it possibly peculiar to Sylvania ballasts? More importantly, is there a work around?

 

DALI Dimming Curve

Hiya,

DALI Ballasts use an "inverse-square dimming curve for better control of the lighting intensity in response to the human perception of brightness" (Ref: http://ecmweb.com/mag/electric_digital_dimming_dali/). If you google "DALI dimming curve" you will be able to find more references.

The aim is to achieve dimming that is linear to the Human Eye, not a Lux Meter. I am pretty sure the curve is part of the DALI standard, so I doubt you will be able to find a ballast that does not use it.

Sorry, i don't see any easy way to get around it atm.

Cheers,
Mark.

 

Hi there,

Yes this is a common problem. DALI is a "relatively" straight dimming curve, C-Bus is not, if you dim DALI fittings using C-Bus levels you will find that all the dimming takes place between 50 - 100%.

Taken from some of my findings on a particular job, DALI 100% = C-Bus 100% - lux level 478.
DALI 50% lux level 269, C-Bus 89% lux level 257.

The only suggestion I have is to write an algorithm if you have logic i.e. colour touchscreen, Schedule Plus etc and "convert" a C-Bus level to a DALI level.

I have also found that directly trying to dim a DALI fitting from a C-Bus key input using a dimmer function does not give the desired result. Better to tell DALI to go to a particular level than trying to ramp to a level.

If anyone has discovered a better way please let me know, I have a large DALI job coming up and want to get the best out of C-Bus and DALI.

Cheers Ian

 

More Info from the original poster

Using a Tridonics DALI gateway, I was able to investigate the problem further. It appears that Sylvania may have made a design choice in their T5 BIAX DALI ballast that is contributing to the "dead travel" problem. While their linear products are 1% ballasts, the compacts (T4 and TT5) are only 3% ballasts. The DALI standard has 127 defined levels below 3% (square law dimming). Sylvania did not adapt their dimming curve relative to the ballast's performance so if you send it a command below 3% it does absolutely nothing. While there may be some valid reason for this, I think it was a poor choice. The ballast ends up with choppy dimming at its low end and it appears to the controller that it is only a 50% ballast (ie, you can't set any level below 50%). In practice, I've measured lumen percentages as low as 7 or 8% but nothing lower. The dimming range is acceptable for the application but the "dead travel" will make adjusting the controls by hand a problem. Scenes with preset levels above the 3% barrier will have to be used for best results as mentioned previously.

I don't believe there is currently any workaround as the DALI gateway doesn't have a Min/Max adjustment per zone like the dimmers do. There is also no way to decouple or offset the mapping from the Clipsal side to the DALI side. It looks like we will have to do our best to hide the issue by presetting the levels.

Also, I discovered by accident that the Remote On functionality also works with the DALI gateway. Nothing was dimming today even though everything else checked out. Turns out my electrical contractor had miswired the RJ45 jack into the unit so that CBUS + was shorted to the REMOTE ON pin. It would really be helpful if the status pages of all units that reacted to this functionality would show it. I spent an hour scratching my head trying to nail this down.

Hopefully this post will save someone else from encountering the same problem.

 

What's in a percentage????

That's OK Ashleigh. Dimming ballast levels often have no relationship to LUX(FC) either. Other than Lutron ballasts, most manufacturers use the ballast factor min/max as their measure of percentage. Lumen-wise this does not match the percentages posted for the ballasts. A 1% ballast by ballast factor is often much brighter at low-end than a 1% ballast by lumen output.

Dimming fluoros is a bit like doing the limbo, it gets more difficult the lower you go. Just FYI.

 

May be there are some dimming curve definition differences between your DALI ballasts and Cbus.
we are DALI ballast manufacturer, our ballasts have 254 dimming level(2%-100%), at the 120th level brightness not 50% , but near 15%.