Ian King
Hi all,
I've been reading this forum for a while now. There are a lot of useful ideas, some of which I've adopted into my Loxone house plans.
I thought I'd post this in case it's of use to others.
Lighting is a major part of my design and I decided to use LED lighting almost everywhere and wire back centrally to the loft and drive from a bank of DMX dimmers driven from a pair of 600W Meanwell HRPG-600-24 PSUs.
I have around 100 downlights in my house design so I wanted a DMX dimmer with a large number of channels.
I've seen a few people mention LTECH for DMX decoders.
LTECH do a 16 channel 350mA CC decoder - LT-880-350 which can be had for about £100 from AliExpress. That's about £6/channel, excluding power supplies. They also do a 16 channel 700mA version - LT-880-700.
When I first prototyped my lighting, I had 1 LT-880-350 and 1 unit of the downlights I intended to use throughout the majority of my house. The downlights required a current of 350mA and a 26V forward voltage.
I intended to use the HRPG-600-24 PSU with its voltage adjusted to 26V.
The prototype worked and I was happy with the dimming performance, so I moved on to other matters.
When the time came to order the rest of the dimmers and downlights, I discovered too late that the downlight design had changed and they are now 250mA with a forward voltage of 36V!
I was stuck with 96 channels of 350mA drivers and 250mA lights. I wasn't going to change my entire lighting scheme at this point, so I decided to open up the dimmers and see what I could do.
It's easy enough to open. Just 2 screws on either side and a further 5 screws hold down the PCB. I couldn't find an easy way to detach the power cables, so in order to get the board out I had to gently bend the edge of the case back to flip the board over while still attached. Easy enough and it bends back without issue.
See the attached pictures.
I have a total of 6 LT-880-350 units.
2 out of 6 have the incorrect case. It appears to be the case from the CV decoder.
1 has wrong coloured power cables. Black is connected to DC2+ rail.
So, some minor quality control issues despite the "QC Passed" stickers on the back!
The resistors on the rear of the board labelled RB and RC are the current sense resistors which set the desired constant current.
R680 is 0.68ohm. The spare pad will be for another R680 to set the current to 700mA for the LT-880-700 variant.
There is an LED driver chip just poking out from underneath the white gunk holding the inductors in place. This chip is a HV9910B. From the datasheet, we see that the internal reference voltage for the current sensing circuit is 250mV.
The chip supports trimming the reference voltage, but I couldn't see any other resistors for that purpose.
250mv / 0.68 = 368mA. Which is about 5% tolerance of 350mA.
I need a constant current of 250mA.
With a solder rework hot air gun, I replaced the R680 (0.68 ohm) with 1R00 (1 ohm) to set the current to 250mA.
In addition to my downlights I also have various different styles of lights that I'm using for feature and niche lighting, all of which have different current requirements.
I have now modified 6 boards. 5 of them are all 250mA and the final one has a mix of currents, all set with a combination of resistors:
250mA - 1x 1 ohm
300mA - 1x 0.82 ohm
350mA - 1x 0.68 ohm
600mA - 2x 0.82 ohm giving 0.41 ohm
700mA - 2x 0.68 ohm giving 0.34 ohm
I solved my downlight problem and this makes it more convenient to control different types of lights from a single box instead of having lots of individual decoders for each current.
If you are willing to void the warranty and don't mind a bit of surface mount rework, it's possible to get a 16 channel CC DMX dimmer with independent current control per channel.
I did have to switch to 36V PSUs to order to reach the required voltage of my downlights.
As a consequence, I now have 2x HRPG-600-24 PSUs for sale.
They've only been powered up a handful of times for testing.
They were purchased new in July 2019 for £320.
I'm willing to let them go for £100 each + postage.
Ian
