Converting AC led fixtures to operate on 12VDC  FlexBlock? - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: July 01, 2013, 07:57:13 pm
content: Hi guys...long time no chat. I have been working building a flatbed truck camper (sort of an expedition vehicle).I am at the point where I want to investigate 12V lighting. I do not like any of the RV type 12V led fixtures on the market (poor quality, expensive, dim....pick any two). I have come across several AC led fixtures at the local hardware stores which are much nicer. In particular, for the ceiling lights, I bought a Sylvania recessed/surface mount light kit for about $36 which has a nice metal frame dome, very bright leds (about the light output of a 60 watt bulb), and is very low profile. I took the cover off of the little box which is an AC input buck converter and measured the voltage going to the led string. It was 42VDC. Playing around a little bit with my adjustable supply, I found the light to barely come on at 32.8VDC 1mA and be suitably bright at 39.4VDC 129mA and drawing around 221mA at the 42VDC the factory AC powered driver outputs.The mostly opaque dome is sealed, so I can't count the individual led elements, but I would guess there are about 10 or 11 in series. Could be two parallel strands of 10 or 11, but there are only two DC supply wires.Anyway, I was looking around for a suitable boost current source and came across something called the FlexBlock. Yes, I could build my own but I have so many other things to do with the camper there is just no time. Have any of you used this FlexBlock driver or know anything about it? (emi, reliability). The idea would be to embed this small module on the backside of the light where the AC driver was and thus convert the led fixture to run on 12V nominal.Here is the link to the FlexBlockhttp://www.luxdrive.com/content/FlexBlock_A011_V1.pdfIt is about $20 at digikey. Thanks ahead for any help.

author: richard.cs
author_rank: Super Contributor
timestamp: July 02, 2013, 12:14:47 pm
content: So you plan to buy the 350 mA module and use the dimming control to wind it down to 220 mA or so? Sounds reasonable. The suggestion in the datasheet about boost-only mode would be applicable to you if you haven't spotted it already.

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2013, 06:48:05 pm
content: I have some 12V MR16 LED lamps that use a small module on them to drive the LED's. the module seems the same across either a 3 or 10 LED cluster, driving it with 3 or 9W of power. Could be worth looking at using some of those for the lower power/light applications.

author: KTP
author_rank: Frequent Contributor
timestamp: July 02, 2013, 07:44:29 pm
content: Thanks..that is an idea...robbing the electronics from an existing 12V led fixture. I ordered a couple of the FlexBlocks though and will see how they work. I just bought the 700mA version since it was the same price as the 350mA and 500mA (probably they are all three identical inside but for a resistor value. Since I will need to set the current to lower than even 350mA, might as well get the max capability and adjust that one down.It is amazing how cheap the LED stuff is getting. Bulbs now with 20 0.5 watt leds are under $10...I remember paying $4 for a single 20mA white LED about 15 years ago.

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2013, 07:59:36 pm
content: I bought some via Frankie, and the only issue i had with them was the rather inadequate heatsink. ( not Frankies fault, I was intending to strip them down anyway to use like that) so now they are on rather more substantial spreaders, old Pentium ones from dead computers without the fan. Went from 90C die temp to 30C, so they will have a lot longer life.

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 02:45:47 am
content: Well the Flexblock dc-dc modules arrived and seem to work well. I bought the 700mA version since it was the same price as the 350 and I was going to need to dial down the current anyway.I used a 4.7k resistor which runs the lights at about 150mA and around 40VDC across the led lamp module. This level is plenty bright and is significantly underpowering the lamp so life should be excellent and heat management easier. At this 6 watt output from the Flexblock I am seeing a draw of .51 amps at 12.5VDC (6.375 watts). This puts the efficiency at 94%. I am running in boost only mode, which is supposed to be more efficient.The regulation is *ok*. I see 145mA output at 11.5VDC input and 156mA output at 14VDC input. I think it is supposed to be a bit tighter if you are running closer to the rated output of the module, but this +/-4% over the expected range of my RV batteries is fine.