High voltage constant current led driver - Page 1
Forum section: electronics / beginners

author: RayOfLight
author_rank: Newbie
timestamp: April 20, 2015, 11:54:15 pm
content: First of all, hello to everyone, I've just registered because I'm a little out of ideas trying to fix a 20W LED fixture and I feel it should be a really easy task, I'll provide a much information as possible:I've recently bought a 20W LED fixture at my local home improvement store. It consists of a non isolated 230VAC input, 230VDC output adapter and a strip of 72 LED inside. The problem is that it flickers a lot (100Hz) just because apart of not having a transformer, it has no filtering at all after the full bridge rectifier.Basic schematic in datasheetSo after putting a 150µF 400V electrolytic capacitor, I've somehow managed to destroy it. It was a really simple design, consisting of two paralleled SM2082C ICs, each providing 66mA through two paralleled 18? each IC (9?) (Datasheet states that setted mA = 0.6V / Radj) [Total BOM: 1 MOV, 1 Full 0.5A Bridge, 2 Constant current ICs, 4 18? Resistor, 4 little SMD Caps and a 1A Fuse][Added a 150µF cap and 2 1M? bleeding res.].The problem is easily located at one of the ICs, so apart from buying an SM2082C or B from aliexpress... 6€ 10 units and trying my luck soldering SMD bits and restoring a "badly designed PCB", I'd like to build a proper driver/ballast/power supply for this fixture, so far I've managed to check those values:I have 230V AC at home, which means 325V DC at the filtering cap, the LEDs are 2.75V @ 10mA, 3.12V @ 66mA, 3.35V @ 133mA and I guess they were expected to run at 133mA.There is a single series 72 LED strip, meaning the max voltage drop would be 3.35V·72=241V, and 198V the minimum.This means that I have to get rid of 127V to 84V DC and I have no idea how to do that without going out and ordering a custom made transformer.To keep the current constant, I've thought of using an LM317 with the constant current configuration, but it only allows a +40V max differential between input and output, meaning it would release lots of magic smoke.If I put a resistor in series, calculating what voltage do I have to drop, It'd be a >5W ResistorWhat would you do? Would you put a smaller resistor and a 60V zener diode? Have got any ideas?I've also looked at already made LED drivers which promise high voltages and then they only provide 55..75V, no mention of higher voltages anywhere, so this is why I'm looking at a small PCB designed by me... or by you!Thank you for reading all of this and creating some discussionTLDR: How to make a (non-isolated?) AC/DC Constant current supply, 230VAC goes in, 133mA goes out, but with 180-250V.

author: Seekonk
author_rank: Super Contributor
timestamp: April 21, 2015, 12:05:01 am
content: Series capacitor before the rectifier.

author: RayOfLight
author_rank: Newbie
timestamp: April 21, 2015, 12:17:23 am
content: Thanks for reading this, and proving that the solution was really simple in factD'oh, I didn't think of that simple trick...Do you know how to calculate the capacitor value for this situation?Should I consider the capacitor as a standard resistor/impedance like this: Xc=1/(2·?·50Hz·C) And then calculate 100V/0.133mA = 752?And finally, C=1/(752?·100·?)=4.23µF? seems too easy...Thank you!Some info I found