Creating voltage source from the output of an constant current LED driver - Page 1
Forum section: electronics / projects

author: noddi
author_rank: Newbie
timestamp: November 30, 2019, 11:33:38 am
content: Hi,I'm wondering if it is possible to create a constant (ish) voltage source of a constant current source (i.e. a LED driver)? I'm in a situation where the expected input from the supply I'm hooking up my system to was +48V, but recently learned that it was the output of a LDU4860 (Depended on some misleading documentation). There's also a challange with the changing system load, which changes between 100mA to 200mA peak when supplied directly from a +48V lab supply.All input and help is strongly appreciated.

author: dom0
author_rank: Super Contributor
timestamp: November 30, 2019, 03:35:12 pm
content: 48 V Zener or a shunt regulator (e.g. TL431 with extra pass element).

author: magic
author_rank: Super Contributor
timestamp: November 30, 2019, 08:54:54 pm
content: What's the output current from that thing and can you take it apart for modification?I mean, zener will work, I wanted to jokingly suggest resistor, but power dissipation may become a bit crazy if the output current is high.

author: ocset
author_rank: Super Contributor
timestamp: November 30, 2019, 11:28:08 pm
content: I mean really its not good to try and do it…but since you cant get inside the LDU4860, then maybe put a big cap on its output…then just on/off switch it to keep the cap at approx. 48v…so a comparator looking at the cap voltage and feeding into the ldu4860 on/off pin…

author: noddi
author_rank: Newbie
timestamp: December 02, 2019, 09:03:51 am
content: Thanks for all the replies!Quote from: magic on November 30, 2019, 08:54:54 pmIt can supply 1.75A in short circuit. Can't modify the LDU circuit at all, so I think trying to go the zener diode road. I'll give an update during the week on how it plays out.

author: magic
author_rank: Super Contributor
timestamp: December 02, 2019, 12:09:25 pm
content: Quote from: noddi on December 02, 2019, 09:03:51 amHow so? It's a constant current regulator, it should be outputting constant current, whether into load or into short.If it really is 1.75A then a 48V shunt regulator will be dissipating 84W of heat under no load, are you sure you can handle it?Actually, a series regulator like LM317 (or some high voltage variant thereof) would perhaps be better, depending on the maximum open circuit voltage the LED driver can output. Heck, even a switcher might pay for itself.

author: noddi
author_rank: Newbie
timestamp: December 02, 2019, 02:42:32 pm
content: Quote from: magic on December 02, 2019, 12:09:25 pmSorry, I was a bit quick. I had a 48V isolated switching regulator in between, that gave the short circuit current. I was a bit quick on the measurement as well, and can't seem to reproduce it.A special remark that might be an important point. on the output of the LDU is connected with a 110 Ohm power resistor in series. When short circuiting it I'm now measuring 430mA short circuit current and +48V over the resistor. The problem is that I can't touch those components, and have to find a way to live with it.Quote from: magic on December 02, 2019, 12:09:25 pmI'm open to everything. Not really in finding the cheapest solution.