getting 1MEG+ multiturn potentiometer for LT3080 lab supply - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: February 10, 2016, 11:41:45 am
content: Hi :)I'm trying to assemble the power supply from the datasheet (please see attached picture). And after I designed everything and drew the PCB (there is much more than just two LT3080, I added pre-regulator, separate supply for Vctrl pin, etc) I found out there are no multi-turn pots for 1MEG+ on digikey :(. Only one-turn carbon pots.So, is there an easy solution to make such pot with precise control? I wouldn't want to add opamp or something unless there is no other way because the board space is very limited (I already have the desired enclosure, hence, it will be hard to fit more components).

author: Dave
author_rank: Super Contributor
timestamp: February 10, 2016, 11:53:51 am
content: You could use an additional current source (wired between Vcontrol and Set) to increase the current through the pot and therefore reduce the needed resistance of the pot.

author: tszaboo
author_rank: Super Contributor
timestamp: February 10, 2016, 12:08:23 pm
content: Probably it is easier to switch to LT3081, which ahs 50uA of current instead of 10uA. That would mean, you need 200K pot for the same voltage range, which they actually make.It is possible that the page 26 circuit of the LT3081 will work with the LT3080.

author: exe
author_rank: Unknown
timestamp: February 10, 2016, 12:33:16 pm
content: Quote from: Dave on February 10, 2016, 11:53:51 amThank you very much, I think this is the best solution in terms of cost, board space, complexity and idiot-proofness. And it works in LTSpiceQuote from: NANDBlog on February 10, 2016, 12:08:23 pmIt oscillates both with LT3080 and LT3081. I'm not sure why, may be I missed something. If I figure out what's going on I, for sure, will go this way.

author: Dave
author_rank: Super Contributor
timestamp: February 10, 2016, 05:59:13 pm
content: Try adding some series resistance to C3, like an ohm.Putting a 10uF MLCC capacitor (incredibly low ESR) on the output would make a real life circuit go crazy, too. Adding an ohm in series would bring it closer to an electrolytic capacitor.Go with NANDBlog's solution, because it makes no sense to add unnecessary parts into your circuit if you can just pick a different part and solve the problem that way. As you probably figured out, I'm not too familiar with LT voltage regulators.

author: exe
author_rank: Unknown
timestamp: February 10, 2016, 07:37:08 pm
content: Quote from: Dave on February 10, 2016, 05:59:13 pmFor modern LDOs low-ESR capacitors are absolutely fine (and recommended or even required!), this is mentioned in the datasheet. Surprisingly, MLCC is also fine for plain-old regulators like lm317 (if I'm not wrong) because of their output cascade (common collector or voltage folower, I don't really know what I'm talking about, I'm more into MCUs) does not have voltage gain and, thus, stable with MLCC or event without a capacitor.Concerning using different part. Besides its slightly worse haracteristics (like PSRR), it also does not have separate supply for built-in circuitry. This greatly reduces dropout. LT3080 has like ~0.35V dropout, LT3081 is around 1.5V. This makes some difference because I've got a pre-reg and can significantly reduce heat dissipation (I'm trying to fit a 50-100W PSU in a relatively small box (just for fun and to save some space on the bench)).Anyway, both parts work unstable in the simulator under some conditions. But I'll recheck tomorrow, may be I did something wrong (meanwhile, there was also a mistake with voltage double, I fixed it).

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 11, 2016, 02:05:15 am
content: Bourns makes a 3.75 turn 1 megohm cermet potentiometer, 1 or 2 watt depending on taper. Mouser stocks them:http://www.mouser.com/ProductDetail/Bourns/3856A-282-105A/?qs=tR5HdW2QqnGrhC196i8R6Q%3d%3d

author: edavid
author_rank: Super Contributor
timestamp: February 11, 2016, 02:08:40 am
content: Quote from: alsetalokin4017 on February 11, 2016, 02:05:15 amNon-stocked (Minimum: 495)Looks like only Quest has them:http://www.findchips.com/search/3856A-282-105

author: hugo
author_rank: Regular Contributor
timestamp: February 11, 2016, 02:45:13 am
content: QuoteJust zip the .asc file so we all can see / help ...

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 11, 2016, 08:14:12 am
content: Quote from: edavid on February 11, 2016, 02:08:40 amOh, sorry, I misread the page, I thought they had 495 in stock. Thanks for the correction....

author: C
author_rank: Super Contributor
timestamp: February 11, 2016, 09:05:20 am
content: Think you should look at iSet in data sheet again.Constant current through resistor = voltage reference.Also look at absolute maximum ratings for iSet to OUT.

author: exe
author_rank: Unknown
timestamp: February 11, 2016, 12:21:42 pm
content: Quote from: hugo on February 11, 2016, 02:45:13 amHere it isThe scheme is directly from datasheet's "Using a Lower Value Set Resistor" picture. BTW, Dave was right about the ESR, it helps, but, to me, this is a bad sign. It was always stable with ceramic capacitors. Now it's not, I'm not happy. I'm also very unhappy that I would need to perform circuit analysis (open-loop response, closed-loop response, all that phase and gain margins) to be sure that it is really stable. I don't like messing up with feedback loops...

author: hugo
author_rank: Regular Contributor
timestamp: February 12, 2016, 01:03:47 am
content: You have to increase C1 to 100u for 1A load ...Take a look at this doc from LT:http://cds.linear.com/docs/en/lt-journal/LTJournal-V24N2-2014-07.pdf

author: uncle_bob
author_rank: Unknown
timestamp: February 12, 2016, 02:18:53 am
content: HiThe 3080's have worked quite well for me and been both quiet and stable. I've never needed to stray very far from what is on the data sheet. Always keep in mind that modern regulators (of any sort) are sensitive to input, output, and (if present) reference capacitors. I've spend an amazing amount of time chasing parts with output cap issues that magically went away when I replaced the defective input cap.Bob

author: exe
author_rank: Unknown
timestamp: February 12, 2016, 01:47:00 pm
content: Quote from: hugo on February 12, 2016, 01:03:47 amWow, that's interesting. The datasheet says " The LT3081 is stable with or without input and output capacitors.". But, yeah, I added more capacitance and now it works.Actually, I might have made a mistake using lt3080 because 1) I need two of them to regulate current and voltage 2) For current sensing where is a 1Ohm resistor that creates significant dropout that I forgot to account. So, at the end, two lt3080 and the resistor 1) cost more 2) require current source for Vset pin from a 10k resistor 3) require separate supply for Vctrl (or voltage drop will be huge) 4) occupy more pcb space. But I already put my order.

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2016, 02:31:40 pm
content: Quote from: exe on February 12, 2016, 01:47:00 pmI dont think you made a mistake. Making a nice CV-CC power supply is not as trivial as it seems. Even if everything is working on paper, reality can give you some surprises.Please keep us updated on the project!

author: exe
author_rank: Unknown
timestamp: April 03, 2016, 09:55:21 pm
content: Quote from: Dave on February 10, 2016, 11:53:51 amJFYI, I used lm234/lm334 and it seems it's not very stable. The one that supplies ~1mA works okay, but the one supplying ~5mA (my fault, it should be 1mA) drifts quite a bit when switched on (voltage changes from 8.85V to ~9.07 within 5minutes or so according to the panel meter). I think it just heats up. I don't have a spare part to make the experiment. I can't say it's a big surprise, the curves in the datasheet speak for them selves, just a bit disappointing.