Need help adj trim pot on buck converter (picture) - Page 1
Forum section: electronics / projects

author: billbyrd1945
author_rank: Regular Contributor
timestamp: April 30, 2019, 08:24:29 pm
content: Can you tell me how to set up meters and so forth to adjust the current trim pot on this buck converter? I'm going to be drawing about 400mA. (The red one is for volts)Thank you

author: golden_labels
author_rank: Super Contributor
timestamp: April 30, 2019, 09:22:14 pm
content: With the plethora of details provided, a reasonable answer is: by turning the potentiometer.What is this board? Do you have any documentation or schematics for it? If no, what are the chips on the bottom? I guess that is an op-amp and the controller; but this is only my guess and without part numbers this information isn’t worth much.If everything else fails and no information is available, after making some assumptions¹ it may still be possible to set it up reliably:Attach a 10kΩ resistor across the output. This is to provide minimal load if the converter misses an internal one.Attach avoltimeter across the output (mind setting the right range).Turn the CV pot down, until you reach few volts — round numbers are preferred to make calculations easier. Let’s call that voltageV_foo.Calculate resistanceR_bar=V_foo/ 400mA, and powerP_bar=V_foo· 400mA.Find a resistor, that have resistance a bit lower (15–20%?) thanR_barand power rating considerably higher thanP_bar. You may need to use a few resistors in parallel to handle the power.Connect the resistor(s) across the output, puttingammeter in series with them (mind setting the right current range).Turn the CC pot until you are at 400mA.Remove everything.Connect the 10kΩ resistor across the output.Attach thevoltimeter across the output (mind setting the right range).Turn the CV pot up, until you reach the desired voltage.Remove everything.If you are brave, you trust that converter and you know it can drive voltage to nearly 0V, put a paperclip across the output and measure current — but be warned that if the converter fails to set the right voltage fast enough, you will have your ammeter fried. Having a fast 500mA fuse in series is not a bad idea.Steps 1–3 and 9–12 may be skipped if you have load that can handle 400mA at the desired voltage. But for, let’s say, 20V this would require a ~40Ω resistor with >10W power rating — this is hardly a jellybean element — or an electronic load.___¹ At minimum: CC/CV pots are doing what CC/CV pots are expected to do; it can supply low voltages; it can do that without producing considerable ripple; current regulation is independent of voltage.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: April 30, 2019, 09:35:08 pm
content: I know it's poor form to ask for help without providing info. I was thinking that there was probably some standard way of rigging up to adjust trim pots for current. I suspected there had to be a load, but I wasn't sure if a resistor would be the way to go, or what. The link below shows the device from ebay. I'm going to put it between a 12V power supply and an 8-pod 5v relay module which, in turn, will drive a couple of 4 amp thermo-electric wafers and fans. The relays will be triggered by an Arduino Uno, but I'm told that a separate 5v source is needed for the relay pack. So I figured I could tap onto the 12v PSU with the 5V buck.https://www.ebay.com/itm/60W-DC-Buck-Boost-Voltage-Converter-Constant-Current-Module-Step-Power-Up-Down/142624381499?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2648

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 30, 2019, 09:41:47 pm
content: "If you are brave, you trust that converter and you know it can drive voltage to nearly 0V, put a paperclip across the output and measure current — but be warned that if the converter fails to set the right voltage fast enough, you will have your ammeter fried. Having a fast 500mA fuse in series is not a bad idea."This is very true. If the converter has significant output capacitance and/or a slow compensated current loop, the surge current can be several times the set current and the surge itself can last hundreds of milliseconds....enough to damage a meter. Or at least to cause it to drift out of spec.If you have access to a meter with a fast bargraph display in addition to the numbers, your first test should be in the 10 amp range, and while you perform the test, watch the graph for an instantaneous jump, and then settling down.Of course, if the DMM has a min/max feature, it is even better.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: April 30, 2019, 09:44:18 pm
content: I had no idea I was getting into all of that. I'll just get a 5v wall wart and call it done. Thanks for the help.

author: golden_labels
author_rank: Super Contributor
timestamp: May 01, 2019, 12:14:39 am
content: Hey! Those 12 steps are not “all of that”! I was just describing the procedure precisely to avoid any ambiguity. It’s just connecting some resitors to the output.Don’t give up. Making interesting things is all about getting over obstacles! In the worst case you lose $0.10 worth of resistors (except for step #13).If you want a 5V output, ten 100Ω 1/4W resistors in parallel are all you need — you may skip the 10kΩ part and getting voltage to the lower values, as it is already sufficiently low. Just don’t run the test for too long — a few seconds to read the current and that’s all. 100Ω 5% resistor at 5V±5% is <300mW. THT carbon resistors do not evaporate instantly because one exceeds power rating by 50mW for a moment.Also note, that JacobParts Inc. seems to accept quetions at <support@jacobsparts.com> (taken from their website). You may ask them for “official” recommendations.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 01, 2019, 12:25:16 am
content: Thanks for the spiritual boost. I needed it. I didn't mean to belittle your list. I'm not in the least bit lazy when it comes to learning. That's why I come here. But the list would require me to do things that I don't yet know how to do. That's very frustrating. And with a DOB of 1945, I have to find shortcuts when they're safe and otherwise advisable. Don't have a lot of time for taking on large chunks. I love taking on the small chunks though. I did email ebay with my question. And now-- I'll use the address you gave me and go directly there. In the mean time, I'm sitting next to my project with the current trimmed to some unknown value. So far, no heat to speak of. Not very scientific, but all I have for now. Thanks Golden. I really do appreciate it.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 01, 2019, 11:47:11 pm
content: As suggested, I contacted JacobsParts who answered me immediately. When using with my relays, turn current pot all the way clockwise until you hear it click. I hope they have more things that I'll need in the future. My kind of guys. Thanks to all.