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Need help adj trim pot on buck converter (picture)
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billbyrd1945:
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
golden_labels:
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 a voltimeter 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 voltage V_foo.
* Calculate resistance R_bar = V_foo / 400mA, and power P_bar = V_foo · 400mA.
* Find a resistor, that have resistance a bit lower (15–20%?) than R_bar and power rating considerably higher than P_bar. You may need to use a few resistors in parallel to handle the power.
* Connect the resistor(s) across the output, putting ammeter 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 the voltimeter 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.
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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.
billbyrd1945:
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
schmitt trigger:
"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.
billbyrd1945:
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.
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