I always have a problem in projects that I need to supply a whole variety of different voltages, 8V, 5V, 3.3V etc. So I found these small DC-DC Step Down Converters and got few.
I plugged one into a power supply, and gave it 8V and 15V. Converter's output stayed fixed at 4.8V and 10V respectively.
I connected voltmeter directly across the outputs.There is a micro-pot and I turned it tirelessly for 30 sec counter-clockwise and than 60 sec clockwise. During all that turning, output voltage stayed fixed. No change.
Thank I attached 3.8kOhm resistor and tried again. This time voltage was lower, but it still stayed fixed, no mater how much I turned the micro-pot.
I was using a steel screwdriver to turn micro-pot. Should I use plastic?
Does it mean that these converters are broken and I should send them back? Or I am doing something wrong?
We cannot comment on your post because you didn't link the vendor so there is no way to obtain any information about these converters. Link the vendor who sold you the parts.
That micro pot has about 270 degrees of motion during which it is connected - those are not multiturn pots. During that other ~90 degrees, it'll either be shorted to one side or the other, or not connected at all. It doesn't matter if you've turned it for 30 or 60 seconds if both times you ended it with it in the not-connected part of the rotation. You should be turning it fractions of a revolution.
I've used a number of little DC-DC converters like that, and they've always "just worked" out of the box...
No, it only cares about the current position, If you turn it 360 degrees, you are back where you started. There's no screw thread - there's no space for one in such a small pot.
The way those things are structured is - those two pins on the same side are the two ends of the pot. These are connected by carbon composition going around the pot in a ~270 degree arc. The third pin is connected to the part that rotates, which has a wiper that contacts the carbon comp, thus giving you a potentiometer.
You may have damaged the wiper by running it off the end and back so many times, though. Check with multimeter.
The ones I've used have not required a load to get to the right voltage.
Those pots usually have end-stops to prevent you from rotating them by more than 270° so you might have broken it. Try one of the others - gently. A metal jeweler's style screwdriver is fine.
Yes, you may need to give it a minimum output load, depending on the design. Try a 1 kΩ resistor.
Breaking the endstop in no way affects the resistance . If the unit is not adjusting it is likely not due to the pot (this is of course contingent on the user doing the required resistance check with a DMM. One might wonder why no resistance measurements on the pot were done prior to posting on this forum
I connected voltmeter directly across the outputs
Obviously the OP has a meter. How difficult is it to change the mode to resistance and measure the pot with the power off ?
note that those tiny smd trimmers are not meant to be turned too many times, even if you do not turn past 270 degrees left or right. If you look at the specs for one, I think it will say the resistance will change after X number of turns. I can't remember exactly how many turns, but it is not high, like maybe 5.
you will know if the trimmer is bad if you measure the resistance between the middle and one of the end terminals and rotate at the same time to see if the resistance is changing.
it seems like you may have turned it way too many times already, and probably all resistance material has been scraped off the trimmer already.
what I am getting now is, no matter what I throw in resistance doesn't change?
I tried Philips/cross screwdrivers down to 1mm dia. Tried 0.7mm alen hex wrench. Tried sharpened steel scraper and toothpick. Nothing turns this SMD trim pot?
If doughboy said that hey are meant to be turned only 5 times, they are not of much use? I thought of using them maybe 100-200 times minimum.
Anyway, how do you turn them? Tiny Philips screwdrivers don't cut it?
if you want something that is really meant for end user adjustment, then don't get one that uses the small trimmer. Like I said, according to the spec, it is meant really to adjust (essentially once), and then that's it.
I use the same buck converter, and adjusted it once to 5v output and that's it. It is not designed to be your "variable voltage" power supply.
I just use a precision screwdriver.
I use the same smd trimmer on my analog circuit, and using metal screwdriver skews the output.
Is the mechanism on the top of the pot rotating? If it is, and the resistance between one of the ends and the wiper (that would be between one of the pins on the side with 2 pins, and the single pin on the other side) isn't changing, the pot is damaged. The resistance between the two pins on the same side will always be constant.
If the implement you're using is not rotating the top of the pot, you must use one that does so. Phillips screwdrivers don't work in those, because they're pointy, and there's nowhere for the point to go - use flat-head screwdrivers, or the ends of tweezers, or whatever. Visually examine the pot to ensure that you are turning it.
The typical spec is a X many turns guaranteed without a loss of accuracy - they can be used for a while after that, but the resistance may not vary smoothly, or total resistance may be off. Since it's a voltage divider, and you need to watch output while adjusting anyway, this isn't fatal - but it doesn't bode well for long-term use.
The larger DC-DC converters use those blue boxy trimmer pots; those have a much longer rotational life.
WOW! Three hours spent trying to turn a screw! Rookies are so bad.
I was fixated on turning that hollow round pin deep in the middle.
Anyways, I now know what to expect. I managed to turn the pot and get varying voltages out.
But voltage drop is very limited. 12V on input, was showing between 11.1V and 7.6V on output. I was hoping to drop from 12V right down to 3.0-5V range. Even from 8V one can't really reach less than 5.5V.
And, yeah, I wore one out quickly. 5-6 turns and it was goner. It cost me only about $2.0, so it was worth it.
I need something that can distribute several different voltages. Say, from one input of 10-12V, I need to get 3.0V, 4.0V, 5.0V, 5.5V and 8V. That's all for a different sub-circuits.
Is there such a thing, better than these tiny step downs?
those mini smd trimmers are also very hard to adjust (fine adjust that is), particularly if it is closer to the lowest or highest end. you need a very steady hand that can make really small adjustments. It is just what it is and works fine if used as intended.
unless you need a really small footprint, I think it is just better altogether to get one that has those blue trimmer resistor blocks.
I bought these just to try them, and liked them because my workbench is really small. But I am now inclined to agree with you.
@raschemmel
Yeah, I started with LM317, but in every project I need 4-5 different voltage levels, which means 4-5 LM317s. It takes a ton of time to wire them up, they dissipate lots of heat and need heat sinks. At least for me, they become cumbersome to manage and distract from the main project.
@jack wp
I am wiring them according to the + & - signs on the bottom. Maybe pots got worn out, because I fiddled so much with them. Are you getting bigger voltage ranges?
If I supply 6V on the input side, the range I get on output is 5.85V to 5.533V. That's with 2.2kOhm resistor accros output. So total adjustement range is about 300mV?
Shouldn't this thing give me a bit more of adjustment range to play with? A couple of volts at least?
