this post was submitted on 03 Nov 2023
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Adjusting SMPS output voltage (discuss.tchncs.de)
submitted 1 year ago by Rolive@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de
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Does anyone recognise these power supplies? They're cheap AliExpress led drivers and I want to change its output voltage to around 22V from 12V. I've read that the way to do this is to adjust the REF voltage on the IC that controls it. It's a KA3845 but I don't understand where that reference voltage is regulated. One voltage is feedback from the output where then other should be a reference.

What would be the best way to approach this? I can't find any schematics on these boards unfortunately.

Thanks.

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[–] cmnybo@discuss.tchncs.de 7 points 1 year ago

You might be able to increase the voltage 10-20% by changing the feedback, but you will not be able to double it without changing the transformer.

[–] BigDanishGuy@sh.itjust.works 6 points 1 year ago

As others have said, increasing the output by 5/6 is unrealistic. And even it would work, it'd only be for a short while. Best case is that it works and you're home and awake when it catches fire.

Better buy a dedicated powersupply. How expensive something is, is in the eye of the beholder, but it is worth it. Most of the ones on this list can be adjusted 10 or 15% and that should get you there from 24V. And they're actually meant to do it. https://dk.rs-online.com/web/c/stromforsyninger-og-transformere/stromforsyninger/switch-mode-stromforsyninger-smps/?pn=1&rpp=100&sortBy=P_breakPrice1&sortType=ASC&applied-dimensions=4294965650,4290297838,4291523693,4294965001,4293335399,4293589165,4294472088,4294478231,4294273800

[–] renormalizer@feddit.de 4 points 1 year ago (1 children)

It's probably not a good idea for actual use, but if you'd like to experiment: looks like the 3-pin devices U4 and U5 on the upper right provide feedback through the optocouplers next to the class Y cap north of the transformer. I bet those are LM431 voltage references (or similar). The passives around them provide filtering, but two of the resistors should form a resistor divider for the Ref pin (lower right pin if the single pin is on top). That divider sets the voltage.

[–] Rolive@discuss.tchncs.de 2 points 1 year ago

Thanks. I ordered the 24V model in hopes of adjusting it down to 22V. I will use that to keep a 7s Li-Ion battery at a minimum charge level whereas a solar panel array may increase that voltage higher. It looks like the 24V model's capacitors need to be changed as well since they can't handle the Li-Ion batterys' max charge voltage.

[–] Rolive@discuss.tchncs.de 1 points 1 year ago

Replying to myself for informational reason. Modifying voltage was more or less successful. Both optocouplers transmit a reference voltage, so both need to be adjusted simultaneously.

This can be done by changing the value of R19, a 23.2k smd resistor close to the output terminals.

I've attached a 10k pot with a 10k and 6.8k resistor in series and successfully modified the voltage down to 22.5.

The power supply itself is a piece of crap though. It claims to handle 400W but anything over 150W causes the short circuit protection to activate, never mind overheating at 150W very quickly.