How to change the voltage output of a smps - Page 1
Forum section: electronics / beginners

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 20, 2023, 12:19:09 pm
content: I've seen a few youtube videos of people changing the output voltage of their smps by changing a resistive divider on their boards. I'd like to do this with a 24v/5a power supply that came with my soldering iron, I want to change it to 21v output.I think the resistor I need to change is r18? I think it needs to be about 17kohm for a 21v output but I haven't messed much with smps's so I'm not very sure, and I can't find an online calculator like for the lm317. lm317 calculator doesn't seem to match what my power supply is outputting so I can't use that.Here's a link to a power supply I'm pretty sure is identical to mine.http://www.icstation.com/step-down-power-supply-ac110v-245v-voltage-converter-p-16037.html

author: tunk
author_rank: Super Contributor
timestamp: April 20, 2023, 12:29:20 pm
content: Or possibly R19 to 2.7k.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 20, 2023, 12:55:39 pm
content: Which is better to change? I have trimpots in both ranges.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: April 20, 2023, 01:06:37 pm
content: You could also try connecting 120k in parallel with R18. This should give equivalent of 17.1k of resistance. Output voltage should be >21V.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 20, 2023, 01:20:53 pm
content: You can change either, or replace both of them with a single pot. Really its up to youHere's a quick sim to see how varying R19 would work, from 1k to 20k

author: tunk
author_rank: Super Contributor
timestamp: April 20, 2023, 02:01:04 pm
content: 2.7k is a standard resistor value.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 21, 2023, 12:32:07 am
content: Success!I decided to change R19 because it was easier to remove and replaced it with a 5k trimpot, 21v works fine and the trimpot should be ok safely tucked between the control board and the secondary on the psu.

author: tunk
author_rank: Super Contributor
timestamp: April 21, 2023, 12:40:13 pm
content: There's one potential problem - what will happen ifyou accidently short the trimpot? I think most SMPSsuse a trimpot on the "high side" (R18).

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 22, 2023, 01:20:44 am
content: Quote from: tunk on April 21, 2023, 12:40:13 pmYes I think I should've installed the trimpot on the high side like with the lm317(it needs between 100-1000ohms on the low side), my smps can't cope with voltage less than 18v set with the trimpot because I think the trimpot value becomes too high(seems to cause wild oscillations). I don't think shorting the trimpot is a realistic possibility(I shrink wrapped the leads and the screw isn't connected to any of the pins) and it works fine at 21v so I'm not going to bother changing unless somehow I repurpose the smps for a different purpose.

author: Jwillis
author_rank: Super Contributor
timestamp: April 22, 2023, 09:16:04 am
content: The chip on board is most likely CR6842s PWM .https://pdf.dzsc.com/88888/20086179315462.pdfThe circuit looks very familiar. Check the pin outs and traces to be sure.

author: MrAl
author_rank: Super Contributor
timestamp: April 22, 2023, 09:35:35 am
content: Quote from: bobbydazzler on April 20, 2023, 12:19:09 pmHi,Is that an LM431 chip?A lot of wall warts use that.Plenty of info on the data sheet.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 23, 2023, 07:30:56 am
content: It is indeed a TL431 programmable reference. Its used in isolated power supplies to provide feedback through the optoisolator to set the correct output voltage

author: MrAl
author_rank: Super Contributor
timestamp: April 23, 2023, 03:13:20 pm
content: Quote from: BadeBhaiya on April 23, 2023, 07:30:56 amHi,Yes that's why i asked and mentioned wall warts.Ok then output is easily adjustable as per data sheet.

author: G-son
author_rank: Regular Contributor
timestamp: April 23, 2023, 11:15:07 pm
content: Is it common that such SMPS has any additional voltage control functions, for example to disable them if the voltage goes too far above or below the factory standard? In other words, can I adjust the TL431 resistor divider as much as I want (within the voltage capacitors etc. can handle) or could I run into other circuits meant to protect against incorrect voltages?

author: MrAl
author_rank: Super Contributor
timestamp: April 24, 2023, 02:17:35 am
content: Quote from: G-son on April 23, 2023, 11:15:07 pmHi,Yes sure, there could be comparators that sense voltage and detect over and under voltage conditions and shut down the supply, just like in most computer power supplies.The only way to know for sure is either find a schematic, draw up a schematic yourself, or just try to modify the LM431 by a small amount and see what happens.You can sometimes draw a schematic yourself by tracing out the circuit traces and trying to figure out what IC's are being used, and hopefully they are marked. You can try to follow the traces by eye or by taking photos, and also can use an Ohmmeter. I did this on several circuits in the past and although it takes time and patience it works.What power is this supply maybe a new one of the right output would not cost that much.

author: Jwillis
author_rank: Super Contributor
timestamp: April 24, 2023, 05:36:51 am
content: As I mentioned before. It's a CR6842 SMPS. Not much to them. The component values may be different but the general diagram is similar.

author: MrAl
author_rank: Super Contributor
timestamp: April 25, 2023, 11:49:42 am
content: Quote from: Jwillis on April 24, 2023, 05:36:51 amHi,Ok then R18 and R19 control the output voltage.Vout=2.5*(R18/R19+1)However, i would not worry too much about going DOWN in voltage but i would be careful about going UP in voltage as the other components on the output may not be able to handle the increase. That includes the output caps and even the output LED current via it's resistor.

author: Don Warr
author_rank: Newbie
timestamp: April 10, 2025, 06:02:07 am
content: This was great,,, But the schematic and the formula do not match up... The schematic shows 12v with R18=20k and R19= 5.1k...But pluging in those values to the formula gives me a voltage output of 9.801 v,,,I have a 24 volt output smps, same schematic but r18= 20k and R19= 2.32k... and it gives me 24v... Plugging in those numbers in theformula the voltage should be 21.5v...Can someone Help me out with this, as I hate trial and error stuff.... Any thoughts would sure be appreciated...

author: tunk
author_rank: Super Contributor
timestamp: April 10, 2025, 10:21:36 am
content: I think you're missing the "+1" in the formula.

author: Don Warr
author_rank: Newbie
timestamp: April 12, 2025, 10:49:48 am
content: Yes, I was tired, but, excited to have found the info here, and was adding the 1 to R19 only... The smps's that I have follow the namingconvention of the schematic exactly... If anyone Knows where I might find a good filter to go on the output, please let me know... and thanks tunk for saving my mind... lol.