How to modify output voltage with a potentiometer of a printer smps?

Papabravo

Joined Feb 24, 2006
16,156
The link is broken and leads to a 404 error.
For the potentiometer, you would need a schematic diagram of the board to see if this is even possible. Just because an online source claims something can be done does not mean that you can actually do it. The schematic diagram is the only way to know for sure.
 

Papabravo

Joined Feb 24, 2006
16,156
It looks like the pot(?), if that is what is is, goes in place of C108. No idea what value you need or where you can find one that will fit the holes. Having a schematic seems kinda critical unless you're just messing around and don't care if it works or not.
 

Tonyr1084

Joined Sep 24, 2015
5,977
This is a 12V Wall Wart (actually it's a brick) I modified to output 13.8V. This is the solder side and the symbols of the components on the other side are represented.
1608591754877.png
Located in the "Transformer" region of MY board, I found the TL431 programmable voltage reference and the voltage divider that sets the program. In MY mod, I use a 4.7KΩ resistor and a 5KΩ pot in parallel with a 1.2KΩ resistor ("1201" on the PCB). Adjusting VR502 allowed me to tune in to 13.8V. The SPDT slide switch enables me to take my mod out of circuit and return the PS to 12V.
1608591925857.png

You probably have to identify what part of your circuit controls the output. In my case it's the TL431. Changing the voltage divider network (the 1201 and the 1401 PCB resistors) will change the output. Just be sure that if you UP the voltage output your caps need to be big enough to handle it without exploding. The caps on MY board are rated for 16V.
 

Thread Starter

thedark1811

Joined Dec 20, 2020
8
This is a 12V Wall Wart (actually it's a brick) I modified to output 13.8V. This is the solder side and the symbols of the components on the other side are represented.
View attachment 225575
Located in the "Transformer" region of MY board, I found the TL431 programmable voltage reference and the voltage divider that sets the program. In MY mod, I use a 4.7KΩ resistor and a 5KΩ pot in parallel with a 1.2KΩ resistor ("1201" on the PCB). Adjusting VR502 allowed me to tune in to 13.8V. The SPDT slide switch enables me to take my mod out of circuit and return the PS to 12V.
View attachment 225576

You probably have to identify what part of your circuit controls the output. In my case it's the TL431. Changing the voltage divider network (the 1201 and the 1401 PCB resistors) will change the output. Just be sure that if you UP the voltage output your caps need to be big enough to handle it without exploding. The caps on MY board are rated for 16V.
I can't find the tl431 on my board, this board has a soft23 with a4tfl code without datasheet and I don't know if it's a diode or a transistor
 
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Dodgydave

Joined Jun 22, 2012
9,953
I can't find the tl431 on my board, this board has a soft23 with a4tfl code without datasheet and I don't know if it's a diode or a transistor
IC201 is the Tl431 regulator on the trackside, with the resistors ,I've marked them in white. With the values shown 27K, 3K+820 ohms should be 20V, .

Polish_20201222_111454107.jpg
 
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Dodgydave

Joined Jun 22, 2012
9,953
I have a 10k potentiometer but the tester says it is 11k, how do I connect it? I do not speak English
If you want to Lower the voltage, then put the preset across the 27K resistor pads,(remove the 27k first) , this will give up upto 10V, if you need more to vary from 2.5V to 16V, then replace the preset for a 22K.

Some switchmode PSU s don't like being variable and maybe unstable and give out whining noise.
What language do you speak?
 
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Thread Starter

thedark1811

Joined Dec 20, 2020
8
If you want to Lower the voltage, then put the preset across the 27K resistor pads,(remove the 27k first) , this will give up upto 10V, if you need more to vary from 2.5V to 16V, then replace the preset for a 22K.

Some switchmode PSU s don't like being variable and maybe unstable and give out whining noise.
What language do you speak?
ok I'll try, I speak Spanish
 

Thread Starter

thedark1811

Joined Dec 20, 2020
8
If you want to Lower the voltage, then put the preset across the 27K resistor pads,(remove the 27k first) , this will give up upto 10V, if you need more to vary from 2.5V to 16V, then replace the preset for a 22K.

Some switchmode PSU s don't like being variable and maybe unstable and give out whining noise.
What language do you speak?
If it works, Thanks. Originally the voltage is 24v after modifying. It reaches a maximum of 11.6v but when I put it almost to the minimum it reaches a minimum 3.68v but when I lower it more the poteiometer jumps to 5.6v. Also when I turn it on I hear a spark and the lamp on my table dims for a few milliseconds, what is happening?IMG_20201222_152119.jpg
 
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