Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Changing voltage of smps

Not open for further replies.


Advanced Member level 3
Feb 5, 2002
Reaction score
Trophy points
Activity points
I have a lot of switch mode adapters rated 15v-1A. I want to reduce the voltage to 12Vdc with. Is that possible without degrading the performance of the unit. The design uses a TL431, and optocoupler feedback for regulation. It also has some zener diode near the output circiutry.

Any lead would be appreciated



There are normally a comparator which sets the voltage you want by lowering this level you can change the output. But regarding TIs datasheet the normal (1+R1/R2)Vo is valid in some applications so perhaps this is the easiest way. Check here:

By changing the Vref that the comparator uses to determine the switching instances you can lower the Vo.

Good luck

Would doing that (reducing the output voltage from 15 to 12v) degrade the performance of the design. I mean to say that would this change put any extra burden on any semiconductors in the ciruit.



you cant

add a 12 v regulator is my advice

it isnt so easy
as the switching transformer will still supply the top voltage
even when the stages drive is reduced

so the lt will always remain the same

the regulation stage in one of these switching psu is purly to give the current

the transformer dictates the output the tl chip is purly there to "regulate" these rails {keep them steady} so either add a regulator
or give up
otherwise youll get wierd results .....UNLESS it already has adjustments of each rail and post switching transformer regulation to enable change

Just search a resistor what coming from 15V out to TL431. Identify the value and decrease it 10...12% with paralel resistor.


I don't think that by fiddling the values in the feedback comparator's circuit will have much of a affect on the performance. 12v is close enough to 15v

All that will happen now is that the switching duty cycle will be slightly reduced (That is how a smps normally regulates). Some smps's do exactly that, they have a pot that you can adjust to set the output voltage.

You can add a 12v regulator but it is easier just to change the resistor value.

If the voltage diff was much more, say 5v from 15v I will remove some of the windings on the XFMR as well as changing the resistor value.

The TL431 is just an adjustable zener and the opto provides isolation between the Prim and Sec side. The extra zener is for crowbar protection should the regulation cct fail and the volts shoot up.


Not open for further replies.

Part and Inventory Search

Welcome to