How to Modify an SMPS Circuit

SMPS stands for switch mode power supply and its the modern and most compact/efficient way of acquiring low voltage DC from mains AC source. However making an SMPS at home may not be as simple as making power supply units using traditional iron core transformers. Also getting an SMPS with custom specs may not be that easy, in fact impossible if the voltage/current specs are far away from the regular values.


So does it mean that we have be satisfied with the SMPS specs which are typically set, and available in the market? For example how do we get an SMPS having an output of say 13 volts or 14 volts or 17 volts which are definitely not the normally accepted voltage ranges?

Since making such a customized unit may not be an easy task (due to complex layouts and part configurations) it would be a lot better if we could find ways of modifying a readymade one through some simple steps.

I have studied a few standard SMPS units and hopefully cracked ways of modifying the voltages and current as per individual choices. Lets learn it in details.

When you open any standard SMPS unit, you will come across the following things over the enclosed assembled card.

The populated PCB can be primarily divided into two sections by the presence of the center ferrite transformer.
The side of the transformer where the mains chord makes its entry is the input AC section while the other side from where the low voltage DC is derived is the DC section.

We are not interested in the AC section because we do not want to modify the input voltage so do not pay any attention there, moreover the AC section is POTENTIALLY VERY DANGEROUS TO TOUCH IN SWITCHED ON CONDITION, THEREFORE KEEPS YOUR HANDS OF IT WHILE TESTING.

The DC section will mainly consist of a couple of chokes, a couple of filter capacitors, a diode and a few other components.

Search for a transistor shaped component in this section. If you find a couple of them, one will be actually a transistor, probably for limiting the output current, however the other one will be definitely THE PROGRAMMABLE SHUNT REGULATOR.

This shunt regulator is the component which fixes the feedback voltage to the AC section mosfet and in turn determines the output voltage.

This programmable shunt device is set up through a couple of resistors, changing which will instantly change the output voltage as per ones wish.

Try to locate the resistors connected with the leads of this shunt device. One of them can be simply varied for changing the output voltage as per your preferences.

Take an external resistor of any value may be a 4k7 1/4 watt, now step-wise go on connecting this resistor across the resistors which are associated with the shunt regulator device.

Verify the output voltage each time you do the above step.

The moment you find a change in the output voltage either becoming low or high, you might have just found the one which we are looking for.

Now through some trial and error you may find out the exact value of resistor which could be replaced in place of the particular shunt resistor.

Thats it, its as simple as that, once you do it, the output voltage would get adjusted to that particular value permanently.

But do remember to remove the zener diode if theres any at the output of the power supply before you do the above procedures.