Making an Adjustable SMPS Circuit

This informative article makes an effort to find out a technique during which any SMPS could possibly be produced into a variable power supply for acquiring any required voltage level from 0 to maximum.

SMPS represents switch mode power supply, as the name recommends this power supply concept employs high frequency switching pulses for upgrading mains AC voltages to a selected particular low voltage DC for instance 12V or 24V.

I have discussed this in details in one of my prior content ways to realize SMPS circuits

The SMPS theory today provides almost entirely modified the traditional iron core transformers that have turned these kinds of units into a much compact, light weight and effective power adaptor choices.

On the other hand due to the fact SMPS units are generally accessible as fixed voltage devices obtaining a desired voltage in accordance with the users application needs to have evolves to be very hard.

As an illustration for charging a 12V battery one could might need an output voltage of around 14.5V, yet this value being quite odd and nonstandard we could find it hard to acquire an SMPS rated with one of these features sold in the market.

Despite the fact that variable SMPS circuits can be purchased in the market, these kinds of could be more costly as compared to the ordinary fixed voltage variants, for that reason getting a technique of transforming an existing fixed voltage SMPS into a variable type seems to be much more attractive and appealing.

By looking into the theory somewhat I was capable to locate a straightforward technique of developing the same, let’s figure out how to carry out this modification.

You will see one popular 12V 1amp SMPS circuit during my blog which in fact carries an in built variable voltage benefit.

It is clear that that it utilizes a shunt regulator circuit stage for carrying out the variable voltage benefit in the design.

Yet another attractive characteristic is that this shunt regulator device executes the aspect by regulating the input of the opto coupler of the circuit.

Currently since a comments opto coupler stage is obviously utilized in all SMPS circuits, by starting a shunt regulator one can conveniently alter a fixed SMPS into a changeable counterpart.

As a matter of fact one can also make a variable SMPS circuit making use of the same principle as discussed above.

You really should read more about what’s a shunt regulator and how well it works.

Making a Adjustable SMPS circuit

Making reference to the following illustration circuit, we can manage to come across the specific location of the shunt regulator and its configuration particulars:


Observe the bottom right side of the diagram noted with red dotted lines, it displays the variable part of the circuit everyone is focused on. This portion evolves into liable for the planned voltage regulation behavior.

Here the resistor R6 might be replaced with a 22K pot for producing the design variable.

Magnifying this section delivers the best view of the associated particulars:




If in case you have a fixed voltage SMPS circuit, open it and just watch out for the optocoupler in the design, it could be generally placed just around the central ferrite transformer, as could be used in the following image:


As soon as you have identified the opto-coupler, clean up by removing all the elements related on the output side of of the opto, indicating across the pins which might be towards the output side of the SMPS PCB.

And attach or integrate these pins of the opto with the assembled circuit making use of the TL431, proven in the prior diagram.

You can actually assemble the TL431 section on a small piece of general purpose PCB and glue it on the main SMPS board.

If your SMPS circuit does not possess an output filter coil, you can still simply short the two positives of the TL431 circuit and join the termination to the cathode of the SMPS output diode.

In spite of this imagine your SMPS already consists of the TL431 circuit with the opto coupler then simply find the position of the R6 resistor and replace it with a pot (see R6 location in the first diagram above).

Don’t forget to add a 220 ohms or 470 ohm resistor in series with the POT otherwise while adjusting the pot to the upper most level could quickly destroy the TL431 shunt device.

That’s it, now you realize in what way to convert or make a variable voltage SMPS circuit making use of the above talked about actions.

In case you have an extra problems relating to the design or the clarification, feel free to convey by way of your views.