Potential divider

Question

I am an student and I have made a power supply which can provide either 18v AC or 28v DC(i use a switch to change the output between ac and dc)

Now I want to add something to allow me to vary the voltage but the only way that I found to be working is to connect it to a potentiometer and use it as a potential divider.so I bought 3 potentiometers (1k,5k,10k) to allow me to change the voltage with different accuracies.It is as if you change the voltage in 2 branches which are connected in parallel and when its providing max output voltage one of the branches is almost shorted out(very low resistance)(assuming no load is connected) so I'm worried if it is going to damage/burn the potentiometers.

here is a picture of the power supply(I made 2 of these to provide the above mentioned values)(and sorry for my terrible drawing abilities)

The capacitor is an electrolyte 4.7 micro farad and the diodes are rated for 6 amps but I'm only drawing less than 1 amp (lets just assume maximum is 1 amp in this case) and I use 1 single pole double throw switch and 1 double pole double throw switch to safely discharge the cap and change between AC and DC.And this is what I designed as my potential divider.

^{simulate this circuit – Schematic created using CircuitLab}

My question is do I need a fixed resistor connected in series (between R3 and ground ) or not? And is there a better way to have something to provide variable voltage for both ac and dc or not?

Answer 1

A potentiometer is the wrong way to go about changing the voltage of the power supply. It will create a variable voltage, but with very little current capability. It's just not the right way to do this.

The simplest way is to use something called a linear regulator. You can get them in various fixed-voltage versions, and some you can adjust with a pot. To find adjustable ones, look for adjustable linear regulator or just adjustable regulator.

Linear regulators are inefficient when the input voltage is a lot higher than the output voltage, however, they will do the job, and in any case will be lots better than a pot. You may have to put a heat sink on the regulator so that it can safely get rid of the power it wastes.

There are circuits called buck converters that can drop a voltage more efficiently than a linear regulator. However, those are more complicated, and you should understand linear regulators first.