Hello...im quite new to multisim...but im trying to design a power supply with 5v, 9v, and 12v fixed voltage and 0-20v at 1amp voltage regulator altogether...can someone help me...

 

I almost always start a project like this with looking in the junk box to see what I already have on hand. Most critical is the transformer, second is availability of filter capacitors, then an enclosure to put it in. Everything else is cheap and easy to get...

 

Transformer, rectifier, filter capacitor, LM7812, LM7809, LM7805, a 24V zener diode to a pot that drives an NPN Darlington transistor for the 20 volt end. Or, use the +12V supply to drive the potentiometer and use an op-amp to amplify that voltage by 5/3 to drive the 20 volt output transistor. Or use an LM317 for the 20 volt output if you can accept the idea that an LM317 chip can't go below 1.25 volts.

Sorry about the Multisim part. I don't know how to do that.

 

Would you consider downloading LTSpice? It's free and more people use it (I think).