Designing a power supply

[fikui]

I was Designing a variable regulated power supply simulated my circuit in multi I faced many problems I  was able to solve sum of them but sum of them still remain used positive regulator LM317k  the output waveform observed is shown in the figure there is transient curve before the constant voltage???

and after adding capacitor to act as filter the output is constant without any ripples????

there should be sum ripple but graph shows constant voltage,why is this so???

and im using multisim 10.

im attaching my schematic and graph,plz take a look and try to figure out a way to solve these problems

[fikui]

and im using multisim10

[Tom__Sedlack]

Try adding a load to the output?  Can you attach your ms10 file?  It's easier to evaluate the actual circuit.

[fikui]

these are the ms10 files

why would adding a load would make any difference when we have voltage regulator in the circuit??/

[Tom__Sedlack]

You added capacitance to smooth the ripple from the bridge rectifier.  The capacitors charged to the input voltage and then held the voltage at that level because there was nothing to discharge the capacitor.  Adding a load will cause the filter capacitors to start to discharge on the ground going side if the cycle. 

"the output waveform observed is shown in the figure there is transient curve before the constant voltage???"

The capacitors act as a short until they start to charge. That's why the voltage starts at 0 and ramps to the steady state voltage.

"and after adding capacitor to act as filter the output is constant without any ripples????  there should be sum ripple but graph shows constant voltage,why is this so???"

The regulators do hide the ripple until the ripple voltage on the input falls below the regulator input minimum voltage.  For example, the 7805 5V regulator needs 7 volts at the input to regulate properly.  If the ripple voltage at the input dropped below 7 volts, the output regulation could become unstable and the output voltage would also drop (passing the ripple to the output).  

[Tom__Sedlack]

Connect the scope probes to the input of the regulator and vary the load to see the effects of load on the filter.