help please.

Hello friends.

This is my first post here. I am a newbie.While surfing the net i found this forum very active and good source of help.

It is almost a month since i started to take interest in electronics.I took my time to understand the basic components and made few simple circuit mostly just transistors based to learn from these.

Now i think i need a power supply that should be adjustable to test the circuits those i want to make in future. There is lot of circuits on the web available. But i can,t figure out which one i can select for my hobby.

Please help me to design my supply. The specification i need with my supply are as uder...

OUTPUT voltage=0V-12V adjustable
OUTPUT current=lowest possible-3A adjustable.(must have)

i found this simple one.Variable DC Power Supply - RED - Page36. But with different specifications. Can it be modified for my needs.I have a very nice Transformer 18v 5A. But it not center tapped.
I will be much thankfull if i could get some help.Thank you all.

The circuit you found is inexpensive. So its voltage regulation is not very good. The output voltage will drop when it is loaded. Its output transistor gets extremely hot when the output voltage is low or shorted and its current is max at 2A.

The circuit shows a 36VAC center-tapped transformer and two rectifier diodes. You can use your 18VAC transformer with 4 diodes or with a rectifier bridge.

Before i could get the reply from Audioguru, i made the circuit on veroboard, placed the 2N3055 transistor on large heat sink. Applied the 18v supply. Every thing worked well with minor load.Increasing the load caused problems in voltage regulations and serious drop of voltage at the out put(at 2A load 9.8V reamined at output). Then i changed resistor value of power resistor to 0.5R/5W.Gradually incresing the load i managed to reach 3A but again the same voltage drop and loss of voltage regulation.The only thing that surprised me that after 10 minutes of operation at 3A load was the heat generated by the transistor, it was not too much.I could hold the transistor in my hand without burning my fingres.(larger heat sinks are better but not beautifull).

After that i tried another circuit that used a couple of lm350 v-regulator. one for current control and other for voltage control. But nothing better than the previous cicuit.Again a Large voltage drop.

Now i am stuck. Which way sould i go?

The transistor did not get too hot because it had a fairly low voltage across it because the output voltage was set high. The transistor will experience max heating when the output voltage is set low.

The LM350 has excellent regulation. Its output voltage drops typically only 0.05V when its load changes from 10mA to 3A but only if it has 10V or less across it. so in your circuit it gives 3A from 13V to about 21V. as the output voltage is reduced it limits the output current to about only 300mA.

Maybe your transformer cannot supply 4.24A AC so that the power supply can supply 3A DC.

With your 18v transformer use a bridge rectifier (or four diodes) as AG said. Use at least 1000uf capacitor after the bridge.
See if the voltage stays higher.
I would use LM317 ICs for both voltage and current regulation.
Use pass transistors for higher current as needed.