light weight power supply

hello guys
i am planning on make a general purpose lab power supply. (3v - 12v, upto 5A). as using normal transformers for the mentioned specifications would make it very heavy i plan on using an unisolated SMPS to initially feed the rest of the circuit in which i am planning to use linear regulators ( LM338) to get the necessary voltages.

my question, is there any problem in using an 'unisolated' supply, isolated supplies are expensive, thats why deciding to use an unisolated one. pls mention the troubles i could be expecting and is it advicable?

thanks

Any form of unisolated supply is dangerous........

Or build a buck reg circuit

Isolate it. You don't want mains shooting right through it to your circuits, and then to you.

Isolating a buck converter is pretty trivial. Just make sure you use isolated switch drivers, a flyback transformer of some sort, and opto-isolators for feedback lines.

Third the motion. An unisolated lab supply will likely make you eligible for a Darwin Award. Don't build it.

Fourth it. Use a transformer.

I built one for my brother thats about in the same range you need. I just used an off the shelf LCD monitor power brick that was rated at 12 volts four amps and tied a external potentiometer into the voltage feed back circuit in order to get a 5 - 15 volt output while still maintaining the 4 amp rating.
One resistor and one pot is all it took to change it from fixed output to a variable.

Weight isn't a concern for lab power supplies, certainly not nearly important as safety.

For a wide range of an adjustable power supply I would not recommend using a three terminal regulator. The internal circuitry monitors in and out voltage as well and output current and if the ratings are close or exceed the power dissipation curves it will shut down regardless of the size of the external heat sink.
I was making a homemade power supply using LM338 and or LT1083CP and ran into this problem. As soon as I went to an external pass transistor and a LM723 as a control IC my power supply worked fine. Course I had to use a pair of low value resistors to externally limit the current.