Is there a regulator bigger than a lm350?

There are all kinds of linear regulators, for example you can look here:

http://uk.farnell.com/ldo-voltage-regulatorshttp://uk.farnell.com/linear-voltage-regulatorshttp://www.digikey.com/product-search/en/integrated-circuits-ics/pmic-voltage-regulators-linear-ldo/2556290and select at "output current" 3A or more (because LM350 is rated for 3A).

Then you can enter "ADJ" in the search box on the left side to see only the adjustable regulators capable of more than 3A.

A good linear regulator I worked with is LT1084, it can do up to 5A. LT1083 can do 7.5A , LT1085 can do 3A...

Just be aware that just because a linear regulator says it can do 3A or more, it doesn't mean in practice it's possible.

A linear regulator works by dissipating the voltage difference between the input voltage and output voltage as heat.

So, for example, if you have 30v at the input and you want 12v 1A at the output, the linear regulator will generate (30v-12v)x1A = 18 watts. Most linear regulators can only do 15 watts, some can do up to 25 watts.

Linear regulators simply aren't designed to work with such wide voltage difference.

If you really want to have 0-18v or something like that, your best bet would be to find a transformer that has two secondary windings or one that has a center tap.

For example, get a 18v AC transformer with center tap, or one that has two 9v AC secondary windings.

When you want a voltage between 0 and 8v DC, connect the center tap to the bridge rectifier, so that the bridge rectifier receives 9v AC. The bridge rectifier converts this 9v AC to 9x1.414 = 12.5v, minus about 2v in the bridge rectifier, so you have about 10.5 volts.

The LT108x has a voltage drop of about 1.1 volts, a LT350 or similar needs about 2 volts, so you can reliably output 0-8v DC.

Now, if you want - let's say - 3.3v, then the regulator will dissipate 10.5v - 3.3v = 7.2 watts. Since the regulator can dissipate 15 watts at best, this means the regulator can do 2 A at best, even if it's rated for 3-5 A.

The design I attached in one of my previous posts, uses those LM195 power transistors for this instead of dissipating power into the LM317 linear regulator... each one of those can dissipate more watts than a simple linear regulator could, and since they're several of them in parallel, that's how that design can do 5 A like it says in the picture.

PS. A TIP147 transistor like the one in your schematic has a 125w power dissipation rating, much better than 15-25w of a linear regulator... but the wasted power issue still applies. You need a HUGE heatsink to keep that TP147 below 150 degrees.