Hello, I have bought a power supply kit from my local store that has CC and CV. It came with an 18v/1a transformer. I was wondering whether I can use a higher rated transformer to its input (let's say, 24v/3a) since that in the schematic, the TIP42c can handle at least 10a maximum and the lm317 IIRC can still regulate up to 28v

Attached below is the schematic

 

Here's the problem: "Ratings" are the maximum a chip can do, and that's OK if you can keep the chip cool enough. Your kit was designed for 1 amp. If you try to get 3 amps out of it, you are going to triple the heat on the regulator chips and they will not survive very long. Adding about 8 volts to the main filter capacitors will also increase the heat in the regulators, but I don't feel like doing the math right now.

It's a kit because everything has been calculated to balance out before anything smokes. If you mess with that, you have to do all the design work over and that pretty much ruins the idea of, "convenience" which you paid for when you bought the kit. Treat it like a kit or do it over, your choice, but you can't just jack up the power without protecting the regulators.

 

Here's the problem: "Ratings" are the maximum a chip can do, and that's OK if you can keep the chip cool enough. Your kit was designed for 1 amp. If you try to get 3 amps out of it, you are going to triple the heat on the regulator chips and they will not survive very long. Adding about 8 volts to the main filter capacitors will also increase the heat in the regulators, but I don't feel like doing the math right now.

It's a kit because everything has been calculated to balance out before anything smokes. If you mess with that, you have to do all the design work over and that pretty much ruins the idea of, "convenience" which you paid for when you bought the kit. Treat it like a kit or do it over, your choice, but you can't just jack up the power without protecting the regulators.

If I'm not wrong, the Lm317 offloads the current to the TIP42 at a certain milliampere rating by using R3 in the circuit. Does that mean that the Lm317L in the board still receive a fair amount of current even though it sends it to the TIP42 after a set current mininum? Btw, the lm317 looks like a BJT. First time i saw one in this kind of package.

Also, I've apparently worded my thread title wrong. Maybe it would be more appropriate to title it "Why does my psu kit use a 1a transformer".

 

The 317 only passes about 20 ma to 25 ma and the TIP42 does most of the work. That is where the heat will show up. It's a very basic Watts Law equation where P = I (current through the TIP42) times E (voltage across the TIP42). If you try to increase the current, the 317 will have to pull harder on the base of the TIP42, which heats up the 317, and the TIP42 will have more current going through it. Everything is calculated to survive at 1 amp and several parts won't survive very long at 3 amps.

 

The two things likely not to survive a significantly higher current are the bridge rectifiers (D1) and the heat generated in the TIP42.

Also the ripple voltage from the bridge output will also increase proportional to the current.

And note that a transformer must be derated about 60% due to the high peak RMS currents from the bridge rectifier.
Thus a 3Arms transformer can only safely deliver about 1.8Adc output from the supply without overheating.