Semi-homemade 19V Adjustable bench PSU

[antolanca]

Hello everybody.

Right. I want to build a Bench PSU using a laptop charger and connect a couple of LM317's to adjust current and voltage.
The charger has got 19V 3.95A (from my toshiba satellite) and I want 1A at at least 12V.

Now comes the tricky part.
I'm expecting to have a voltage drop because of the LM317's so the max. voltage will probably be 12 to 15V.
It's the amps that mess me up.  Acording to Ohm's law I would need a 19R 19W Load Resistor to get 1A which is my target. But the output of the transformer is 75.1W.

Now, how much power and resistance do I need to get the 1A ?
I've laded a schematic to give you a better idea. the load resistor is on the left.
Thanks

[Zepnat]

The 19v power supply charger should be regulated and not need any load resistor just get rid of it it will waste power. If the charger has 3.95A available the limiting factor will be the LM 317 which I think output up to about 1.5A before self limiting.

[mos6502]

Bad, bad, bad idea to use the LM317 for current adjustment. The droput voltages add up. You'll end up getting maybe 10V out of your 19V. In addition, the current sense resistor will dissipate a lot of heat.

Either use one LM317 and forgo the current adjustment. That will get you a decent, usable 1.25-15V/1.5A supply with your 19V input. Or use a different regulator. The LM723 would be a good choice. It has the advantage of having the pass element separate so you can choose that for the current you need. It also has a current limit settable by a resistor. You could make the current limit switchable with a toggle switch by using different resistors.

[rdl]

You would get 12-15 volts out. The problem is that you have made the current limit adjustable. That 5k pot is going to have to be capable of carrying the maximum current. It is probably better to use a pass transistor with an op amp for the current limiting, something like what is shown in the data sheet on page 21.