Original question:
Hi Arvidn,
Yes I can help. It looks like you want this circuit. This is in the applications section of the LM317 datasheet. They call it a current limiter, but in really this circuit just places the device into a constant current mode. The current flowing through R1 is equal to 1.2/R1. And the current cannot flow into the adjust pin so it must go to the load.
You won't be able to use the LM317 for this. 1. It can only provide 1.5A without possibly entering current limit, and 2. it will dissipate a lot of power if you maintain the minimum headroom of 3V.
The LM1085 is a better option - it has 3A load current and has 1.5V of required headroom. If you use this device, make sure you get the adjustable version and configure it the same way as you see for the LM317 above. You will also want a heat sink to dissipate the 3W. Other LDO's with a floating reference that could also be used are the LM150 and LM350, but they have higher dropout voltages so I would use the LM1085 if possible.
The circuit above simply changes the regulation from voltage to current. If you need to essentially build your own discrete lab supply where you can adjust the output voltage and current limit dynamically, you will want a more complex circuit. One option is provided in the LM150 datasheet applications section. I have built this and can confirm that it works but I would use modern components (modern BJT, diodes, Op-Amp, etc). This circuit will allow you to set the current limit using R2 and the regulated voltage using R8. You could set R8 with a fixed resistor to set Vout = 5V if you don't need to adjust that.
Thanks,
Stephen
