Lm317.
The formula for constant current is 1.25/R. If I want 300mA current. 1.25/R=0.3. therefore R=4.1ohm. But I want to be able to adjust current from 150mA to 300mA. How to attach trimmer? What size trimmer and how to calculate?

 

a 10 ohms 1 Watt preset will give you the range you need,with the 4 ohm resistor in series rated at 1 watt .

 

But do I get small Smd 10 ohm trimmer. My board is very small.

 

What is the starting voltage Vin? What is the min/max output voltage Vout (compliance)?

At 300mA, you will likely be dissipating so much power that you will need substantial heat-sinking, which is not compatible with a small size, anyway...

P = (Vin-Vout)*0.3

Suppose Vin = 12V and Voutmin = 1V, then P= (12-1)*0.3 = 3.3W is huge...

 

Vin is 6v. Vout is 2.5v.

 

But only reference voltage passes through resistors which is 1.25v. Power on resistors = ixixR = .3*.3*4.1=0.369w.

 

Simple sim. Top plot pane is load current vs R1 for 0V<Vout<3.5V Note that the regulation stops at ~Vout=3V (Vdo+1.25V).

Middle pane is power dissipation in the LM317. Note you will need lot of copper on the pcb or TO220 LM317 with tab.

Bottom pane is power dissipation in R1, which is a fixed resistor plus a trim pot.

 

You could use a 5Ω trimmer in series with a 4Ω resistor.
But the trimmer should be at least a 1/2W device (to carry the maximum of 300mA current) which may be difficult to find.
An alternate would be to use a 10Ω resistor in parallel with a 10Ω 1/4W pot (along with the 4Ω resistor in series with both).

 

I thank everybody for the good information and help.