# variable current

Discussion in 'General Electronics Chat' started by imraneesa, Apr 12, 2015.

1. ### imraneesa Thread Starter Member

Dec 18, 2014
189
4
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?

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,171
777
a 10 ohms 1 Watt preset will give you the range you need,with the 4 ohm resistor in series rated at 1 watt .

3. ### imraneesa Thread Starter Member

Dec 18, 2014
189
4
But do I get small Smd 10 ohm trimmer. My board is very small.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
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...

5. ### imraneesa Thread Starter Member

Dec 18, 2014
189
4
Vin is 6v. Vout is 2.5v.

6. ### imraneesa Thread Starter Member

Dec 18, 2014
189
4
But only reference voltage passes through resistors which is 1.25v. Power on resistors = ixixR = .3*.3*4.1=0.369w.

7. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
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.

8. ### crutschow Expert

Mar 14, 2008
13,523
3,392
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).

9. ### imraneesa Thread Starter Member

Dec 18, 2014
189
4
I thank everybody for the good information and help.