Using LM317 as current regulator

[nvd]

Current regulator

I am using LM317 as a current regulator. But I do not have the variable resistance required (0 to 100 ohms) for the current regulation. Is there any other simpler method so that I can use variable resistances of higher values like 2k.

 

[IanP]

Re: Current regulator

What you can do is to connect a resistor (example: 1kΩ + 500Ω variable) in parallel with a power resistor (say, 2Ω) and supply the ADJ pin of the 317 regulator from the variable resistor. By doing this you will be able to set almost any current (withi 317 limits).
Choosing a potentiometer you have to remember about minimum current that has to flow to the ADJ pin ..

 

[nvd]

Re: Current regulator

I knew about it but I think this will give a non-linear control over current. Am I correct?

 

[VVV]

Re: Current regulator

Actually, the solution IanP suggested will work nicely. I was about to suggest the same thing before I read his reply more carefully (don't want to be accused of plagiarism).

Essentially the low ohm resistor will develop a voltage across it proportional to the current. Since the pot + fixed resistor has a much higher resistance, it will work just like a voltage divider and thus the adjustment will be pretty much linear. The only term that will introduce non-linearity is the ADJ pin current.

 

[batdin]

Re: Current regulator

But the voltage drop is going to be quite substantial at high currents,am I not right?

 

[nvd]

Re: Current regulator

But this is how current regulator will work. LM317 tends to maintain 1.2V across
VOUT and ADJ pins. If higher current output is required, smaller resistance is used so that the voltage drop is again 1.2V :wink:

 

[batdin]

Re: Current regulator

That's right, but if you use a voltage divider to ADJ pin,then the volatage drop may exceed 10V or more depending on the divider ratio.
If you power this circuit with 9V supply,it just won't work correctly.

 

[nvd]

Re: Current regulator

Can anyone post the schematics of the discussed circuit. I am not getting the idea.
Thanks.

 

[suromenggolo]

Re: Current regulator

you can repalce with L200 cuurent regulator dedicated from ST semiconductor
https://www.st.com/stonline/books/ascii/docs/1318.htm

 

[BOHDA]

Re: Current regulator

Take a look to National Semiconductors application AN 178.It has a nice current regulator with LM317 10mA-1.2A.
If you are not depending on LM317 the better solution is a simple Mosfet regulator controlled in gate.

 

[nvd]

Re: Current regulator

Can you please provide me with the link?

Regards.

 

[IanP]

Re: Current regulator

Here i is:
**broken link removed**

 

[nvd]

Re: Current regulator

Hi IanP,
You discussed about the circuit but I am a bit confused about it components layout? Can you please provide me with the schematics?

Thanks.

 

[IanP]

Re: Current regulator

Attached is the drawing of the adjustable 317-based current source ..