How to calculate current in R2 with an LM317

[sam_sam_sam]

Re: How to calculate current in R2 with an LM317

Why not limit the current and adjust the voltage it would seem to me to be easier if you are going to use a digital POT you could also have a (switchable current ranges or use a rheostat type of POT ) if you are wanting different current ranges

Or use a Servo to control the rheostat type of POT ( if you are going to use a micro controller )
with a current sensor ( any way )

If I were doing this project this would be the way I would do it
I have giving this some thought about doing this type of project myself

but my new problem is

I now have a new job and work ( 6 days a week until Halloween and then go to 4 days a week ) at night shift 11 pm to 7 or 9 am so I have very little free time any more

I am going to go off topic a little bit —>

I have a power tool that I use at work that the running time is some what short I have some LG 18650 batteries that I have tested that are 3 amp hour rating that I am going to use instead of the LG 1.5 amp hour ( I have a use for these batteries for another project) that are in it right now these batteries are not bad but every three of four days I have to charge them which is not convenient for me unfortunately this tool only offers this type of battery and no other options

https://www.harborfreight.com/12v-li...ery-56566.html

This is the tool that I am using

https://www.harborfreight.com/12v-li...nly-56564.html

The reason I have use this type of tool is I have to limit the torque to the bolts on one of our machines that the bolts are easily stripped if you apply to much torque but if you do apply enough torque then the plates leak product

I also have this tool that I have to figure out how to limit it torque the speed controller does have a very small control step range to it so it not convenient for me the way it is now but I need the reach of this tool

https://www.harborfreight.com/12v-li...nly-57276.html

I will be back on topic

[EasyGoing1]

Re: How to calculate current in R2 with an LM317

now the question is....what is this information for?

Well ... some time ago, I wanted to make my own battery charger circuit for 18650 batteries using an Arduino, a current sensor, and at the time, I wanted to use a LM317 with R2 being an adjustable POT on a chip. That way I could read the current going into the battery, then adjust the voltage being fed to the battery by means of the digital POT. HOWEVER, those digital pots are very limited in the amount of current they can withstand ...

So how did I get here? Well, I was placing an order on Mouser and I like to try and maximize my shipping fee by getting things I either need or might need in the future and I realized that my LM317 supply was down to 1, so I was looking at various LM317's and looking at the datasheet for one of them and that triggered my recall back to that project and then my wondering about the current in R2 because I still have digital POTs and I might attempt this circuit again only this time ... with more knowledge going into it.

I don't know at the moment, what the current limits are on the digital POT, but if I attempt it again, I will make sure that the POT can handle what I'm going to ask it to do.

And now that I'm thinking about it in a little more detail ... this does seem to me to be a crude way to accomplish the charging of a battery.

But that should answer your question.

[redwire]

Re: How to calculate current in R2 with an LM317

I R1 is constant as the LM317 always maintains 1.25V between the OUT and ADJ terminals.
1.25V/datasheet 240R for R1 is 5.2083mA, which must flow through R2 plus the ADJ pin current ~50uA which is so small it is usually ignored to simplify math. Exactly I R2 would be 5.2583mA
This is from using Kirchoff's current law.

Note the LM317 needs a minimum load which is why R1 is set to a low value.

[eccerr0r]

Re: How to calculate current in R2 with an LM317

now the question is....what is this information for?

[EasyGoing1]

Re: How to calculate current in R2 with an LM317

Actually I screwed up, should be (1.25/R1 + a bit, usually around 60µA or so), math isn't cooperating with me nowadays. Sigh. I knew something was wrong with the previous answer when I tried dividing by zero, something was definitely wrong... But yes it should not matter what the load is, as long as the LM317 is operating within its design point - both the IC and the entire circuit.

My calculus instructor back in jr. college had a gift for connecting math concepts to things which anyone could comprehend ... for example when he explained dividing by zero ...

He said ... picture yourself with a 5-gallon bucket and a hole in the ground that is the same size as the inside of the 5-gallon bucket.

Next, fill the bucket to the top with water then pour the water into the hole. How many times did you have to fill the bucket in order to fill the hole?

Next, fill the bucket halfway, and each time it is half full, pour it into the hole then tell me how many times you had to fill the bucket halfway before the hole was full.

Repeat for ¼ bucket water etc.

NOW, leave the bucket EMPTY and pour its contents into the hole... how long will it take you to fill the hole when the bucket is empty...

And THAT is why dividing by zero is infinity!

[eccerr0r]

Re: How to calculate current in R2 with an LM317

Actually I screwed up, should be (1.25/R1 + a bit, usually around 60µA or so), math isn't cooperating with me nowadays. Sigh. I knew something was wrong with the previous answer when I tried dividing by zero, something was definitely wrong... But yes it should not matter what the load is, as long as the LM317 is operating within its design point - both the IC and the entire circuit.

[EasyGoing1]

Re: How to calculate current in R2 with an LM317

IR2=Approximately 1.25/R2

Regardless of the current being drawn by the load?

[eccerr0r]

Re: How to calculate current in R2 with an LM317

IR2=Approximately 1.25/R2

[EasyGoing1]

Given this circuit:



How can I calculate IR2?