LM317 current limiter help please

[RogerTango]

I am wanting to use an LM317 variable voltage regulator in current limiter setup.

I understand fully how to wire it up, that part is quite simple.

However, even after much searching with Google, I have failed to find a page that would describe is "dummy terms" how to calculate the resistor value.

Can someone provide a simple explanation how to determine the resistor value?

Id like an example for 12v input, and 5v input, I *think* the R value changes depending on the input voltage(?).

I am wanting to drop current to 30ma (.03a) and 1a, two different applications.

Id rather have an explanation so I can learn, rather than just the R value given to solve the problem.

Many thanks,
Andrew

 

[KeepItSimpleStupid]

page 18 of the datasheet https://www.electro-tech-online.com/custompdfs/2011/05/LM117.pdf shows how to connect. When you read the datasheet carefully there is a mention of a 1.2 V reverence, so 1.2 Volts/ohms = 1 amp, so that's where the 1.2 came from. You also have to pick the resistor wattage. Care to guess how?

The other important notes are, the regulator needs about 3V to work and the power dissipation is (Vin-Vout)/current, however, one needs to be aware that under short circuit conditions in the voltage regulator mode the max current will be drawn. Thermally, the voltage regulator will shut down.

Current is current. The power dissipation of the regulator changes with Vin, Vout and current expected.

 

[ronv]

KISS, That was excellent!

 

[RogerTango]

Can someone provide a simple explanation how to determine the resistor value?

I was hoping... for an example or two to understand the formula, I already looked at the datasheet before I posted.

Andrew

 

[crutschow]

The LM317 will adjust the current to maintain the voltage across the resistor at the 1.2V nominal reference voltage. Thus the constant current from ohm's law is simply I = 1.2V/R. Solving for the R gives the required resistor value of R = 1.2V/I for a desired constant current value of I.

This current is largely unaffected by the supply and output voltage as long as the supply voltage is 3V or so greater than the output load voltage.

 

[RogerTango]

Chow, thanks for the additional info.

I just got down banging out some numbers on my calculator and examining some known figures from various pages, and understand how the calculations are done.

Your information on the +3v was helpful, and I have made a note of that.

Thanks again to all..
Andrew