How to limit current flowing through LM317

Thread Starter

iinself

Joined Jan 18, 2013
98
What determines the current? When you say you are looking for 7A, is that under all conditions? Or is that just the max current at the minimum value of the load impedance?

What is the maximum voltage you can live with at the output of the circuit?
It is the maximum current the amp will draw. Most probably never unless I drive it to about about 50 Watts of output, I am only looking at about 20W mostly. Just designing for the worst case.
 

WBahn

Joined Mar 31, 2012
29,976
I was hoping the change in the emitter/collector current will cause the base current to adjust to maintain the hfe ?
But you are using the second LM317 specifically to deliver a constant 10mA of base current (until the regulator hits its limits, which by definition means you are out of control), so how can it adjust?
 

Thread Starter

iinself

Joined Jan 18, 2013
98
But you are using the second LM317 specifically to deliver a constant 10mA of base current (until the regulator hits its limits, which by definition means you are out of control), so how can it adjust?
The datasheet calls that second topology current limiter, so I was hoping it will allow less than 10mA as need by the base which will adjust based on the Ie/Ic which will depend on the load. I did think about the difference between current limiter and constant current source, finally went with the what the name meant.
 

dl324

Joined Mar 30, 2015
16,844
Hi iinself,
Hi,
I am to reduce Vin to Vout but my current requirements are higher than the 1.5A that LM317 can supply. [snip] Since the current is being provided by the transistor I do not want to put a large heat sink on the LM317 also and thus would like to reduce the current through it to a bare minimum enough t drive the base of T1.
Attached is the circuit I would use. You use the ratio of R1 and R2 to set the current division. Since the LM317 has built-in thermal protection, you should let it provide it's max current and install the regulator and Q1 on the same heatsink (taking care to consider that the tab on the regulator and the collector of the power transistor are at different potentials). If the regulator goes into thermal protection, it will protect your pass transistor. I used a classical PNP power transistor substitution (Q1, Q2, R3) since you used a power NPN in your original circuit. The diode connected Q3 is to match the Vbe of Q2 to so the resistors set current division.

You'll need to make some component substitutions. If you run 1.5A through the regulator, Q3 needs to be substituted. You could go with a diode, but matching with Q2 won't be as good.

HTH,
Dennis
regCurDiv.jpg
Edit: I left out a resistor...
 
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