LM317 Variable Voltage + Variable Current

crutschow

Joined Mar 14, 2008
31,552
..................will R1 get hot during current limiting ? all in know LM317 is acting as a voltage regulator and of course will get hot , but which component is responsible for current limiting ?.............
The voltage across R1 is indeed used to determine the current limit.
Power is I²R, thus the maximum power dissipated in R1 is 1.2² * 0.1 = 0.144W so a ½W resistor would be more than adequate.

ShayanFiroozi

Joined Nov 18, 2015
41
Below is the LTspice simulation of an LM317 with an LM339 comparator that senses the voltage across shunt resistor R1 due to the load current.
The comparator reduces the regulation pin control voltage for the LM317 when the current exceeds the threshold set by the voltage reference U5 and pot U2, thus performing the current limit function.
The current limit can vary from 1.2A to essentially zero current by varying pot U2, as shown for pot positions of 0%, 50% and 100%.
Pot U4 controls the maximum output charging voltage.

View attachment 105479
Mr crutschow
would you please indicate which pins of LM339 should i use ?

crutschow

Joined Mar 14, 2008
31,552
Mr crutschow
would you please indicate which pins of LM339 should i use ?
The LM339 data sheet (page 3) shows you the package pinout.
You can use any one of the 4 compartors in the package.
Also connect power and ground to pins 3 and 12 respectively.
The unused pins can be left open.

ShayanFiroozi

Joined Nov 18, 2015
41
The LM339 data sheet (page 3) shows you the package pinout.
You can use any one of the 4 compartors in the package.
Also connect power and ground to pins 3 and 12 respectively.
The unused pins can be left open.

Thank you
i made this circuit on breadboard (i attached the file)
but there is a problem
i set voltage to 13.7 volts but when i connect VCC of LM339 voltage drops to 2.8 volts !!! i don't know why
also when i wanna measure the output amperage of the circuit my multi meter indicates 2.5 AMP and then my LM317T will not work again !!

my transformer is 16 volts 3 AMP

it seems circuit doesn't limit the current

now i have 4 damaged LM317T on my trash

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crutschow

Joined Mar 14, 2008
31,552
You likely have an error in your circuit breadboard.
Carefully trace out every connection as compared with the schematic to determine where that is (I use a highlighter to mark each connection on the schematic as I trace it on the circuit board to make sure I check every connection).

ShayanFiroozi

Joined Nov 18, 2015
41
You likely have an error in your circuit breadboard.
Carefully trace out every connection as compared with the schematic to determine where that is (I use a highlighter to mark each connection on the schematic as I trace it on the circuit board to make sure I check every connection).
i checked it out
many things !!
1- when i remove 1nf capacitor from LM339N circuit voltage will be ok ! i don't know why , when i put it back circuit volage is under 4 volts
2-many pressure on LM317T because of current limiting and voltage providing(IC won't survive even with big heatsink)

is there any way to ise 2 LM317T ? one as current limiter , one as voltage provider(as well as LM339N or C945 for cutoff) ??

thank you again