lm 317/117 working

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swordfish12

New Member
hi everyone,
I am not completely able to understand the working of lm 317,i took a look at the datasheet but could not understand the "constant current output" part and the output voltage calculation ....... a little help needed here.....

ronsimpson

Well-Known Member
The LM317 wants to have 1.25 volts from ADJ to OUT.

When power is first applied there is INPUT and no OUT voltage. So there is 0V from ADJ to OUT. This causes the LM317 to turn on hard and pass current from IN to OUT. This causes OUT to come up in voltage.

Then OUT has good voltage on it, the voltage divider will put 1.25V from ADJ to OUT. This causes the part to keep that amount of current flowing from IN to OUT.

If the OUT is too high there will be more than 1.25 from ADJ to OUT and the part will stop passing current. The OUT will fall in voltage.

The part will do what ever it takes to get 1.25V from ADJ to OUT.

ronsimpson

Well-Known Member
Thanks Mike.

I never answered the question on constant current.

The output CURRENT (not voltage) comes from OUT and passes through a resistor. The LM317 tries to pass enough current to get 1.25 volts across the current sense resistor. (The current is not coming from the ADJ pin)

swordfish12

New Member
thank you...

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