LM317: how to Switch(on/off) voltage regulator LM317

The only way to completely disable the output is to disconnect the input.

What is the input voltage, and the maximum current?

Hi Natesh,

there's no simple way to turn the LM317 off. But you can decrease the output voltage to about 1.25V when short-circuiting R2 by the help of a transistor.

Kai

Dear clemens,

input voltage is 12v,

max current 2A

The BC557 is intended for signals, not for large loads; it cannot switch more than 100 mA. It would be possible to replace it with a large P-channel MOSFET (e.g., CSD25501F3 or DMP3099L) or a dedicated load switch (e.g., TPS22810), but as Ryan said, it would be a better idea to use a voltage regulator with a built-in enable function.

Dear Clemens,

i am try another method, i switch output ground of lm317. this method i get 1.5v at my transistor base is 0.
but when i triggered my transistor base to high i get 2.8v only. what is the problem is there. please suggest any idea to get output with out voltage drop.
my design with lm317 is must. i can't change another regulator for my module

Please show the schematic of your circuit.

Dear Clemens,

i shown my connections at above these two circuits give same output 2.8V. if you find any mistakes in my circuit please point that.

thank you

Hi Natesh,

you want short-circuit the output of LM317?? That's no good idea. First, a huge output currrent will flow and then the LM317 will go into thermal shut-down. That's the best method to ruin the reliability of LM317 and/or to kill the NPN transistor.

Kai

Dear kai

Sorry i made wrong circuit. i was switch output ground. i shown my correct schematic at above  

this circuit i get 1.5v at transistor base is 0.

2.8V at base input is 1.

can you please what is mistake 

thank you

In the last circuit, the feedback resistors (R1,R2) are not connected correctly.

Hi Natesh,

as Clemens already mentioned, R1 and R2 are wired in the wrong way in your last schematic.

Yes, of course, you can use a transistor to switch the signal ground return current of load. Eventually, you can use a n-MOSFET with a low gate-source turn-on voltage. This would allow you a smaller voltage drop across the switch.

You can put the switch also to other locations. At the input line of LM317 you can use a PNP transistor or p-MOSFET. The advantage of having the switch in front of the regulator is, that the output voltage of LM317 isn't influenced by the voltage drop across the switch.

Another option is to use a PNP transistor or p-MOSFET at the output line of LM317. But the voltage drop across the switch again influences the regulated voltage across the load then.

Kai

Dear kai,

thanks for your reply, can you please tell to which is wrong connect R1 or R2. if you can please post correct wired schematic.

once again thanks for your detailed explanation.

 

Dear clemens,
thanks for your support

with regards
kannannatesh