PROBLEms of LM317

[dumbnish]

ppl i need urgent help....im makin a 0-30v power supply using lm317 but its regulated output is from 1.2-37.....i need it from 0v not 1.2 v.....hw can i make it work...is der neways by using opamps etc....plz help me fast

[indulis]

Yes you can, you have to "off-set" the adj pin lower resistor to -1.2V.
Now... how you get the -1.2V is another story. The key is to be able to have the adj pin go to zero.

[audioguru]

Yes you can, you have to "off-set" the adj pin lower resistor to -1.2V.
Now... how you get the -1.2V is another story.

Simply use two LM317 ICs. One is the main regulator and the other makes a perfect -1.2V for it. Get them with the same date code so they are nearly the same.

[dumbnish]

is der a better way by using opamps or smthing like dat....also can we use zeners?

[Theatronics]

Not to be blunt, but - no.

The -1.2 V is the answer.  It regualtion comes from the LM317.
if you feed it the negitive ref voltage, you will get 0 volts out.

I have heard of Boostraping OpAmps with high power transistors to make high power amps and also using Ziners and high power amps to create power supples.
Can it be done? I don't see any reason why not.  But how would you very the voltage output of the device with a zener as the source voltage?

I think AG's solution is the occoms razor in this case.

-Mike

[dumbnish]

ppl temme dis........wen u say apply -1.2 at adj pin.....u mean like applyin it using another lm 317.......wont der a problem of feedback(sourcing)...also wats d best way of producing -1.2 for dis lm317.............according to u guys?

[ante]

This is one way to do it that works fine:

[dumbnish]

hey ppl i need help wid d above ckt..........can sm1 plz explain d use of IC1..is it only for current limiting.....also wats d input to IC2??
guys d ckt is a gr8 find thnx......but i need further help.......

also temme hw to include short ckt prevention.....plzzz

[MP]

You just need to make sure your negative lead on the power supply output is 1.2 volts lower than the common connection of the LM317. The output of the power supply does not have to have it's zero potential connected to the "ground" pin of the LM317. Does that help? There are a number of ways to do this. You can purchase a -1.2V precision reference IC, use voltage dividers, op-amps, additional regulators, transistors, etc.


BTW....LM317 is a positive voltage regulator. If you are going to use another regulator for a negative voltage reference, you would want to use the LM337 for this. It is a negative adj. voltage regulator.

MP

[ante]

After decoding this:

hey ppl i need help wid d above ckt..........can sm1 plz explain d use of IC1..is it only for current limiting.....also wats d input to IC2??
guys d ckt is a gr8 find thnx......but i need further help.......

also temme hw to include short ckt prevention.....plzzz

I gathered you want to know about the following:
Yes IC1 is the current limit part.
The current limit does prevent short circuiting.
You can find a complete explanation here: http://www3.telus.net/chemelec/Projects/Power-Supply/Power-Supply.htm