LT317 voltage ref problem

[orienteraren]

Hi! I just got my LT317 voltage regulators. I do need a very precise output on 1.25V so I did the design found in the datasheet ( http://www1.elfa.se/data1/wwwroot/assets/datasheets/07331713.pdf ) under typical applications 1.2-25 v adjustable regulator. I connected everything and feed it eith a +12V source. On the output I get 11.52V no matter what I do. I tried to feed it with a 5V supply but the problem is tha same, the output is just a fraction of the input no matter what I do on the trim potentiometer (I´ve tried 0-5kohms with no change in output).

What am I missing? I suppose I´m misusing the componen but how?

Regards

Fredrik

 

[karesz]

Hi,
Whats exactly your circuit pls?
You will never have an output of 1.25V with these IC_remember pls; Uout=Urefx(1+R2/R1)! :-(
It will for you deliver Uref (1.25V) + some(depending of relation R2/R1) to the output.
K.

 

[IanP]

If you connect the ADJ pin to 0V then the V[out]-pin will produce 1.25V ..
Make sure you have the pin configuration right ..

IanP
:|

 

[arefeidi]

Did you measure the voltage between Vout and ADJ? It should be 1.25V. The voltage ADJ to GND should be near 0V.

What is your resistor value for R1? R2/R1 needs to be near 0 to get 1.25V output, so you should set R1 >> R2 (or ground ADJ directly).

I guess there's no problem with the resistors though, do you have an oscilloscope to check if the regulator is oscillating? Try adding a smaller value one (some nF) to the input and output (additional to the 1uF one).

 

[IanP]

Attached is a circuit based on the LM317 that allows for small output voltage adjustment and in the same time provides a lightweight "dummy" load ..

IanP