curious observation in using LM350/LM317

Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
I am fiddling with developing 1.2 V to 30 V DC power supply using LM350/Lm317, circuity being same whether one uses either IC.With the transformer(12-0-12) 220 t0 12 V ac, I get converted 34 V dc. I am using fix resistance of value 222 between Iadj and Vout reducing maximum Vout to 30 V. I am using 5K pot between Iadj and earth. The circuit works fine for both IC with a curious observation- As POT resistance goes down, a stage comes for both ICs, when Vout goes on increasing! If I use transformer to get 220 to 12 v ac( actually, with the transformer I got I get 17 Vac!) and reducing to 15 v Dc as input for both ICs, the above observation doesn't hold. Why so? Can any expert find a reason for this curious observation?
I checked my POT and it is working OK.
I am using circuit given in data sheet of LM350/LM317.
sureshparanjape
 

bertus

Joined Apr 5, 2008
21,448
Hello,

The 220 Ohms at the output to Adjust pins is to high.
The regulator needs to be loaded with 10 mA minimum.
A resistor of 120 Ohms between output and Adjust will do this.
Also decoupling capacitors are needed for stable working.

Take a look at this drawing made by Bill_Marsden:



Bertus
 

Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
Thanks a lot.
I will try with less value, as you have suggested, for the fixed resistaance and determine maximum vlaue of POT resistance so that I shall get desired resistance and let me see whether I don't observe that I had.
Thanks again.
I will post back this experience.
sureshparanjape
 

bertus

Joined Apr 5, 2008
21,448
Hello,

I have seen your email:

Sir,,
I tried the circuit given in your reply to my query. Fortunately I had all components. Unfortunately the trial run gave same observation when I used 34 V dc as input.However when I used only half of 34 v dc, using middle tap of the 12-0-12 transformer, it worked nice. Vout for LM350/LM317 is given by the same formula. It appears that Vout doesn't depend on what Vin is. It has linear relation to the ratio of R2/R1.Hence there are various combinations of R2/R1, giving the same ratio and the same Vout. One consideration for the choice of pair of R1,R2 is value of current which should be at least 10mA, as you have stated. There will be only one pair of R1 and R2 giving same Vout and current value = 10mA. I couldn't guess any other condition that would give a better pair by observing graphs from the datasheet of either IC- must be my non knowledge. I am now going to consider building continuous power supply between 0-15 v dc using the circuit you have supplied. Would you recommend inserting two 1N4007 diodes between (i)Vin and LM317 in pin,(ii) between Vout and LM317 in pin or any other place in the circuit?
Thanks for help
sureshparanjape
As I do not answer emails, I will try doing it here.

Do you use a heatsink on the LM chip?
If so, does it get warm, when you add the load?
If it gets really hot, the LM chip will go in shutdown.
The maximum rated powedissipation of the TO220 version is only 20 Watts for the LM317.

It would also be nice if you could post a full schematic of your set-up.

Bertus
 
Last edited:

bountyhunter

Joined Sep 7, 2009
2,512
The 20W dissipation rating for a TO-220 is a bogus spec as it assumes an infinite heatsink. In reality, it can only dissipate about 1.5W with no heatsink and it can safely do maybe 8W maximum for the typical kind of heatsinks used with a TO-220 before it goes into thermal shutdown.
 

Jaguarjoe

Joined Apr 7, 2010
767
If you look at a LM117/LM317 data sheet, you will see that the 117 part only requires 5ma flowing through that resistor so it can theoretically be as high as 240 ohms 1.2/.005 = 240.
The 317 part wants 10ma so the the resistor will 1/2 of the 117's value.
 
Top