Help required for Linear Power Supply build using LM317

[IanB]

You need to use dots on where the net connections are joined together. This is just sloppy work.

The presence of dots or otherwise at junctions is an attribute of the schematic drawing package.

However, there is a well understood and standard drawing convention that lines are connected at tees where three lines meet, and lines cross without connection where two orthogonal lines cross over each other.

There is nothing sloppy about using a different drawing convention than the one you are used to. It just means you need to adjust your perceptions and to understand that your way is not the only way.

[JoeO]

It is time to post a picture of your power supply.

[Joule Thief]

If R2 is wired as shown it appears the wiper is not connected to anything. In the schematic shown, the LM317 is set to provide about 58V out. Refer to page 10 in the TI LM317 data sheet for information on setting the regulated voltage of the LM317:

http://www.ti.com/lit/ds/symlink/lm317.pdf
 It's also good practice to place a resistor on the pin 1 end of R2 to set the low end of your adjustment range.

Yes, this ^

using a potentiometer in the "rheostat mode"  can lead to issues with too much current flowing thru the wiper of the component.  I don't recall if you mentioned the wattage of the potentiometer you are using. The fact you saw a flash from the pot makes me suspicious this may be a contributor to your problems.

[AF6LJ]

If R2 is wired as shown it appears the wiper is not connected to anything. In the schematic shown, the LM317 is set to provide about 58V out. Refer to page 10 in the TI LM317 data sheet for information on setting the regulated voltage of the LM317:

http://www.ti.com/lit/ds/symlink/lm317.pdf
 It's also good practice to place a resistor on the pin 1 end of R2 to set the low end of your adjustment range.

Yes, this ^

using a potentiometer in the "rheostat mode"  can lead to issues with too much current flowing thru the wiper of the component.  I don't recall if you mentioned the wattage of the potentiometer you are using. The fact you saw a flash from the pot makes me suspicious this may be a contributor to your problems.

The current in that leg of the LM317 is very small; with that said I wouldn't use any pot small than a quarter watt rating.

[Seekonk]

Should you get this working, take a lesson from some of the linear supply manufacturers. Put a switch in to switch the secondary of the transformer to 12 04 24V  This will reduce heating when operating the supply at low voltages.  Manufacturers have a output voltage detector that does this automatically switching a relay.  You could add this later with a LM431 and a relay.

[MrOmnos]

Hi guys. Sorry for no immediate updates. I was busy as the entire Indian subcontinent is going through festive season. But the good news is I have got the power supply working. It's the same circuit. I just changed the input AC voltage to 15volts. I guess the IC couldn't handle more than 30 volts even though it is rated for maximum 40 volts. So, now with the input of 15volts AC, I am getting about 22-23 volts after rectification. That's pretty close to (15*1.414). I have also added 300 ohms in series with the 10k pot to get an starting voltage of 3volts.
I had to built the thing in modules.  A rectifier module and the regulator module, I wanted to test it with different transformers, different rectifiers and with DC input from other supplies because I wanted to know the cause after frying 5 ICs.

Regulator Module
Front

Back


*soldering skillzzz*

Rectifier Module





If anyone is interested in the ICs, here's a pic.



PS Currently I am getting the max output voltage close to Input voltage i.e 21v since I have a 10k pot. But I am thinking of limiting the output voltage to 15 volts. I have done the maths, to get around 15v of output I would need a pot of around 2.5K (calculated resistance 2420 ohms) but I have searched everywhere and It seems 2.5k pots are very rare and I couldn't find a single one. So I did the maths and found out that if I connect a 3k permanent resistance in parallel with 10K pot I will be able to get maximum resistance of about 2.3k which I guess is good enough. Theoretically It should work but I have a feeling this is not going to work. I have also calculated and plotted the graph of how the total resistance is going to change with when the pot is turned from 0-10k.



How voltage is going to change when the pot is turned with 3K resistance in parallel.



It looks fine to me and I think voltage should be controllable with the 10K pot. What do you think? Will I have a fair amount of control on the voltage.

[AF6LJ]

Good deal, not bad...

[macboy]

Where is the heatsink?! 

[Seekonk]

When you can't find the pot you want, change the resistor on the output of the regulator.

[rdl]

Though probably best not to wander too far below 10 mA minimum load current. LM317 from TI is specified at 3.5 typical, 10 mA max, but not all brands may work as well.

[SharpEars]

And the answer everyone didn't realize for why the original circuit didn't work:

The LM317s he is using (based on the pics above) are cheap Chinese fakes. Is it really surprising that they fail to work at the Max voltage rating from the spec sheet. I am more surprised that you got them to work at all.

So, the lesson learned here is not to just look at the circuit presented, but at the authenticity and operability of the parts used.

[fcb]

Suggest you prove your circuit works using the 0-12V tapping at this stage, before you go the whole hog to 24VAC input (which is getting close to the Vmax of your 317T).

[KL27x]

Where is the heatsink?! 

+1 But he will find that out for himself.

With 22V in, you won't be able to sustain more than a trickle at 3V out. For every watt you draw at 3V, the regulator will have to dissipate 6x that much as heat.

The T0-220 package LM7805 that is inside my 20V PSU (simply to run the panel meter!) can't even do its job without help. Well, to be fair, it worked for a couple years before I had to tweak it with a series resistor between the input pin and the 20V rail.

I've another project where the only standby draw is a single micro and one LED. At 20V in, a Dpak 7805 cannot reliably supply the 5V out, longterm, without help. Again, it worked for a year or two before it started to fail. Series resistor to the rescue, once again.

[MrOmnos]

And the answer everyone didn't realize for why the original circuit didn't work:

The LM317s he is using (based on the pics above) are cheap Chinese fakes. Is it really surprising that they fail to work at the Max voltage rating from the spec sheet. I am more surprised that you got them to work at all.

So, the lesson learned here is not to just look at the circuit presented, but at the authenticity and operability of the parts used.

Yes, they are all fakes. I did some extensive research, read some reviews of similar looking products on amazon and turns out people have had similar problems. I can't imagine a company like ST is letting these Chinese thugs rip off their name like this.

[MrOmnos]

Where is the heatsink?! 

I have it but I haven't screwed it on yet, because I won't be using this power supply for anything useful. I was planing to use it to run the fume extractor fan which I had assembled for my bench, but I won't be using it now. The Ic's are fake and I doubt it is going to last. This project has become more of a test project for me. And I have gained a lot of experience from this single project.
Conclusion : If you need a power supply with an output of 12 volts/1 Amps, go get yourself one of those wireless router DC adapters and save your precious time.