Hi every one, This is for the first time am posting thread here and i really hope to get a solution here.
I am trying to make LM317 Power supply for the first time. I designed a diagram on stimulation software and it goes well.
The major problem is when ever i adjust POT, it gets burnt and not showing any effect on output volts. I have already wasted 12 POTs.

Please take a look at schematics attached. I really dont know where am wrong. I am not an experienced person in electronics.
I am using 12-0-12 / 2 Amp transformer, bridge rectifier ( diode version ), 10 K POT ( B 10K SHARP ).

Output volts - 37v. When the pot is at zero mark, circuit still shows 37V but pot got burnt.

Please help me with this problem. Thanks in advance.

 

pot got burnt.

What's the power dissipation over the pit at 37vout?

 

and are you SURE that regulator is wired correctly? note, pinout is not the same as for 78xx regulators....

 

LM317 Pin out showing 37.2 v.
Vin also showing same volts.

 

and are you SURE that regulator is wired correctly? note, pinout is not the same as for 78xx regulators....

Yes, very much correct. Left Most Pin as ADJ, middle as Output, Right most as V in.

 

What's the power dissipation over the pit at 37vout?

LM317 Pin V out showing 37.2 v.
Vin also showing same volts.

One more thing, Meter shows AC volts also ( 92 V )

 

you need to be more specific - WHERE is your meter connected when you read that? are you sure your transformer is ok (no leak, connected ok)?

 

Vin, isn't it too high?

 

Your pot is wired as a rheostat. If it is set for a low resistance value then 37V (or less) will drive a lot of current through it via R1 if the circuit can't stabilise quickly. Lots of current = lots of heat.
DMMs don't read correctly on the AC range if the voltage has a large DC component (37V here).

 

try to put fixed resistor instead of pot, say 1k

 

1. Make sure the potentiometer is 5k or 10k.
2. Connect a 10K - 0.5W resistor in parallel with capacitor C3.
3. LM317 has TO-220 package ? If so, then numbering pins from schematic is OK.

 

Hey

Try exchanging your R1 for 330 Ohm. This will then set your whole pot range at attainable values (currently your maximum output voltage is 58V!) LM317 Calculator. With a 330 ohm as R1 you would have 1.25 - 39V range. Also the LM317 needs a minimum current draw to operate properly and this is in the region of 10-20mA so you can just use a LED with current limiting resistor (set at maximum input voltage) rather than just a dummy load resistor. Finally check that the regulator itself is still ok because a shot regulator usually does not function at all and Vo is only Vin - dropout voltage, this can be checked with continuity across input and output, if there is continuity the regulator is shot.

 

you need to be more specific - WHERE is your meter connected when you read that? are you sure your transformer is ok (no leak, connected ok)?

I have checked readings at V IN + Gnd, Vout + Gnd and final out put after cap. all same.
I have also tried 3 different LM317 and also tried 12-0-12 2amp, 15-0-15 500mA, Pot burnt in all conditions.

Please suggest.

 

Vin, isn't it too high?

Thats what bothers me, i really dont know why volts are too high, ideally it should be 24v to 32v.
even 15-0-15 / 500mA is giving 43 v.

 

Your pot is wired as a rheostat. If it is set for a low resistance value then 37V (or less) will drive a lot of current through it via R1 if the circuit can't stabilise quickly. Lots of current = lots of heat.
DMMs don't read correctly on the AC range if the voltage has a large DC component (37V here).

I have checked none of resistor and cap is over heated, connected heat sink on LM317 but its cool any ways.

Can you please explain that, as am hobbyist and have least knowledge about circuits, still this circuit is a challenge for me, I mean where am wrong? What should i do to make a clean power supply?
Thanks.

 

try to put fixed resistor instead of pot, say 1k

Sure ill try that and update results,but i want adjustable power supply.

 

1. Make sure the potentiometer is 5k or 10k.
2. Connect a 10K - 0.5W resistor in parallel with capacitor C3.
3. LM317 has TO-220 package ? If so, then numbering pins from schematic is OK.

Yes sir, i am using 10 K POT its a volume Pot. (B 10K Sharp).
I will connect a resistor in parallel and will update result.
Whats TO-220 package? I am using LM317T also checked data sheet.
Transformer is of 2 amps can this be a issue?

 

Hey

Try exchanging your R1 for 330 Ohm. This will then set your whole pot range at attainable values (currently your maximum output voltage is 58V!) LM317 Calculator. With a 330 ohm as R1 you would have 1.25 - 39V range. Also the LM317 needs a minimum current draw to operate properly and this is in the region of 10-20mA so you can just use a LED with current limiting resistor (set at maximum input voltage) rather than just a dummy load resistor. Finally check that the regulator itself is still ok because a shot regulator usually does not function at all and Vo is only Vin - dropout voltage, this can be checked with continuity across input and output, if there is continuity the regulator is shot.

Sure ill update circuit and will reply.
Thanks

 

What is the power rating of the pot?

 

Hi

I think the LM317 is damaged. Cant be 37v out with 38v input.
Should only be about 5mA thru R2 (ADJ) resistor