LM317 exploding electro

[John Morris]

I know someone is going to make me feel stupid for asking this, but
....

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

I have checked the circuit a dozen times and cannot find a fault.

Can anyone offer a clue? I have heard some brands of LM317 have
differing pinouts.

Thank you,

John Morris (the moron)

 

[[email protected]]

"John Morris дµÀ£º
"

 

[Phil Allison]

"John Morris"

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

I have checked the circuit a dozen times and cannot find a fault.

** That 10uF electro MUST have been installed with reverse polarity.

Maybe it was wrongly labelled.




....... Phil

 

[maxfoo]

I know someone is going to make me feel stupid for asking this, but
...

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

I have checked the circuit a dozen times and cannot find a fault.

Can anyone offer a clue? I have heard some brands of LM317 have
differing pinouts.

Thank you,

John Morris (the moron)

This is a schematic of what your circuit should look like.
http://www.national.com/images/pf/LM317/00906301.pdf

max input voltage is 37Volts on a LM317.
Try using a resistive voltage divider to cut the input
voltage a bit.

Also the max current output is 1.5 amps, put a series current limiting resistor
with the POT, most likely the min resistive value on the pot is your problem.

 

[Phil Allison]

"Phil Allison"

** That 10uF electro MUST have been installed with reverse polarity.

Or it is really a 6.3 volt cap.



...... Phil

 

[maxfoo]

"Phil Allison"


Or it is really a 6.3 volt cap.



..... Phil

hehehe... I once had a board that had a tantalum cap in ass-backwards, the thing
blew up making a loud BANG! and then caught on FIRE!, scared the SHIITE out of
me. The flame was an inch high and the whole lab stunk for hours, put a nice
black spot on the pcb. Damn a-hole assemblers got me again.

 

[Eeyore]

I know someone is going to make me feel stupid for asking this, but
...

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

I have checked the circuit a dozen times and cannot find a fault.

Can anyone offer a clue? I have heard some brands of LM317 have
differing pinouts.

I rather doubt that.

What's this *standard circuit* anyway ?

Have you made sure you have local decoupling for the regulator ?

Graham

 

[Ancient_Hacker]

I know someone is going to make me feel stupid for asking this, but
...

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

I have checked the circuit a dozen times and cannot find a fault.

Can anyone offer a clue? I have heard some brands of LM317 have
differing pinouts.

Thank you,

John Morris (the moron)

There's no way that should happen. Even if you get the electrolytic
backwards, the adjut pin on a good LM317 only supplies 1.37 milliamps,
which is unlikely to blow a capacitor in 20 seconds.


Use your voltmeter and check that the LM317 is getting a PLUS voltage
to its pin 1 (the leftmost pin). Not Minus. Not AC.

My bet is it's AC.

 

[John Popelish]

John Morris wrote:
(snip)

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

(snip)

Either the capacitor was in backwards (the - terminal goes
to ground) or the regulator is unstable and is oscillating
at high frequency, heating the capacitor. You should look
at the data sheet for the recommended (minimum) ceramic or
film capacitors that should be connected as close as
possible to the pins of the LM317, to stabilize it.

See page 3 of:
http://www.fairchildsemi.com/ds/LM/LM317.pdf

 

[John Devereux]

There's no way that should happen. Even if you get the electrolytic
backwards, the adjut pin on a good LM317 only supplies 1.37 milliamps,
which is unlikely to blow a capacitor in 20 seconds.


Use your voltmeter and check that the LM317 is getting a PLUS voltage
to its pin 1 (the leftmost pin). Not Minus. Not AC.

My bet is it's AC.

Perhaps he's looking at the pin view (i.e., backwards).

 

[ehsjr]

I know someone is going to make me feel stupid for asking this, but
...

I have built the standard LM317 PS circuit with variable pot, etc. on
a solderless proto board. Input is 37VDC from a plugpack.

It works fine for about 20 seconds, then the 10uf 63V electro in
parallel with the pot heats up and explodes.

There should not be a cap in parallel with the
pot. Post your schematic so we can see what you
are talking about. No "standard" LM317 circuit
blows capacitors.

Ed

 

[Phil Allison]

"ehsjr"

There should not be a cap in parallel with the
pot. Post your schematic so we can see what you
are talking about.

** You had better look at the original Natsemi app notes for the LM317.

The schem with the additional protection diodes and 10 uF cap across the
pot.

Improves ripple rejection by around 15 dB, according to the published data.




...... Phil

 

[ehsjr]

"ehsjr"





** You had better look at the original Natsemi app notes for the LM317.

The schem with the additional protection diodes and 10 uF cap across the
pot.

Improves ripple rejection by around 15 dB, according to the published data.




..... Phil

Thanks! You're absolutely right. I did not consider the
circuit with the protection diodes (or any LM317 vr circuit
with other than the input and output caps and the voltage
divider) the "standard" LM317 circuit the OP asked about,
but perhaps I should.

Ed