LM317 exploding electro

"John Morris =D0=B4=B5=C0=A3=BA "

"John Morris"

Maybe it was wrongly labelled.

...... Phil

This is a schematic of what your circuit should look like.

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"

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.

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 Morris wrote: (snip)

(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:

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

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

"ehsjr"

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