Subject: Why do "short proof" votlage regulator chips short out?
From:
[email protected] (larrymoencurly)
Date: 4/12/2004 7:42 PM Central Standard Time
Message-id: <
[email protected]>
I was using an LM317 to charge up a big capacitor. I connected it
after I turned on the power, and the LM317 blew out instantly. I
realize that it was stupid to turn on the power first, but why didn't
the LM317's built-in short-circuit and thermal protection work?
The LM317 was hooked up in linear mode, and I had bypass capacitors on
the input and output, just as National Semiconductor recommends (.1 uF
ceramic in parallel with 10 uF low-ESR Sanyo OS-Con, plus 2,000 uF
filter on the input side). The regulator was mounted on a fairly
large heatsink, roughly 3" x 3" x .75", and I think that I followed
proper layout recommendations.
I'm assuming here that you mean you connected the cap to the circuit after you
turned the power on.
Is it possible that you had a fairly long wire going from your power supply to
the cap? All wires have inductance. At the moment the cap is starting to make
contact with the power supply, you're forcing current through the inductor to
the cap. If there's any contact bounce, your inductor is going to start trying
to backfeed the LM317, and the voltage at the output pin (V1) will rapidly
exceed the voltage at the input. That might have been what killed the LM317.
Can't do that. You'll notice when you touch a cap with a battery wire, you
frequently get a bit of a spark. That's what's going on here.
Switched Cap Load
____ SW1
.-------. | | V1 ___ _/
o-----o~ +o----o----| 317|---o----o---UUU--o/ o----.
| | |____| | | |
AC | BR1 | | + | | | |
| | ### | .-. | |
| | --- | | | | | +
o-----o | | | | | --- ###
|~ -o----o | '-' --- ---
'-------' | | | | |
| o------' | |
| | | |
| .-. | |
| | | | |
| | | | |
| '-' | |
| | | |
'------o-----------o-----------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Get another LM317, and next time, connect the cap before turning on power. If
you can't do that, get a 1N5402, and connect the anode to the LM317 output, and
the cathode to GND. Get another 1N5402 and connect the anode to the input pin,
and the cathode to the output. This will protect the LM317 against reverse
polarity as well as Vout exceeding Vin. Not bulletproof, but the protection's
a bargain for the price.
317 Protection
.-------|<--------.
| ____ |
| | | |
o----o------| 317|-----o-------o
|____| | |
| .-. |
| | | |
| | | |
| '-' |
| | |
o--------' -
| ^
.-. |
| | |
| | |
'-' |
| |
o-------------o----------------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Good luck
Chris