Why do "short proof" votlage regulator chips short out?

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.

 

Good Question,

To answer your question as its worded, the 317 is short circuit protected,
which means it will current limit if the load goes short. This doesnt mean that
the device itself wont ever go short circuit. A 317 is as liable to go short as
any other semi-conductor device.

But as to why your LM317 went short in your circuit? Its a good question. Did
you check that the cap wasnt actually holding a reverse charge, or even a high
forward voltage! Maybe the LM317 was short before you used it! Using a series
resistor, maybe 10ohm, 5W would have been better, and allow the cap to charge
in a controlled fashion for you, rather than relying on the 317s internal
current limiting, which is a fail safe, not a useable feature as such. As for
the heatsink and thermal protection, if it blew instantly, theses werent
factors. .. John.

Drop QRM off my addy to e-mail me.

 

Subject: Why do "short proof" votlage regulator chips short out?

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

 

Hi!

Did you forget to add protection diodes? The bypass capacitors are for
improved performance and the large heatsink provides adequate power
dissipation for long term reliability. These do not protect the LM317
against reverse current flow due to transient capacitor discharge. The
capacitor you've connected probably has higher potential than the
regulator's programmed output voltage thereby driving current into the
regulator's output pin/junction.

Jovit

 

The obviously missing part of your story is the input voltage.
mike

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And the size of the "big capacitor"...

 

I didn't word the title right. My LM317 went bad, and apparently it's
output transistor is now shorted because it now always puts out almost
the full input voltage, regardless of the voltage setting.

I shorted both its leads against the aluminum case of the power
supply.

This LM317 had been working normally the day before. I'm sure I
ruined it with the capacitor.

 

I have one between input and output and between output and ground for
reverse voltage protection, but I don't have one between output and
the adjustment pin because I don't have a capacitor across the
adjustment-ground resistor.

Do the bypass capacitors on the output also help prevent excessive
voltage from inductance in the wiring, and are a .1uF ceramic and 10uF
electrolytic enough?

 

I'm using a transformer rated at 18VAC, and the maximum I once
measured at the LM317 input with no load was 31VDC. The capacitor I
was charging up is rated 50V, 3000uF.

 

Did you check the diode connected between the input and output for
short?

The decoupling capacitor on the regulator output is for stability of the
regulator's feedback loop and improve the regulator's output impedance.
Im sure it's not intended to protect the regulator output.

 

31V is near the max that you can put on an LM317. Combination of high
voltage and high current can melt the junction before the current limit
can respond. Thermal protection is useless for transients. Output
transistor can pop long before the whole die heats up. The output cap
limits the rate of rise of the current to something the current limit
can keep up with. You didn't have one to slow down that transient.

I don't recall your output voltage, but you really don't want a lot
of volts on the chip if you can help it. Check out the derating curves
in the spec...then derate it some more...like twice more.

I wouldn't do what you did, but I would have expected it to
survive...usually. Maybe you're just lucky. Try it again.
mike

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I had 10uF in parallel with 0.1uF on the output. Was that too little?

My output voltage was at the maximum, about 27VDC, and I thought that
feeding 31VDC into the LM317 was OK because National warns against
exceeding 40V across it. The LM317 definitely blew because the
voltage adjustment pot would no longer lower the voltage. I tried
another LM317, same T-220 package, and it didn't blow when the same
capacitor (also 0V) was connected across it, but it's a different
brand, National, instead of ST like the first one.

 

Your regulator thought so...

This is a popular misconception. Just cause the maximum sez 40V,
doesn't mean you can do anything useful at that voltage. Make sure
the spec you're reading is for the actual part you have. I pulled up
a random spec when I first read your post and it said 37V.
Semiconductors get extremely fragile near their breakdown voltage.
Read up on secondary breakdown and current crowding. The junction
can melt before the current limit can take over. At higher voltages,
maximum power dissipation is often NOT the limiting factor.

TheLM317 is particularly vulnerable because you can float the thing
at an arbitrarily high voltage as long as you don't exceed 40 volts
across it. But you have to make sure it doesn't blow up on shorted
output or during turn-on.

Another thing to worry about is where you got it. Direct from the
manufacturer, it's likely to meet it's specification...BUT the critical
parameter in your application may not even be on the spec. If you got
it from Radio Shack or a generic replacement or at a swapmeet or any
other place, you may
have a part that was rejected somewhere along the line.

Bean counters like to buy from the lowest bidder. Different vendor
parts vary over time.
mike


The LM317 definitely blew because the

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I think you are misreading the spec. A 317 cannot exceed 37v difference
between the input and output voltages. There is obviously going to be an
upper limit, but 40v ain't it. I am using a 317 with about 70v on the input
to put out 48v without any trouble at all.