LM317: Can you kill it with an output short

L

L.Chung

Guest
Hi,

I saw a circuit on the web that puts overcurrent protection (via
current sensing resistor and transistor)to the input of the LM317 to
"protect" it from output short circuit.

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.

So if the LM317 is operating within its input voltage limit, will it
survive an output short? Or, put simply, have you killed one recently?

Regards

L.Chung
 
<L.Chung> wrote in message news:v1cp61tiuu0o4rlund8kjoiur61roon78d@4ax.com...
Hi,

I saw a circuit on the web that puts overcurrent protection (via
current sensing resistor and transistor)to the input of the LM317 to
"protect" it from output short circuit.

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.

So if the LM317 is operating within its input voltage limit, will it
survive an output short? Or, put simply, have you killed one recently?

Regards

L.Chung
Click to expand...
Did this whole arrangement also have an additional "output" series
transistor to increase the current rating of the power supply? Like
the output of 317 going to the base of say 2n3055?
If so, short-circuit protection would not work for the whole circuit.

--
Siol
------------------------------------------------
Rather than a heartless beep
Or a rude error message,
See these simple words: "File not found."
 
On Mon, 25 Apr 2005 11:26:24 +0200, "SioL" <Sio_spam_L@same.net>
wrote:

Did this whole arrangement also have an additional "output" series
transistor to increase the current rating of the power supply? Like
the output of 317 going to the base of say 2n3055?
If so, short-circuit protection would not work for the whole circuit.
Click to expand...
No. Just a plain LM317 on its own.

The question remains is whether a LM317 along with the manufacturer
recommended protection diodes can survive an output short.

L.Chung
 
L.Chung schrieb:

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.
Click to expand...
Yes, the spec say output is short-circuit protected. But current limit
is higher than 1.5 A depending on package (TO-3: max. 3.4 A). Perhaps
additional circuit switches off at lower current.

Regards
-
Michael Redmann
"It's life, Jim, but not as we know it." (Spock)
 
AFAIK, the LM317 is short-circuit proof, but there is no reason that
external circuitry can't be added to limit the current to some lower
value (eg to protect some other circuitry). The only "protection
diode" usage I know of is for reverse voltage protection, not
short-circuit protection.
Tony

On Mon, 25 Apr 2005 12:39:33 +0100, L.Chung <> wrote:

On Mon, 25 Apr 2005 11:26:24 +0200, "SioL" <Sio_spam_L@same.net
wrote:


Did this whole arrangement also have an additional "output" series
transistor to increase the current rating of the power supply? Like
the output of 317 going to the base of say 2n3055?
If so, short-circuit protection would not work for the whole circuit.

No. Just a plain LM317 on its own.

The question remains is whether a LM317 along with the manufacturer
recommended protection diodes can survive an output short.

L.Chung
Click to expand...
Tony (remove the "_" to reply by email)
 
Michael Redmann wrote...
L.Chung schrieb:

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.

Yes, the spec say output is short-circuit protected. But current limit
is higher than 1.5 A depending on package (TO-3: max. 3.4 A). Perhaps
additional circuit switches off at lower current.
Click to expand...
The LM317's high intrinsic current-limit value means it may be under
severe thermal stress during current limiting, so that the thermal
temperature-limit circuit is also activated. There's some evidence
the chip's life is shortened if it spends lots of time at the thermal
limit temperature. Furthermore, other components in the power supply
(transformer, rectifiers, etc.) are also more severely stressed when
at the chip's intrinsic current limit. OTOH, if an external current
limit circuit is added, this circuit will be under severe stress.


--
Thanks,
- Win
 
On Mon, 25 Apr 2005 10:03:18 +0100, the renowned L.Chung <> wrote:

Hi,

I saw a circuit on the web that puts overcurrent protection (via
current sensing resistor and transistor)to the input of the LM317 to
"protect" it from output short circuit.

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.

So if the LM317 is operating within its input voltage limit, will it
survive an output short? Or, put simply, have you killed one recently?

Regards

L.Chung
Click to expand...

With both thermal and current limiting, it should be able to withstand
an output short provided the input voltage is within the limit. It may
not do much for the long-term reliability to depend on the thermal
limit, especialy for more than a momentary short-circuit, as the die
under these conditions will exceed the abs max operating Tj by a
considerable margin.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message news:0cpp61tm94fnh7nasngi1cdkgtrbrjk46h@4ax.com...
On Mon, 25 Apr 2005 10:03:18 +0100, the renowned L.Chung <> wrote:


So if the LM317 is operating within its input voltage limit, will it
survive an output short? Or, put simply, have you killed one recently?

Regards

L.Chung


With both thermal and current limiting, it should be able to withstand
an output short provided the input voltage is within the limit. It may
not do much for the long-term reliability to depend on the thermal
limit, especialy for more than a momentary short-circuit, as the die
under these conditions will exceed the abs max operating Tj by a
considerable margin.


Best regards,
Spehro Pefhany
Click to expand...
To sum it up, if its very likely that the circuit will encounter a short, it probably
is better not to rely solely upon current protection inside the LM317.
It is probably manufactured by a multitude of vendors today and you can't rely
on how a particular lot will perform, unless you order from the same vendor every
time.

--
Siol
------------------------------------------------
Rather than a heartless beep
Or a rude error message,
See these simple words: "File not found."
 
On Mon, 25 Apr 2005 12:39:33 +0100, L.Chung <> wrote:

On Mon, 25 Apr 2005 11:26:24 +0200, "SioL" <Sio_spam_L@same.net
wrote:


Did this whole arrangement also have an additional "output" series
transistor to increase the current rating of the power supply? Like
the output of 317 going to the base of say 2n3055?
If so, short-circuit protection would not work for the whole
circuit.

No. Just a plain LM317 on its own.

The question remains is whether a LM317 along with the manufacturer
recommended protection diodes can survive an output short.
Click to expand...

Tony wrote:
AFAIK, the LM317 is short-circuit proof, but there is no reason that
external circuitry can't be added to limit the current to some lower
value (eg to protect some other circuitry). The only "protection
diode" usage I know of is for reverse voltage protection, not
short-circuit protection.
Tony
Click to expand...
Hey, Tony please put your answer below to not disrupt the discussion.

The protection diodes of the OP are against an *Input* short at higher
voltages with big caps on the regulator and output. They prevent the charge
flowing backwards through the regulator and eventually destroying it.

--
ciao Ban
Bordighera, Italy
 
Thank everyone for their inputs. I am referring to a momentary output
short situation.

I guessed that it should survive but prolonged short would put it in
great stress which is likely to damage it as Winfield Hill stated.

Have anyone ever fried one with a momentary output short?

Regards

L.Chung
 
L.Chung wrote:
Thank everyone for their inputs. I am referring to a momentary output
short situation.

I guessed that it should survive but prolonged short would put it in
great stress which is likely to damage it as Winfield Hill stated.

Have anyone ever fried one with a momentary output short?

Regards

L.Chung

You can fry one with an output short if there's a big filter capacitor (>= 10
Click to expand...
uF) on the adjustment pin.

Another reason for an auxiliary current limit is cost-benefit analysis. The
circuitry powered by the LM317 may be much more valuable than the regulator.
In a $10k system, reducing field failures from 5 per year to 3 per year
might easily justify another $2 in parts. Wearing a belt and suspenders costs
something, but if you do it right, it can really reduce the number of
midnight phone calls.

Cheers,

Phil Hobbs
 
On 25 Apr 2005 19:01:41 -0700, the renowned bigcat@meeow.co.uk wrote:

L.Chung> wrote in message
news:v1cp61tiuu0o4rlund8kjoiur61roon78d@4ax.com...
Hi,

I saw a circuit on the web that puts overcurrent protection (via
current sensing resistor and transistor)to the input of the LM317
to
"protect" it from output short circuit.

However, form the data sheet of LM317, if the protection diodes are
fitted, it appears to me that the LM317 can survive an output short
without damage.

So if the LM317 is operating within its input voltage limit, will
it
survive an output short? Or, put simply, have you killed one
recently?

Regards

L.Chung

An IC needs safe area protection as well as current limit to be
properly killproof.

NT
Click to expand...
Which the LM317 has. "Included on the chip are current limit, thermal
overload protection and safe area protection."



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On Mon, 25 Apr 2005 13:34:12 GMT, "Ban" <bansuri@web.de> wrote:
On Mon, 25 Apr 2005 12:39:33 +0100, L.Chung <> wrote:

On Mon, 25 Apr 2005 11:26:24 +0200, "SioL" <Sio_spam_L@same.net
wrote:

Did this whole arrangement also have an additional "output" series
transistor to increase the current rating of the power supply? Like
the output of 317 going to the base of say 2n3055?
If so, short-circuit protection would not work for the whole
circuit.

No. Just a plain LM317 on its own.

The question remains is whether a LM317 along with the manufacturer
recommended protection diodes can survive an output short.

Tony wrote:
AFAIK, the LM317 is short-circuit proof, but there is no reason that
external circuitry can't be added to limit the current to some lower
value (eg to protect some other circuitry). The only "protection
diode" usage I know of is for reverse voltage protection, not
short-circuit protection.
Tony


Hey, Tony please put your answer below to not disrupt the discussion.

The protection diodes of the OP are against an *Input* short at higher
voltages with big caps on the regulator and output. They prevent the charge
flowing backwards through the regulator and eventually destroying it.
Click to expand...
I missed that post. The diodes may be the same ones I was thinking of
- across the reg (prevents excessive reverse in-out voltage across the
reg, eg under the conditions you mentioned), and to common (prevents
reverse voltages to common) (should have clarified)?

Tony (remove the "_" to reply by email)
 
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