LM317: Can you kill it with an output short

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

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

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.

You can fry one with an output short if there's a big filter capacitor (>= 10 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

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

I read somewhere that some 3 terminal regulators could be killed when they when into thermal limit due to excessive input voltage from leakage inductance of the power transformer. The solution to this was a much larger input capacitor before the regulator (n*1000uf, not just 10 uf or 100 uf).

T> AFAIK, the LM317 is short-circuit proof, but there is no reason that

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.

L.Chung schrieb:

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)

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.

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

to

it

recently?

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

NT

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

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

Best regards, Spehro Pefhany

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)