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Old 14th Jan 2021, 3:37 pm   #1
vinrads
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Default Lm317 and similar regs

Question , has anyone had this type of reg fail giving full output ,or are they fail safe, as I have built quite a few power supplies using this reg . Mick.
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Old 14th Jan 2021, 3:40 pm   #2
agardiner
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Default Re: Lm317 and similar regs

I have used these a lot over the years. Had a couple fail short circuit to ground, but never to output.
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Old 14th Jan 2021, 3:43 pm   #3
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Default Re: Lm317 and similar regs

Ah that's good to hear, so a fuse on the input would be a good idea, Mick.
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Old 14th Jan 2021, 4:47 pm   #4
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Default Re: Lm317 and similar regs

Quote:
Originally Posted by agardiner View Post
I have used these a lot over the years. Had a couple fail short circuit to ground, but never to output.
Can you clarify? The LM317 does not have a ground pin, it has a very low current reference pin, which is intended for connection to ground via a resistor, with another resistor from output to reference.

If the thing failed short from output to reference, the rest of the IC would pull the output really high, to re-establish the internal reference level. And that wouldn't do our load any good...
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Old 14th Jan 2021, 4:54 pm   #5
Beardyman
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Default Re: Lm317 and similar regs

Similar regulator but more current capability (with suitable heatsink) is the LT1085CT (3A), LT1084CT (5A) & LT1083CT (7.5A).
We use these a lot where I work for providing local power supplies in our ROV's. Hardly ever get a failure, very rugged devices.
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Old 14th Jan 2021, 7:10 pm   #6
vinrads
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Default Re: Lm317 and similar regs

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by agardiner View Post
I have used these a lot over the years. Had a couple fail short circuit to ground, but never to output.
Can you clarify? The LM317 does not have a ground pin, it has a very low current reference pin, which is intended for connection to ground via a resistor, with another resistor from output to reference.

If the thing failed short from output to reference, the rest of the IC would pull the output really high, to re-establish the internal reference level. And that wouldn't do our load any good...
Especially if it is the filament volts on some expensive 4 volt Telefunken valves , just for piece of mind I have fitted a fuse on the output then a zener diode . Mick.
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Old 14th Jan 2021, 7:18 pm   #7
agardiner
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Default Re: Lm317 and similar regs

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by agardiner View Post
I have used these a lot over the years. Had a couple fail short circuit to ground, but never to output.
Can you clarify? The LM317 does not have a ground pin, it has a very low current reference pin, which is intended for connection to ground via a resistor, with another resistor from output to reference.

If the thing failed short from output to reference, the rest of the IC would pull the output really high, to re-establish the internal reference level. And that wouldn't do our load any good...
Yes, sorry for the confusion. I have had them occasionally fail, shorted between input and the reference pin. However, I don't know if there is an internal failsafe, but on these occasions there has been no output.

I think I would use a fusible resistor on the input for safety.
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Old 14th Jan 2021, 7:49 pm   #8
dave cox
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Default Re: Lm317 and similar regs

I've seen them fail short circuit.
Certainly not indestructible on thermal overload, especially if you forget the bolt that holds them onto the heatsink!

dc
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Old 15th Jan 2021, 7:35 am   #9
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Default Re: Lm317 and similar regs

If your powering valves a 317 can do odd stuff like going into protection due to the high current surge. A crowbar circuit would be better than a zener for protection.

Andy.
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Old 15th Jan 2021, 10:05 am   #10
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Default Re: Lm317 and similar regs

Just to clarify there is a fuse before the zener 5.1 volt 1 watt. it's only running a 250ma max .it is working fine , Mick.
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