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Old 31st May 2011, 06:12 PM   #11
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The 317/337 generally are happier with Vin about 35V (Max). The datasheet clearly shows this.

The simplest way of exploiting higher voltages is to pre-regulate.

Any linear regulator will be dissipating quite high power under these conditions so your heatsinks will need to be big enough to get rid of the heat.
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Old 31st May 2011, 06:15 PM   #12
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It would make much more sense and be far simpler to build a low power supply especially for the pre-amp.
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Old 31st May 2011, 06:46 PM   #13
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The 317/337 generally are happier with Vin about 35V (Max). The datasheet clearly shows this.
I'm not sure what you are seeing. I'll (maybe) look after I type. The 317 doesn't care what it's input is. It's the in-out differential that matters. And it certainly won't overheat and blow up!
With a 25V drop and 100mA current (which is getting near worst case for the OP), it's only 2.5 watts dissipated by the regulator.
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Old 31st May 2011, 06:53 PM   #14
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The danger is when under initial start up conditions. You must keep Vin-Vout less than 35V. Anything above this is a recipe for failure.
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Old 31st May 2011, 07:04 PM   #15
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The danger is when under initial start up conditions. You must keep Vin-Vout less than 35V. Anything above this is a recipe for failure.
There must be more to the story. Where are you getting this 35V figure from? What start up conditions do you infer from the OP's app that make this relevant?
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Old 31st May 2011, 07:05 PM   #16
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Please read the datasheet. We are here to help, not to do the hapless homework.
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Old 31st May 2011, 07:13 PM   #17
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Oh geez. I have the NatSemi July2004 datasheet in front of me. Please kindly answer the previous two questions.

Even worse worst case: 30v drop and 250mA is still only 7.5W. With appropriate heatsink that 317 will run "happily" til the cows come home.
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Old 31st May 2011, 07:19 PM   #18
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LM317? It says nothing in the NatSemi datasheet other than input-output voltage 40V. I searched every reference to voltage.

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Old 31st May 2011, 07:23 PM   #19
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Quote:
Originally Posted by Andy5112405 View Post
The danger is when under initial start up conditions. You must keep Vin-Vout less than 35V. Anything above this is a recipe for failure.
I agree, this is incorrect. The total difference between input and output voltage cannot exceed 40V, not the input voltage.

"Since the regulator is “floating” and sees only the input-to-output differential voltage,
supplies of several hundred volts can be regulated as long as
the maximum input to output differential is not exceeded, i.e.,
avoid short-circuiting the output."

"Input-Output Voltage Differential: +40V"
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Last edited by BrianDonegan; 31st May 2011 at 07:26 PM.
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Old 31st May 2011, 07:24 PM   #20
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Correct. And I'm sure there's a lag, as capacitors charge etc, for output to settle, but there's no dire warnings or example of such a thing I've ever run across.
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