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Old 17th January 2005, 12:53 PM   #1
ZERS is offline ZERS  France
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Default Vin MAX for LM317

Hi all,

well the datasheet of the LM doesn't point out what is the max voltage input of the LM317.
It is noticed that the max output voltage is 37V as the max output current is 1.5A, but what about the max input voltage ?

I entend to use a LM317T to down the voltage froml 42V to 18V (or 12V).

The calculation for R1=240R gives R=3216.

Will the LM317 explose as VIN is 42V ?

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Old 17th January 2005, 01:00 PM   #2
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No V(in) max is defined as the 317 doesn't have a direct 0V connection.

The spec defines a maximum Vin - Vout of 37V.

Now in your case, the max voltage across the chip when it is running is OK, but you need to check what happens at switch-on. If the input comes up instantaneously, and the output has a capacitor to 0v, then you may transiently exceed the max voltage spec and damage the 317. If the input itself rises relatively slowly then you'll be OK.
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Old 17th January 2005, 01:18 PM   #3
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42 vin and 12 volt out = 30 V and this is Ok I think.

I think the turn-on problem is a minor problem if the total capacitance at the output isn't too high but this has to be verified and if you can't do that just add a 36-39 volt zener across the regulator.

An anonther way to get 12 volt out in safe way is to use a LM431 plus a series transistor. Then you can choose a 50-100 V transistor. Check the datasheet for LM431 how such circuit looks like. This circuit isn't equally simple but not so complicated either. The good thing is better stability!
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Old 17th January 2005, 01:36 PM   #4
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LM317HV = 60Volts MAX i/p.
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Old 17th January 2005, 02:07 PM   #5
ZERS is offline ZERS  France
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here is my schematic.

The idea is to previously reduce the voltage down to 18V after filtering the PSU, before getting the other voltages (5v, -21v, 9v)
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