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Using the LM317 with higher voltages
Using the LM317 with higher voltages
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Old 25th October 2019, 01:54 PM   #1
carlmart is offline carlmart  Brazil
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Default Using the LM317 with higher voltages

Some time ago I had seen a variation of an LM317/337 power supply in The Audio Amateur but applied for higher voltages that it was designed for.

In theory, the LM317/337 regulators, as well as the LT1083, accept up to 37v inputs. But in fact what it can't accept is a 37v difference between input and output.

So you can feed higher voltage at the regulator's input as long as the difference with the output voltage does not exceed 37v. Right?

What you does lose is protection, particularly from output short-circuit.

If I remember well, and that is what I am asking a suggestion for, what you had to use was to add a zener diode voltage between the regulator's output and input. If I remember well that zener was between 37v and 39v.

Do you agree with such a regulator as safe to use?

In that TAA article the regulator was used on a Hafler power amp, feeding a higher regulated voltage to the low current stages, leaving just the power output unregulated.

Most amplifiers use such a solution, but sometimes the regulator they used there is not that good, and I think a 317/337 pair would work well.

What do you think? What would you suggest?
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Old 25th October 2019, 02:03 PM   #2
carlmart is offline carlmart  Brazil
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This is one high voltage regulator example I found.

What do you think of it?
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Old 25th October 2019, 02:47 PM   #3
osscar is offline osscar  Latvia
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there is 317HV version also - s up to 60V if i remember correctly.
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Old 25th October 2019, 04:00 PM   #4
Mark Tillotson is offline Mark Tillotson
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That topology wants a current limited main supply to be robust - 1W zener isn't going last long on overload. Perhaps use it to drive an emitter-follower transistor (on the heatsink)?
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Old 25th October 2019, 04:17 PM   #5
leadbelly is offline leadbelly  Canada
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There is nothing wrong with using the LM317 or LM317HV for high voltage, as long as you accept that it will fail if shorted. The zener just helps ensure the LM317 regulates properly, not survive a short.
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Old 25th October 2019, 06:59 PM   #6
multisync is offline multisync  Canada
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I think that at startup when the output cap is at 0Volts the LM317 sees the entire 100 volts. The Zener quickly charges the cap to 100-39 and protects the LM317. I use the same setup to supply a LME49830 Fet amp front end with +/- 75 VDC and the output vertical fets are at +/- 65VDC.
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Old 25th October 2019, 07:17 PM   #7
scottjoplin is offline scottjoplin  Wales
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Yes correct, I got away without it on older LM317s, not recommended.
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Old 25th October 2019, 11:41 PM   #8
FauxFrench is offline FauxFrench  France
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What about this coupling for higher input voltages?
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Old 26th October 2019, 02:33 AM   #9
sgrossklass is offline sgrossklass  Germany
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Here's a similar circuit suggested in NatSemi appnote AN-103:
Click the image to open in full size.
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Old 26th October 2019, 12:32 PM   #10
rsavas is offline rsavas  Canada
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Try using a TI TL783 instead, it is designed for up to 120VDC. As for the -ve, it can be used too but the transformer can not be center tapped, you would need one with isolated secondaries.
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