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Old 4th December 2020, 01:40 PM   #11
zergxia is offline zergxia  China
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Google Translate seems to be unable to fully express my thoughts, haha. I plan to get a voltage of ±60V from the main power supply of the power amplifier because I don't want to add new windings to the transformer. While maintaining good performance, it needs to be reduced to ±15V in the simplest way. At least 200ma current is used to drive 5-6 operational amplifiers. It has a huge heat sink, so it can get good heat dissipation support, and I don't care about power consumption. Based on this requirement, I first thought of lm317/lm337 because it can provide good enough performance and is simple enough, but its maximum withstand voltage cannot meet the requirements. I thought of using two lm317s in series to solve the power consumption and withstand voltage problems, and considered using TVS to protect the lm317 from accidental damage. I don't know whether the whole circuit is strong enough after using TVS, or whether the operational amplifier will be damaged by accidental ultra-high voltage. If this idea is feasible, can lm337 do the same? This is my initial thought.
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Old 4th December 2020, 01:50 PM   #12
zergxia is offline zergxia  China
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Quote:
Originally Posted by JMFahey View Post
WHAT-IS-THE-LOAD-CURRENT?

Are you asking for a solution or confirmation on your already decided plan?
We lack important data.
And not sure about your "clamping" either.

????????


No.
If anything, load will be *under* volted until any capacitor across it charges.



Which overvoltage?


You are running too fast for us to catch up , please supply full details on your load requirements first.

And welcome to the Forum
I made a mistake. When the capacitor starts to charge, LM317 will bear all the voltage, TVS will protect it. The load will not withstand excessive voltage. Thank you!
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Old 4th December 2020, 02:01 PM   #13
zergxia is offline zergxia  China
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Quote:
Originally Posted by bansuri View Post
You need a preregulator to lower the regulator input voltage, it is above the specs.
How much current is required at 15V?
Attachment 899095
This is good to 200mA, it gives 23V out
Using a pre-conditioner is indeed a good way.it is simple enough. Use high withstand voltage BJT to withstand huge voltage difference.
thank you!
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Old 4th December 2020, 04:12 PM   #14
mchambin is offline mchambin  France
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I had this trouble of a too high voltage for a LM317.
My solution was using a TL431 ( a shunt regulator ) and a BJT to deal with the high voltage.
In this solution, the BJT emitter is at the regulated output, the collector at the unregulated voltage. The TL431 has the anode at ground, the cathode at the BJT base, its ref input senses the regulated output voltage with an appropriate voltage divider.
For an improved low ripple, I use a CCS to feed de BJT base and give at least 1mA to the TL431.
This solution is my favorite and is fine for up to 35V positive regulated PSUs.
Very flexible using Skyzlar pair for high current.
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Old 4th December 2020, 09:02 PM   #15
mchambin is offline mchambin  France
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Can one dream simpler ?
1 Pass BJT.
1 Shunt regulator: TL431.
1 Output cap ( about 100uF )
1 Resistor divider: 2 resistors.
1 CCS: 2 cheap BJT + 2 Resistors.

For +15V -15 V. Make two positive regulators, feed them from two independent secondary windings with 2 diode bridges. Connect outputs so to have your central ground.
This is far better than the usual center tap winding, followed by pos and neg complementary stuff that only advantage is purely cosmetic.
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Old 4th December 2020, 09:10 PM   #16
00940 is online now 00940  Belgium
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This might not be possible if you are getting your +/-60v from an existing supply...

edit: btw, if this is for opamps, with decent psrr to start with, one might not need a lm317/337 pair at all. A pair of big bjt (one pnp, one npn), two 15v zeners, two resistors and two output caps are all that's needed if ample dissipation is available. Not efficient though.
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Last edited by 00940; 4th December 2020 at 09:14 PM.
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Old 5th December 2020, 12:12 AM   #17
kokoriantz is online now kokoriantz  Lebanon
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Use LM5001. It is 75v can be used as step down positive voltage, but here is a circuit for negative.step down.JPG

It will always be noisy, this why it needs to follow by a linear regulator.

Last edited by kokoriantz; 5th December 2020 at 12:15 AM.
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Old 5th December 2020, 03:35 PM   #18
zergxia is offline zergxia  China
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Thank you all friends upstairs, I have got several feasible solutions from your replies.
As part of satisfying my curiosity, I tried a simulation. The simulation looks okay. Maybe I can try to make one.

LM337 Tracking Preregulator Circuit
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File Type: jpg 20201205232509705.jpg (120.8 KB, 105 views)
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Old 5th December 2020, 04:32 PM   #19
DUG is offline DUG  Canada
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Don't forget to simulate the first turn-on of the supply.
Use a step or pulse function for the 60V.
Look at all voltages and currents for all devices.
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Old 5th December 2020, 04:52 PM   #20
JMFahey is offline JMFahey  Argentina
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00940 is quite on the spot.

No need for a tour de force using inadequate LM317 and praying they share voltage equally which will NOT fail safe under any problems, use parts which can each stand 60V rails (and more, play it safe), period.

Solution is robust and cheap:

1) use TIP31C-32C with 2k2 2W resistors Collector to Base and 20-22V 1W Zeners Base to Ground as preregulators.
You´ll get 19-21V DC
You may add small 10uF electrolytics from emitter to ground

2) use LM7815-7915 as final regulators, following datasheet suggestions.

That´s it.

Forget the dangerous idea of clamping diodes as you mentioned, they will either "do nothing" or explode.

Simulate it if you wish, not really necessary, I have been doing this thousands of times for decades, in abused Guitar amps and powered mixers.

No need for quirky switchdown preregulators or anything else, everything off the shelf stuff.
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