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LM317/337 ripple/noise/oscillation
LM317/337 ripple/noise/oscillation
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Old 31st January 2018, 08:14 PM   #1
belyakove is offline belyakove  Netherlands
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Default LM317/337 ripple/noise/oscillation

Hello. Probably my question is very newbie but I didn't find answer anywhere else. I've recently got my first oscilloscope and tried to measure output of my dual rail +/- 12V regulated power supply that I use for preamplifier that I'm building. Power supply is built on standard LM317/337 regulators. I've attached picture of what I see on oscilloscope and want to understand what it means.
As I understand I see around of 40mV of fluctuation (high frequency).
I don't really understand what it is ripple, noise or oscillation and if it's ok. Can you please help me with that?
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Old 31st January 2018, 08:22 PM   #2
Mark Johnson is offline Mark Johnson  United States
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LM317/337 ripple/noise/oscillation
It might be your scope probe and its ground lead. To learn more, disconnect your probe from the power supply and instead use it to measure the voltage of a 1K resistor. Ground clip goes to one end of the resistor, scope probe goes to the other end of the resistor. Leave the vertical sensitivity at 20 mV per division. If your measurement apparatus were noise-free, we'd expect to see about 80 microvolts of thermal noise from a 1K resistor. But instead if you see the same 40 millivolts, yet it's not connected to a power supply at all, then you can be confident the 40mV is not coming from the LM317/LM337 ... it's coming from your measurement setup.
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Old 31st January 2018, 08:44 PM   #3
belyakove is offline belyakove  Netherlands
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Tried with 1k resistor (metal film if it's important) and peak to peak signal with it is 16mV
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Old 31st January 2018, 09:37 PM   #4
MarcelvdG is offline MarcelvdG  Netherlands
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Do you use ordinary electrolytic capacitors for decoupling the LM317 output or some high-quality cap? Some brands need the losses of cheap electrolytics to stay stable.
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Old 31st January 2018, 09:43 PM   #5
belyakove is offline belyakove  Netherlands
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Yes, I've read in the data sheet that low esr capacitors cannot be used with 337.
Currently there are 10uF tantalums on the outputs of both LM317 and LM337.
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Old 31st January 2018, 10:00 PM   #6
MarcelvdG is offline MarcelvdG  Netherlands
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Then you've found a better datasheet than I have. Anyway, does the ripple or whatever it may be disappear when you pull the mains plug of the power supply and if so, does it disappear immediately or after the reservoir caps have discharged?
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Old 31st January 2018, 10:34 PM   #7
Mark Johnson is offline Mark Johnson  United States
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LM317/337 ripple/noise/oscillation
Quote:
Originally Posted by belyakove View Post
Tried with 1k resistor (metal film if it's important) and peak to peak signal with it is 16mV
Conclusion: some, but perhaps not all, of the displayed noise originates from other sources that are NOT your voltage regulators.

MarcelvdG's suggestions are excellent, good luck!
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Old 1st February 2018, 06:46 AM   #8
belyakove is offline belyakove  Netherlands
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Tried measuring in on and off state. Not much changes in on and off state and during capacitors discharge. Here are images of these two measurements. First is in on state and second is in off state.
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File Type: png off.png (52.4 KB, 590 views)
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Old 1st February 2018, 07:03 AM   #9
Keruskerfuerst is offline Keruskerfuerst  Germany
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Can you post the schematics?
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Old 1st February 2018, 07:08 AM   #10
belyakove is offline belyakove  Netherlands
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Here are the schematics. As mentioned on it, 100nF output caps were replaced with 10uF tantalum.
It is PSU producing +/- 12V and +5V for micro controller. but for now I'm measuring only 12V part
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