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LM317 load capacitance
LM317 load capacitance
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Old 18th October 2017, 11:53 AM   #1
dotneck335 is offline dotneck335  United States
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Default LM317 load capacitance

Is there any disadvantage to using a larger capacitance (~4,700uF) on the output side of an LM317 regulator?
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Old 18th October 2017, 12:33 PM   #2
gmphadte is offline gmphadte  India
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If the input cap is smaller or the output pin voltage goes higher than the input, LM317 will be damaged. One reverse biased diode from input to output will solve the issue.

Gajanan Phadte
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Old 18th October 2017, 12:36 PM   #3
wintermute is offline wintermute  Australia
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LM317 load capacitance
Big capacitors can cause instability especially since their ESR is lower. The LM317 has a small inductance on it's output which can form a resonant circuit with the output capacitor if there is not enough ESR to damp the resonance.

I've used 1000uF caps on the output, but with added series resistance of around 0.3 ohms from memory. It's probably safer not to go bigger than 200 - 300uF. The bigger the cap value the lower the frequency of resonance. 1000uF is right in the audio range (I can't remember exactly where).

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Old 18th October 2017, 12:49 PM   #4
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Some helpful detail here - Simple Voltage Regulators Part 2: Output Impedance
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Old 18th October 2017, 01:43 PM   #5
infinia is offline infinia  United States
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Quote:
Originally Posted by dotneck335 View Post
Is there any disadvantage to using a larger capacitance (~4,700uF) on the output side of an LM317 regulator?
It's very rarely a good thing to add that much, since you are using a regulator with negative feedback to filter ripple and provide decent control of changing loads. It's not designed for those kinds of capacitive loads.
Various power supply turn off / on sequences can damage semi-conductors with that much energy (0.5C*V squared) stored at an output. Use diodes and such to discharge the thing.

I'd say much more to lose than can be gained, I can't think of any good reason to add a ton of C at the output of any series V regulator. Any capacitive loading should only be device bypassing / local PCB decoupling and smaller capacitor parts works much better there, Using multiples of ~0.01uF and 10uF for audio. Then after some point its probably better to add the regulator devices on each PCB instead of wiring it to various points. Id say no more than a couple of hundred micofarads combined TOTAL at most and you still may not have the data sheet performance.
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Last edited by infinia; 18th October 2017 at 02:09 PM.
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Old 18th October 2017, 02:34 PM   #6
traderbam is offline traderbam  Europe
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Quote:
Originally Posted by dotneck335 View Post
Is there any disadvantage to using a larger capacitance (~4,700uF) on the output side of an LM317 regulator?
I can think of some advantages. But it depends on what you are trying to achieve. What are you trying to achieve?
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Old 18th October 2017, 04:38 PM   #7
DF96 is offline DF96  England
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If a 317 does not do what you want then use a better regulator. Slapping on a big cap might make matters worse, not better.
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Old 18th October 2017, 06:31 PM   #8
mchambin is offline mchambin  France
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Do as they say on the LM317 application note.
Most of the design engineering I fixed were wacky ideas fixed by just following the application note.
Hence, if anything go wrong, first thing to do, is digging the AN.
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Old 18th October 2017, 08:38 PM   #9
AIM65 is offline AIM65  France
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LM317 load capacitance
Biggest cap is not the solution, right cap is...that's what's described in this interesting paper
Attached Files
File Type: pdf Power Supply Frequency-Impedance improvements.pdf (308.2 KB, 325 views)
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Old 19th October 2017, 09:58 AM   #10
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by DF96 View Post
If a 317 does not do what you want then use a better regulator. Slapping on a big cap might make matters worse, not better.
Such as?
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