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How to create a DC power supply filter? Answered

I am programming a microcontroller, and it worked fine. By this time, the microcontroller is powered by the USB port. After programming, i do some testing, i used LM7805 paired with wall wart as the controller's power supply. When using this power supply, the controller's behavior is a little bit unpredictable. Based on observations, the problem is the ripple in the DC supply.

I there a filter method or topology that i can use?
Note: simple capacitor filter won't work.

i have a frequency counter...
it reads 0 Hz at the USB
and reads 60 Hz at the LM7805 powered by a wall wart.

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steveastroukBest Answer (author)2012-04-13

Show your circuit. The real arbiter would be to use an oscilloscope to see what's happening.

I suspect the input to the 7805 isn't smoother. Put a nice fat 100uF cap BEFORE the 7805, put a 100uf cap on your circuit, and parallel that with a 100nF ceramic.

Steve

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codestroy7 (author)steveastrouk2012-04-13

i 'll try that, thank you.

just asking...
i terms of ripple rejection, which is better? LM7805 or PI Filter (LC)?

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codestroy7 (author)steveastrouk2012-04-13

the other end of the circuit is connected to the MCU

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keesv1 (author)2015-03-31

indeed is the first thing a lot of peoples think is increasing the the 10uf , that will only increase the ripple, only a bit large and indeed bypass with a small cap will do the trick and a bit ripple can't do no harm , but use a low imp cap , so it rises fast

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steveastrouk (author)2012-04-13

A 7805 will beat most PI filters ! And neither a 7805 nor pi filter can't beat 100% input ripple.....

Steve

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