How to create a DC power supply filter?

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.

Picture of How to create a DC power supply filter?
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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.

codestroy7 (author)  steveastrouk5 years ago
i 'll try that, thank you.

just asking...
i terms of ripple rejection, which is better? LM7805 or PI Filter (LC)?
codestroy7 (author)  steveastrouk5 years ago
the other end of the circuit is connected to the MCU
keesv12 years ago

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

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