Basic power supply filtering


#1

I typically use 120VAC-5VDC “wall wart” power supplies for my devices. To date I’ve had no known problems, but suspect it would be wise to put some filtering on the power supply before connecting to VIN. It seems something like the following is recommended: 1 uf 50v ceramic, 220 uf 50v electrolytic and possibly a TVS diode (but I don’t know how to spec it). Any general recommendations along these lines? Since I have several boards already designed lacking this feature, I may just design a very small board that I can insert ahead of my terminals where I connect my 5DC power. Thanks for any guidance!


#2

Your “wall wart” as you call it will most likely be using a switching power converter. Depending upon the quality of these the output looks like a sawtooth on each tooth of the saw. Using ceramic capacitors of different values but if one 100nF is a good idea to clean the higher frequency buzz. A large electrolytic cap will smooth out the switching drops - 47uF should be enough. The Transzorb or TVS diode is really only necessary for over-voltage or surge protection and will limit the voltage to 5V. Below is a circuit I use on a board with a Photon. The 5V is the board supply bus.
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#3

Great! I’ll see if I can find a through-hole version of the TVS. Thanks!


#4

They are quite big and easily hand solderable onto pads!


#5

OK. I’m just a bit leery about SMD and usually opt for the through hole if they’re readily available. I assume it goes into the circuit in a reverse-biased orientation.

I found a SA5.0ALFTR-ND on Digi-key.

Thanks, again, for your recommendation. I’ll likely design something similar to yours, but with terminals on either side to facilitate inserting it into existing applications.