I have a 12V 20A DC power supply I'm using to power my some of the accessories on my layout. I'm also using step down converters to reduce that 12V in some cases. Should I be using a filter (capacitor) on the input of these converters?
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It depends on how much ripple there is in the DC output of the power supply. The specs should tell you; if not an oscilloscope will.
Chances are you will not need any filtering. The power supply output should be clean enough to operate accessories as is.
Larry
Can you tell us more about the power supply and the specific DC-DC converters? Some of the eBay models have input capacitance, some don't.
Both the power supply and converters are from Asia vie eBay and are similar to the attached pics.
Thanks for the replies
I recognize the module, it has a 100uf input filter, but that's not enough for real ripple in the DC. Without specifications, "looks like" doesn't help me much. 
