Filter cap needed on regulated supply?

I'm using a 3A 12 vdc switching power supply for my small stepper motor, and will be taking the 12 volts to a 7805 for 5v VCC to Arduino, LED driver and LED power.

Since the 12v is coming from a regulated supply, should I still hang a 1 ufd cap across the input leg of the 7805?

The 12 v is getting to the stepper by way of a Big Easy Driver.

=Alan R.

Yes. Input & Output. Read the datasheet for your particular brand - most require caps.

hould I still hang a 1 ufd cap across the input leg of the 7805?

No a 0.1 uF ceramic and at least a 4u7F cap on BOTH the input and output, at least. As crossroads say read the data sheet for the make of 7805 that you have.

Why not just connect the 12 volt to the arduino external power connector or Vin pin?

That can overheat the regulator. For example: 12V into duemilanove controlling four MAX7219 - overheats & shuts down. 7.5V in - runs fine. 5V into 5V header - runs fine.

The cap has multiple purposes.

One purpose is to "filter" voltage ripple and other unwanted signals coming in from the power source.

A second purpose is to provide a buffer of current capability for brief periods when your device wants a lot of current.

A third purpose is to prevent brief periods of high current demand from your device, from lowering the voltage of the power source and potentially affecting other devices using that source.

These purposes are all inter-related but their conceptual importance depends on the context. It would be unwise to get the concept stuck in your head that these are "filter" capacitors, and that that is the only relevant purpose.

retrolefty: Why not just connect the 12 volt to the arduino external power connector or Vin pin?

I need more 5V current than the Arduino can supply - the project has a seven-segment display as well as a few LEDs. Also the LED driver (NXP PCA 9622) needs VCC as well. I have room for the 7805 so will have a bit of reserve...

Thanks for all the comments - will def go to the datasheet. I though maybe since the 12v input voltage was coming from an already regulated supply, I might not need the caps.

=Alan R.