Consider adding a 5V zener diode as shunt regulator

[scottlawsonbc]

My Gen7 board is currently being powered by an ATX power supply. Using an oscilloscope, I've noticed my power supply has been introducing high frequency noise into my 5V rail. It's possible this noise is the culprit behind the occasional USB communication error that I've encountered.

Nophead points out in this thread:

One of the problems with Gen7 is it uses the PC 5V rail, which is noisy and
unstable, whereas other reprap electronics regulate the 12V down to 5V
locally. That is much more stable and clean and is referenced to the local
ground.

In the case of the Gen7, it would be impractical to replace the 5V rail with a linear regulator like the LM7805. Instead, I recommend adding a single 5.0-5.5V zener diode across the 5V rail as a shunt regulator. This will also provide over-voltage protection for the micro-controller. A zener diode will only add about 20 cents to price of the Gen7.

[Traumflug]

Not so sure on this one. If there are noticeable voltages to regulate down, this Zener diode is target for unlimited current and could easily play "glow in the dark".

To be honest, I'm a bit disappointed. These supplies are designed to and made for supplying digital electronics without further refinement. Not that I've ever seen big caps on PC mainboards, but would a bigger capacitor help here, too?

[scottlawsonbc]

I've been thinking about it a bit more, and I agree a zener would be target for lots of current. I like your description of "glow in the dark". Potentially, a 6V zener might work, since if more than 6V is applied, the zener becomes a short circuit and the power supply should detect the short circuit and shut off (hopefully before the diode melts).

I just have a feeling someone might accidentally stick a 12V rail into the 5V rail (perhaps by soldering the disk headers in backwards) and fry the microprocessor. It would be nice to have some kind of over voltage protection, but I'm not sure what the best solution is.

You need a small ~100nF ceramic cap to filter out the high frequency noise, but it looks like the Gen7 already has those, so I'm not sure what else can be done.

[alfadex]

Have you ever try another power supply? I have notice the cheaper the supply the biggest noise.

[Traumflug]

Feel free to solder such a Zener diode onto your board and measure wether it gives an improvement. Adding something just because it could improve something which isn't even a problem is not so much my taste. 5V supply is within specifications and USB signals run on 3.3V anyways, so there's a voltage regulator already involved.