General power question
June 11, 2007 02:28AM

Is the power coming from an ATX supply clean? Or, do I need to put a 7805/decoupling capacitor in front, to clean it up when working with PICs?


Re: General power question
June 11, 2007 02:54AM
These days I develop my 5v power off of my ATX 12v supply with a voltage regulator/capacitor combination to get it to 5v and clean it up. I think that the standard RepRap boards all do that as well, iirc.
Re: General power question
June 11, 2007 04:09AM
The 5V rail should be resonably clean when it leaves the PSU, appart from some high frequency switching noise, but after it has travelled around the machine next to wires carrying stepper motor currents and DC motor currents I would not use it to feed digital logic. You might get away with it but its not good practice. Also you would need to fuse it. PC supplies can give of the order of 20A on the 5V rail so unless you maintain the wire guage and PCB track widths for 20A you will have a fire hazzard.

Much better, as Forrest says, to bus round 12V and regulate it down locally.

Re: General power question
June 11, 2007 10:18AM
Switching noise issues aside, the other thing that you might want to think about when using the nominal 5v power off of an ATX is that it tends to run about half a volt higher than 5v (on my ATX power supply anyway). That is very near the upper limits of what you should be putting into a PIC chip logic.
Re: General power question
June 11, 2007 12:41PM
Yes see my post in PowerComms + Programmer. PC PSUs are designed to run with massive loads on the 5V and 3.3V rails. When you use the 12V rail only they struggle to regulate. Adding a big dummy load to the 5V rail will probably bring it down into spec and raise the 12V rail as well.

Re: General power question
June 11, 2007 12:56PM
That big dummy load is going to mean that we burn a bunch of 5v wattage in order to get a reliable 12v supply, right?

Boy am I glad I'm using gearmotors with about zip current demand instead of steppers. eye popping smiley
Anonymous User
Re: General power question
June 11, 2007 02:22PM
If you do not care about accuracy and temperature linearity light bulbs can make for relatively inexpensive alternate for high wattage load resistors.

ex. 12V 50W bulb will approximate a 2.9Ohm 50W resistor for ~2$ US.

Just a thought.
Re: General power question
June 11, 2007 03:10PM
Yes you can also use the bulbs to light your machine for photography winking smiley.

You can add a fair amount of current to the low voltage rails without wasting masses of power because the voltage is low. It's worth trying if your 12V rail is too low or you 5V too high or in fact some PSU don't run with no load. They repeatedly start, go over voltage and shutdown again.

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