Archive for December, 2011

Testing and using an ATX power supply outside of a PC

Tuesday, December 20th, 2011

ATX Power Supply Dongle

Here's a pic of a quick little dongle I threw together that allows you to power up a PC power supply and use it as a bench supply, or in my case, to power a few stepper controllers for my CNC setup. You can also use the dongle to test potentially bad ATX power supplies.

You can find tutorials on using ATX power supplies outside of PCs all over the net but all of them are pretty much a variant of what I have here.  To get the thing up and running, you just need to:

  • Connect the green wire on the 20 or 24 pin connector to ground to power on the supply.
  • You'll probably want to supply a load to the supply to keep it up and running, usually just a power resistor on the 5V rail, a 10 Ohm 10W resistor is typically used, creating a 2.5W load.  I've also seen people simply plug in an old PC peripheral like a hard drive to the supply to provide this load as well.
  • Attach an LED to the output of the supply OK signal to get a nice "everything's fine" light.

Here's what you'll need:

  • 30 Ohm 5W Resistor - Mouser P/N: 71-CW5-30-E3 - This puts a nice 0.83W load on the 5V rail.  Don't worry if this warms up a little bit during operation, that's what a resistive load does.  This smaller load seems to work fine for me.
  • 20-Pin Power Supply Connector - Mouser P/N: 538-39-29-3206 - Just the socket we need to connect to the ATX supply where a standard motherboard would be connected.
  • 330 Ohm 1/4W Resistor - Just a current limiting resistor, not needed if you don't put a power on light on your dongle.
  • Green LED - Just a typical LED for your power good light.

Here's a couple close up shots of the dongle on the inside:

ATX Power Supply Dongle

ATX Power Supply Dongle

Check out the Wikipedia entry for ATX and scroll down to the power supply section for a detailed pinout of the ATX connector.  Note that I'm using a 20-pin connector and the connector pin numbers on the Wikipedia article are for a 24-pin.  I'm using the pin numbers from the article, so realize they could be different based on which connector's pinout you're looking up, so pay attention to the actual signal names!

Basically I've connected pin 16 (power on) to pin 15 (ground), put a 30 Ohm resistor across pin 21 (+5V) and 19 (ground), making sure that the long exposed lead is connected to ground, and connected the positive lead of my indicator LED through a 330 Ohm resistor (hidden in the heat shrink) to pin 8 (power good) and the negative lead to pin 7 (ground).  After testing, I wrapped the whole thing in a little electrical tape and installed it.  I'm using the switch on the back of the power supply as my power switch, but if you want to have a more convenient switch wired to your dongle, just replace the jumper between ground an power on with a standard normally open switch.


3D Printing Presentation at Hallmark

Friday, December 16th, 2011

Back near the end of October, CCCKC did a set of presentations primarily focused on 3D printing (mostly DIY 3D printing) at Hallmark in Kansas City.  Here's the video from the event (I'm speaker #3 starting at about 25:00):

I wish they would have recorded the last session we did rather than the first, because it seemed rather polished in comparison by the third time we had presented (and I need all the help looking like I know what I'm doing I can get)!


Making a brushed DC motor controller out of a standard RC servo

Tuesday, December 6th, 2011

Another fun hack you can do by modifying the guts of a servo is converting the controller on board to a brushed DC motor controller.  The resulting controller will work just like a standard RC speed controller, allowing you to make a DC motor turn forward or reverse with it's speed proportional to the throttle position on your RC setup.   A simple controller like this should work great if you're trying to throw together a small RC setup like the Botmobile from some scavenged parts.

All you need to do to build your controller is remove the controller of a standard servo and replace the potentiometer with two equal value resistors (typically 2.2k, but higher values should be OK) and hook up the brushed motor you want to control in place of the servo's DC motor.  You want to connect the two resistors in the center by twisting their leads together connect the outside leads of the resistors to the outside potentiometer terminals and the twisted center to the center terminal. Here's a pic of a controller (specifically the TGY-S4505B controller, see some more pics of the guts of this servo in my previous post) after it's been removed from the servo and the resistors have been installed:

RC Servo controllerI used heat shrink on the two outside leads of the resistor to keep the connections insulated.  Next I put heat shrink over the two resistors:

And tape everything down.  Now I'm ready to test it out on Luis' partially completed turtle shell racer using a standard RC Tx/Rx set by connecting up the DC motor I want to power where the servo's DC motor was previously connected:

If everything works you should not see the DC motor turn (or turn very slowly until you adjust the trim) when applying power to the RC Tx/Rx set but the brushed DC motor should respond as expected when you use the throttle on your controller.  If you've modified a servo for continuous rotation this should be very familiar to you because that's essentially what you've done, you're just not using the mechanics of the servo.