What is an easy way to get 7-7.2vdc from an scavenged atx psu? I had thought about using a lm317 but I'm going to need at least 5-7 amps of current, and the 12v-5v split just sounds like a really bad idea...any thoughts?


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  • \$\begingroup\$ Can you clarify what you mean by "the 12V-5V split"? Also, LM317 is only good for 1.5A in the packages I'm seeing on Digikey and Octopart. You should consider switching regulators instead. \$\endgroup\$ – Stephen Collings Oct 29 '12 at 14:53
  • \$\begingroup\$ I think he means using the 5V as the ground reference. 12V-5V = 7V. I know people have done that to power fans at a lower speed, therefore making them quieter. I'm not sure how using 5V as ground impacts regulation/power delivery in a SMPS. I would guess that it is, in fact, a Bad Idea(TM). \$\endgroup\$ – dext0rb Oct 29 '12 at 15:48
  • \$\begingroup\$ Exactly dextorb. \$\endgroup\$ – JohnnyMadDash Oct 29 '12 at 16:37
  • \$\begingroup\$ Is there a way to make a lm317 drive more current with driver power transistors and filter caps? \$\endgroup\$ – JohnnyMadDash Oct 29 '12 at 16:39

A good "green" solution would be to design a suitable DC to DC converter to step the 12V down to 7.2V. There is a good bit of information for assistance in designing a suitable DC to DC converter at a number of manufacturer web sites but the one that comes immediately to mind is the one here at this link. The web bench tool offers a plethora of design ideas as possible solutions for you.

enter image description here

I do have a sneaking suspicion that due to your use of a re-captured ATX power supply that you are looking for a solution that does not involve going off to design and build a surface mount PC board. If that is the case then you may want to consider the use of a pre-made POL (point of load) module to do the DC/DC conversion. The picture below shows what one of these looks like.

enter image description here

These POL type modules support a range of input voltages and can have their output voltage set through the use of a resistor or two.

Another possible solution is to consider looking into available AC to DC type power supplies and foregoing the use of the ATX power supply. This is likely to be an overly expensive solution so it may be desirable to take a look at what you need the 7-7.2V for and see if there is a way to effectively rethink the problem so that you can actually use +12V or +9V for your design, both of which are way way more readily available in off the shelf product.

  • \$\begingroup\$ Amazingly helpful reply, I can't thank you enough. The application is a custom pan/tilt mount each servo is 7.2v for full torque and needs 2.5-3amps at max for each servo. The atx psu is involved because the tracking software pc is just a stripped motherboard and a hd so i have some extra plugs in the box to use. \$\endgroup\$ – JohnnyMadDash Oct 29 '12 at 15:22

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