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Iam trying to power the circuit mentioned here (v4.3), Will this schematic work?

I would like to be able to use a single power supply that can provide both 5V and 12V DC circuit. I have the following requirements: (1) The 5V and 12V supply must be isolated (2) The 5V rail needs to power the RPI and can work with 2A current rating (3) The 12V, however, must power 12 pumps with a max rating of 1.2A each for a total of 14-15A current rating (the circuit shows only 5 pumps, will be adding in the additional pumps in future) (4) Want only a single AC input in to the enclosure. This must get converted to the two independent DC power rails.

Any suggestions? A off-the-shelf PSU would be great too, but I could not find any that matches my requirements.

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2 Answers 2

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A variety of COTS power supplies will meet your needs, but the cheapest and most readily available option would be an ATX computer power supply; just about any modern ATX supply can put out 15A@12V and 2A@5V at the same time!

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  • \$\begingroup\$ Thanks for your quick feedback. I've already asked the question about the isolation on 5V & 12V rails in typical ATX PSU below..Would appreciate any inputs. Also, can a ATX PSU handle the back EMF that might be generated when so many pumps are powering down ? \$\endgroup\$ Commented Sep 9, 2015 at 4:47
  • \$\begingroup\$ @VijaySeshan -- the back-EMF problem can be handled by using diodes to snub the pump coils (although most ATX supplies will see some back-EMF in their normal habitat, it's not something that they're spec'ed for I suspect). As to cross isolation? Most cheap ATX supplies use a multiple winding flyback configuration which will provide a few hundred volts minimum of cross isolation between the rails. \$\endgroup\$ Commented Sep 9, 2015 at 16:29
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You state at (1) that you want the two voltage rails isolated. Why? Even your schematics is drawn without a galvanic isolation. I would suggest you buy a cheap computer power supply like this one. They will fit your requirements easily.

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  • \$\begingroup\$ Well, in the v4.3 schematic the 5V and 12V circuits are separated by an opto-coupler. Isn't using an opts-coupler the achieving some sort of galvanic isolation? \$\endgroup\$ Commented Sep 9, 2015 at 4:45
  • \$\begingroup\$ @VijaySeshan Yes, but you added to both GND lines the earth symbol which in every simulation software or EDA- Program will connect those two lines together and therefore eliminates a real galvanic isolation. Remove one symbol an we have galvanic isolation, but then i doubt you will find a cheap power supply which fits your requirement. \$\endgroup\$
    – Martin
    Commented Sep 9, 2015 at 5:26
  • \$\begingroup\$ Yeah, thats my bad. I did not mean to add a common ground. Nevertheless, two followup questions: (1) can a ATX PSU handle the back EMF that might be generated when so many pumps are powering down ? (2) The reason for the opto-coupler was to avoid the RPI from getting damaged/reset due to spikes from the pump back EMF. Will this still be concern using ATX PSU with non-isolated DC rails? If so, can anything be done to workaround this and still use the ATX PSU? \$\endgroup\$ Commented Sep 9, 2015 at 5:40

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