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I have decided to build my own linear regulated power supply to provide the power for an upcoming build. Since I'm no pro, I was hoping I could get some feedback on my design before building it, just in case I made some catastrophic mistakes in my design or anything like that. Its pretty simple and based off the LM317 adjustable voltage regulator. Most of the schematic is based on the information found in the LM317 datasheet. The requirements I have for this build is a clean source of about 30VDC at the output. I did the calculations and came up with the details and design listed on the attached photo of the schematic.

If anyone has a moment to look the schematic over for any mistakes or to give any feedback or criticisms of the design, I would be immensely grateful. Thanks![enter image description here]1

LateraLiz
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  • What output current do you need? You haven't considered the voltage drop of the transformer windings, another few volts, remember the peak current during charging of the capacitor, twice each cycle, will be several times the average. Also consider the impact of the mains voltage being low. The regulator will dissipate a lot of power, work it out at worst-case high mains voltage. – tomnexus Aug 21 '19 at 03:47
  • I wasn't really sure how to go about calculating the current. I would like between 2-3A output current, but being my first ps build I would settle for less. I calculated the voltage regulators dropout voltage using the nominal value of 1.5V, since corrected this and used worst case 3V drop out for the LM317. This alone means a 24VAC transformer is not enough to yield the desired 30 VDC with this circuit. Would upgrading the transformer to a 28VAC @ 3.56A be enough for the desired 30VDC? How do I determine the transformer voltage drop? Thanks for your assitance! – LateraLiz Aug 21 '19 at 04:34
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    OK, first, the LM317 is only for up to 1 Amp. The datasheets do have suggestions for how to add large transistors to increase its current ability. But second, you must find a regulator that has foldback current limiting - the 317 with a big external transistor will explode, or destroy your transformer, if you short the output. I recommend 1) reading some more about how to make a 3A linear supply, 2) updating your design, and 3) posting it on electronics.stackexchange.com for advice there. – tomnexus Aug 21 '19 at 14:22
  • Personally, I think C1, at 30mF, is over-specified and that's particularly true given that you're following the rectifier with a regulator IC. Also, while in-rush is a transient event and not usually a problem, it could be as high as 60A depending on where in the AC cycle you switch on; transformer winding resistance and secondary circuit resistance (capacitor ESR). Consider in-rush limiting and specify an appropriate fuse (slow blow). – Buck8pe Aug 22 '19 at 13:12

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