Help designing a power supply
  • So this is not really related directly to the shrut(h)i, but I wanted to pick the brains of some of the guys here (again) since the shruthi does manage to accomplish exactly what I am failing to do.

    Basically I am building a differential line driver for a friend of mine and need a dual polarity supply for it, but I want to run it from just a regular positive DC supply (which will come off of his brick instead of having to tack on another bulky wallwart). I am putting +18 in and need +/-15 out. I’ve tried building a circuit using the 7815 to regulate the positive rail down, and a 2575t-15 along with associated schottky and inductor to invert and regulate the negative rail. (datasheet for the 2575 series).

    In theory I don’t see any problems with my circuit – however in practice nearly all of the input voltage is being dropped somewhere, and I’m only getting about +0.8V out on the positive side and about -1.3V out on the negative side. I can’t find any improper shorts to ground and the chips don’t get overly hot (the 2575 gets a little warm, but not excessively so, and the 7815 doesn’t get very warm at all). For testing purposes I just have 33k resistors on the outputs to ground. This is really confusing me and I’d like to figure out what exactly the problem is here. My schematic is below, if anyone has any advice I would really appreciate the help!

    http://willydavidk.com/mi/pwr15.sch

    Thanks!
    -David

  • Hi David :)

    I'm assuming the negative rail generator is fed by the 18V unregulated input and not the 7815 output?

    I'd start by measuring the actual value of the 18V supply. If it's much under 18V in circuit there won't be enough headroom. If you can easily disconnect the negative rail generator you can debug the positive rail first.

    Do you have capacitors at the input and output of the 7815 and 2575 as per the data sheets?

    Sorry I can't be more help.
  • in the x0xb0x power supply they do it by mirroring down a half wave with some diodes. the schematics are all at adafruit.

  • pin 3 and 5 should be connected to GND, not the negative rail.

  • why not just go with a classic half wave ac-dc? That is the easiest one..

  • @gritz: yes, both the 7815 an 2575t-15 are fed from the unregulated 18V. I’m testing the circuit using Just an 18v rated wallwart, which actually has an unloaded output of roughly 23v. I provided a link to the schematic I drew up where you can see exactly how Its wired. It opens in eagle, you can Download it for free if you don’t already have it.

    @smrl: grounding the feedback pin (4) of the 2575 puts it into inversion (buck-boost) mode, where the output is taken from the ‘gnd’ pin (3)

    And my source voltage is already DC so half-wave isn’t an option. Getting dual polarity from AC is pretty straightforward, generating it using a single polarity DC is not as simple. That’s my whole issue here… :/

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