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Thread: A homebrew power supply

  1. #1
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    A homebrew power supply

    I've built many 12v power supplies before... Just started on another.

    The transformer is a piece of crap I made by re-winding a microwave oven transformer... Dual 12v secondaries outputting 10.8vdc, each individually rectified, then fed into a massive 1 farad 25v smoothing capacitor. Open circuit voltage is 14.2 volts. Under a 20 amp load, it drops to 13.1v. Haven't tested it higher than that.

    This transformer is VERY inefficient, due to the fundamental design flaws (read: cheapness) of a microwave transformer, and the poorly wound secondaries. The primary and core are paired with the intent of a constant, unvarying load, with a fairly low duty cycle. It draws 8 amps @ 120v just sitting there.

    I plan to source a better transformer for this build, but this one was free, so I figured I'd roll with it for now. The whole thing will be properly assembled in a custom cabinet, with fused protection on both sides of the transformer, a voltage meter, cooling fan, etc.


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    Nice job Grasshopper . your kung foo is strong

  3. #3
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    Got some more work done... I drew up an enclosure in Inventor and cut it out of 3/32" aluminum on the CNC plasma table at work. Bent it up on the press brake, and started figuring out where my mounting and component holes will need to go. There will be a rectangular hole cut in the back of the case behind the heat sink, so the rectifiers can mount directly to it.

    Looks like a good fit...



    Cover on...



    On the front of the unit, the power switch will be top left. 120v fuse bottom left. 12v fuses bottom right. Voltage and Ampere meters will be mounted side by side in the center (not yet marked). For outputs I will have a couple sets of binding posts on the front, and screw terminals on the back. I will also be adding a second field of ventilation holes on the left end of the unit, so the cool air gets drawn past the transformer.

  4. The Following 2 Users Say Thank You to anickode For This Useful Post:

    High Voltage Mobile NJ (07-20-2013), the caveman (07-23-2013)

  5. #4
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    Nice project!!!
    Caveman 195 waving a hand in Southern Indiana

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    Almost there... Just have to add a cord, finish up the internal wiring, and add the meters and extra output terminals. I'll probably paint it too.




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    Sweet! Very nice!
    Caveman 195 waving a hand in Southern Indiana

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    Well, it's finished, and all wired up. Did some load testing on it today, and the results are satisfactory, considering I didn't spend a dime on it. Everything was out of my parts bins. The voltage drop under load was very consistent up to 400 watts of resistive load.

    OCV: 15.5v (I added one more turn on the secondaries from the original configuration)
    100w load 14.6v
    200w load 13.8v
    300w load 12.9v
    400w load 12.1v
    500w load seems to have over-saturated the transformer and it dropped to 10.1v This was a 700w microwave transformer with already low efficiency, so considering it's now poorly wound with THHN, I'm not disappointed with the results.

    I added a 120 ohm 5w resistor across the capacitor to bleed it down. 15v isn't much of a safety risk in terms of electrocution, but a 1 farad capacitor holds a hell of a lot of juice... 1 farad = 1 ampere/second per volt... do the math that's 750 amps for 20 milliseconds, which is more than enough to do some damage if inadvertently discharged through something sensitive. I'm hoping the large capacitor will help support the voltage under surge loads.
    Last edited by anickode; 08-17-2013 at 08:37 PM.

  9. #8
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    Quote Originally Posted by anickode View Post
    Well, it's finished, and all wired up. Did some load testing on it today, and the results are satisfactory, considering I didn't spend a dime on it. Everything was out of my parts bins. The voltage drop under load was very consistent up to 400 watts of resistive load.

    OCV: 15.5v (I added one more turn on the secondaries from the original configuration)
    100w load 14.6v
    200w load 13.8v
    300w load 12.9v
    400w load 12.1v
    500w load seems to have over-saturated the transformer and it dropped to 10.1v This was a 700w microwave transformer with already low efficiency, so considering it's now poorly wound with THHN, I'm not disappointed with the results.

    I added a 120 ohm 5w resistor across the capacitor to bleed it down. 15v isn't much of a safety risk in terms of electrocution, but a 1 farad capacitor holds a hell of a lot of juice... 1 farad = 1 ampere/second per volt... do the math that's 750 amps for 20 milliseconds, which is more than enough to do some damage if inadvertently discharged through something sensitive. I'm hoping the large capacitor will help support the voltage under surge loads.
    Nice! Is it getting painted and maybe your own decal design? I have a guy close by that can do any decal design you want or custom decal solutions for a small price. Pm me if your interested.
    Caveman 195 waving a hand in Southern Indiana

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    Quote Originally Posted by anickode View Post
    Well, it's finished, and all wired up. Did some load testing on it today, and the results are satisfactory, considering I didn't spend a dime on it. Everything was out of my parts bins. The voltage drop under load was very consistent up to 400 watts of resistive load.

    OCV: 15.5v (I added one more turn on the secondaries from the original configuration)
    100w load 14.6v

    200w load 13.8v
    300w load 12.9v
    400w load 12.1v
    500w load seems to have over-saturated the transformer and it dropped to 10.1v This was a 700w microwave transformer with already low efficiency, so considering it's now poorly wound with THHN, I'm not disappointed with the results.

    I added a 120 ohm 5w resistor across the capacitor to bleed it down. 15v isn't much of a safety risk in terms of electrocution, but a 1 farad capacitor holds a hell of a lot of juice... 1 farad = 1 ampere/second per volt... do the math that's 750 amps for 20 milliseconds, which is more than enough to do some damage if inadvertently discharged through something sensitive. I'm hoping the large capacitor will help support the voltage under surge loads.

    Man, that is sweet. Totally awesome Bro.




    73 The Fogdog is clear and out of here!

  11. #10
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    Quote Originally Posted by Fogdog View Post
    Man, that is sweet. Totally awesome Bro.
    He could make a serious profit making those
    Caveman 195 waving a hand in Southern Indiana

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