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Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
hey everyone having killed my 4th or 5th computer power supply yesterday i decided to try and build one. i'm going with uzzor's design i've got the entire driver section built and am going to do the power side today. i've got a couple questions though: i'm going to be running this off of U.S. 115vac mains so i've made the necessary power transformer changes with uzzor's helpful smps calculator spreadsheet. i should be able to safely use a 200v capacitor after the bridge rectifier right? would using a larger capacitance do anything for me? the three other capacitors in the halfbridge (3uF, 3uF, and 3.3uF,) what would happen if i used smaller capacitance values?
i don't have enough 3+uF AC caps for the job, but i could parallel up a bank of smaller values if need be.
and about the driver:
there shouldn't be any changes i have to make for 115vac operation correct? since everything samples off the output. what kind of current does the gdt full bridge need to work? i don't have the bd139/140 so i used 2n3904/3906
thanks
edit: for the centertapped secondary on the power transformer, should it be wound up the core, connected back to the start, and then wound up again (like when winding for a zvs), or could i wind up the core, make a loop in the wire at the top and twist that together for the centertap, and then wind back down to the start in the same direction.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
finished the smps. it didn't explode or get hot when i plugged it in so that's pretty nice.
edit: replaced the 27k resistor on the voltage adjust with a 6.8k in series with a 20 turn 25k trimpot so i can adjust the min voltage down to 5v with 0.1v precision. only question left is with regards to the 2x 3uf and 1x 3.3uf capacitors in the mains section. what effect does the capacitance value have on the operation? i used 2x 1uf and 1x 2.2uf and it works, so i'm not gonna screw with it, but i will likely build another unit in the near future and would like to know.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
OKAY. Sorry that no one else has replied to your thread topic.
1. If your mains are 120 v AC then yes, a 200 v capacitor after the rectifier will work fine. More capacitance means more filtering, which is better for heavier loads due to less ripple in the output voltage.
2. For the transistors, as long as they can handle roughly the same power output as the ones Uzzors used, it should be fine, along with they need to have roughly the same gain.
3. For center tapping a transformer, what I usually did was take the wire I'm going to use, strip off the middle section of the wire, then solder a 6 inch or so piece of wire (for extra in case you mess up stripping the coating off) to the center part. Then just wind it normally, making sure the center wire doesn't get wound in otherwise you won't be able to get it out.
4. For changing the output voltage, a lower frequency on the TL494 should reduce the output voltage a bit. Try experimenting with different cap values. Like, instead of using 1 nF, use a 5 nF. Experiment, and see what you get!
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uzzor's smps for 115vac
