DPS5020 dual power supply build

[redwire]

Re: DPS5020 dual power supply build

You short each winding and measure capacitance from pri-sec, pri-core, sec-core.
It's good to know how much the leakage currents might be. Some 600VA big toroids are 100's nF pri-sec.

[Dannyx]

Re: DPS5020 dual power supply build

You short each winding and measure capacitance from pri-sec, pri-core, sec-core.
It's good to know how much the leakage currents might be. Some 600VA big toroids are 100's nF pri-sec.

Does "core" in this case refer to the metal chassis of the transformer where earth would go ?

[Dannyx]

Re: DPS5020 dual power supply build

Got the caps today. As you can see, they claim to be Nippon Chemi Cons

[sam_sam_sam]

Re: DPS5020 dual power supply build

What makes you think that they are not real

[Dannyx]

Re: DPS5020 dual power supply build

What makes you think that they are not real

Ok, maybe I was a bit harsh on Ali with that statement. Funny thing is that I once saw a video of Dave from EEV Blog where the took apart a cheap subwoofer and projector and he found some Elna and NCC caps inside and he was having a very hard time believing they were real. The people in the comments however disagreed with him and pointed out that it's unlikely they'd counterfeit the logos on the caps as well, so it could be the same in my case

[stj]

Re: DPS5020 dual power supply build

i'v not seen fakes with snap-in style base.

i suppose a company could re-sleeve another brand, but it's unlikely - they arent cheap to begin with.

[Dannyx]

Re: DPS5020 dual power supply build

they arent cheap to begin with.

Correct - they were not exactly cheap, so here's hoping they don't bulge after a few hours

[Dannyx]

Re: DPS5020 dual power supply build

Back on this project, I was thinking of adding an NTC thermistor in series with the secondary of the transformer to limit inrush on those large caps after the bridge rectifier.

[stj]

Re: DPS5020 dual power supply build

i wouldnt bother, the Q of the transformer will limit the inrush.

[redwire]

Re: DPS5020 dual power supply build

If the primary fuse is slow-blow and it can handle it, then I would keep it simple.
Above around 20,000uF is when I add a soft-start to a mains transformer, or just listen to the mains wiring rattle in the walls when I turn on power.

[Dannyx]

Re: DPS5020 dual power supply build

Speaking of fuses and stuff "blowing". I once had a freak mishap with my existing supply made from the same UPS transformer: it tripped the main breaker when I plugged it into the wall outlet. To this day I have no idea why it happened, but it hasn't happened since...I thought inrush current caused it, but that means it should've tripped again after that as well, which it hasn't done so far, so I have no idea what that was all about I saw a little spark flying out of the wall socket as well, so maybe it was a faulty outlet, though I remember wiring that outlet myself and plugging in lots of stuff in it before without a care....strange :|

[redwire]

Re: DPS5020 dual power supply build

Big power transformers, they can be magnetized when you switch off power.
The flux alternates between N and S at 50/60Hz but say you unplug it when the core is at peak N. Then you plug it in for peak S, and you get a bigger than usual inrush current. The core has to flip magnetic polarity.
It just depends on the mains sine wave phase when you unplugged it and when you plugged it in again.

[Dannyx]

Re: DPS5020 dual power supply build

Ok, but how long can it remain magnetized like that ? It had been more than a couple of days when it happened

[redwire]

Re: DPS5020 dual power supply build

The core is steel, it can stay magnetized for a very long time.

[Dannyx]

Re: DPS5020 dual power supply build

I see. Any way to "de"magnetize it ? Like shorting out the primary ?

[redwire]

Re: DPS5020 dual power supply build

It happens in all transformers, residual magnetism in the core. The only way to demagnetize is apply a lower voltage sine wave and then slowly dial voltage down. It's kind of like demagnetizing a screwdriver or cassette tape.

The formal name is "magnetizing inrush current" and it's why some transformers literally jump up in the air or blow primary fuses for no reason when they are energized, on the odd occasion. The core has a high (up to 90%) flux, just sitting there and then applying power the sine wave adds flux on top of that. I think some cores can saturate when that happens.

[Dannyx]

Re: DPS5020 dual power supply build

Interesting. Transformers go a bit over my head as far as electronics theory goes - a LOT of math in there, but at least I got the basics One of the more complex electronic components, with a lot of stuff going on in that little heavy metal box - flux, current, capacitance, resistance, turn ratio....etc. it's got it all

[Dannyx]

Re: DPS5020 dual power supply build

Ok folks, I'm back on this project. I started working on the case today (or rather my old man did according to my instructions and tastes, as I'm a bit of a noob when it comes to machining metal stuff at this great precision ) and in the meantime I was gathering whatever parts might still be required. I've hit a bit of a snag however when it comes to those USB ports: remember the two modules can be connected via USB to a piece of software that controls them from a PC....pretty useless in the grand scheme of things, but if it has the option, I'd surely like it to work too. Trouble is, if I connect the two modules to USB, the grounds would become common through the PC or hub or whatever. It's not a problem if I'm powering something like a TV main board which requires two supplies (5 and 12v for instance), since the GND is common by default there, but if I want to go split-supply (+/- 12v for instance), that's not going to work, is it ? I found this isolator thing on Ali which according to the datasheet for the ADuM3160 chip it uses has separate GNDs and might do the job. Pretty pricey for my taste and I'd need two of them ! Am I at least looking at the right thing ?

[stj]

Re: DPS5020 dual power supply build

no, the usb to serial boards are optically isolated.
as long as they are not touching the metal case the pc wont be referenced to the psu's

[Dannyx]

Re: DPS5020 dual power supply build

But doesn't the outer shell of the USB connectors do just that ? Plus, isn't the black GND wire inside the USB lead common across all devices ?