Dual transformer power supply build

[Dannyx]

Good day folks. Digging around my junk pile at work I found a couple of UPS transformers and I thought they might be handy for putting together a lab PSU, which we don't have around here >_>. A while back, I built one at home using a similar transformer and a DC-DC converter module from China. Works alright, does the job...I bet it certainly wouldn't win any awards for best output stability or ripple on the output, but it is there when you need it to charge some batteries or run unsafe sh!t with current limit.

Now I want to do the same thing with these guys but instead of having one output, I was thinking of having two independent DC-DC modules...a bit pricey but worth it IMO, especially for a DIY geek like me. Sounds simple enough: two transformers, two bridge rectifiers, a couple of caps, all nice and separate, but then that got me thinking about a couple of things:

First, what if I wire the secondaries in series like in the schematic I drew...I know I know, there's no reason why I'd ever need an unregulated 96v output but if I have the option, why not. Would it work the way I drew it in addition to powering the modules ?

Also, should I connect the grounds of the outputs together ? On these DC-DC modules, the ground is common between input and output anyway, so I would essentially tie the negative outputs of the bridge rectifiers together and also to chassis GND while I'm at it, since it's going to have a metal case and if I remember my SMPSs, the chassis is almost always GND.

Any thoughts ? I'd love to have a high voltage regulated PSU as well (which I'm aware is not possible with the current setup), but none of the modules out there go any higher than 50v-60 MAX. Cheers.

[stj]

Re: Dual transformer power supply build

that's dangerous, the outputs may be out of phase.
put them in series after the rectifiers for a single higher output.

as for linking grounds, again - if it's after the rectifiers it's fine, but you shouldnt.
that would prevent you having +- outputs for testing amps for example.
like +15, -15

the ruideng modules are good.

[Dannyx]

Re: Dual transformer power supply build

as for linking grounds, again - if it's after the rectifiers it's fine, but you shouldnt

Maybe have a simple switch to link both GNDs together when needed ? Can a split-supply be created with those DC-DC modules ?

If I wire them in series after the rectifiers, that means I'd only have one output instead of 2.

[eccerr0r]

Re: Dual transformer power supply build

not sure why you'd link the two transformers there, just keep them separate and fiddle with the now isolated output DC-DC modules as if they were batteries.

[Dannyx]

Re: Dual transformer power supply build

I was thinking of having, for no particular reason, an unregulated 96v output on the front panel as well, although it would probably never see any use, at least not in an useful way...just a thought...

[eccerr0r]

Re: Dual transformer power supply build

Well, you get to link it there or link them after the converters, but not both...but yes stj's comment about them being in phase, make sure you get it right if you want the 96V.

[Dannyx]

Re: Dual transformer power supply build

Didn't quite understand what you mean by "link". Link what ? You're talking about the two "ends" of each transformers like I pictured ?

[eccerr0r]

Re: Dual transformer power supply build

yes

[Dannyx]

Re: Dual transformer power supply build

Ok, so if I wire the outputs of the bridge rectifiers in series, this removes the possibility of having two separate DC-DC modules.

If I do it before the rectifier, like in the schematic, the only caveat is that the transformers have to be in phase and from what I've seen online, this means the primaries have to have their "start" and "end" connected in parallel to 230v L and N respectively. I've seen Photonicinduction wire two MOTs in series and it seemed to work.

[stj]

Re: Dual transformer power supply build

if you link the secondary windings, the outputs from the rectifiers will still have a relative difference between them.
link the grounds afterwards for maximum excitement - make sure you film it!!!!

[Dannyx]

Re: Dual transformer power supply build

See, it's a good thing I did some research first...

Still trying to wrap my head around transformers in series...it seems simple at its core, but once I try to do funky stuff like this, it's a bit confusing. For instance, phasing: if the 230v inputs are in parallel, wouldn't the outputs also be of identical phase and cancel each other out if series is attempted ? I use a very crude analogy to batteries: T1 is battery 1 and T2 is battery 2. To get them to work in series, - of 1 must go to the + of 2. If you connect a + to another +, it doesn't work. In the case of transformers, the AC in the secondary windings switches between + and -. Essentially, when winding A swings positive, C swings negative on T1, while on T2 winding D is also positive and F is negative (forgot to add the phasing dots on them). Would C linked to D be like + to + and cancel the series effect ? This is what bugs me and from what I've seen online, series works when the primaries are also in series, whereas here I'd have to flip one of them around so when C is negative, D is positive and completes the circuit....maybe. No fire shows on my watch, no sir

[eccerr0r]

Re: Dual transformer power supply build

I've rarely had a need for AC these days, just DC. I do have an autotransformer variac (0-140V) and a isolated low voltage variac (0-20V) for these occasional uses. In all honesty the low voltage variac has seen 0 use so far...

[budm]

Re: Dual transformer power supply build

You can put the secondary of the two transformer in series WITHOUT any load connected and then use AC meter to check the output Voltage to find out if get the transformer windings in correct phasing or not.

[Dannyx]

Re: Dual transformer power supply build

I wasn't planning on using the AC part - I was going to throw a rectifier on that 96v bus. Again, like you said, it will probably never see any use, but it's there when needed...nothing more satisfying than over engineering something

As for the phasing, indeed I was right: I had to flip the wires on the primary of one of them to get 75v to show up across the very ends of the two transformers. When they were the other way around, I got 0v there. 75v will go up to 90+ once I add a rectifier and cap on it I'm sure...

Now the big question is can I have several bridge rectifiers for 96v, 48v, 24v, etc without blowing sh!t up ? :| I spilled some juice over my schematic the other day - no joke - so I'll have to do it again

EDIT: something like this...it's messy, sorry If I were to have JUST the rectifier from A to F for 96v, it would work no doubt. The challenge is getting two 48v outputs as well. When I was working on my previous supply, when I had just ONE transformer and I was trying to get both 24v and 48v outputs, someone had the clever idea of putting the bridge rectifier across the ends of the transformer, use the center tap as the +24v output and the - output of the bridge rectifier as GND for both of them. This got me thinking: wiring the two transformers in series is essentially like creating a larger center tapped transformer, so what if I do it like this: C+D becomes the new center tap, thus the +48v output, and the - of D1 on the right becomes common GND. Of course, this means the 48v output and GND is going to be common between both DC converters all the time, which I'm not sure what impact will have on overall performance or if it's "industry standard"...starting to seem more of a hassle for an extra output...

[stj]

Re: Dual transformer power supply build

that would give you -48v in relation to the 96v output.
with a fuckload of ripple from your single-diode.

[Dannyx]

Re: Dual transformer power supply build

So the trick only works with a single center-tapped transformer ? It certainly did work the last time someone suggested it when I built my first supply. I used the 48v output (ends of the transformer) to run the DC-DC converter and the center tap and negative output of the rectifier to get a 24v output to run the fan circuit, though now that I think about it, I'm not sure how much ripple I'm feeding that circuit TBH...it's been running like that for a while now

[stj]

Re: Dual transformer power supply build

but you got away with that because your 2 circuits did not have to share a common earth.
and the ripple wont harm a fan, but it means you need much bigger caps.

[Dannyx]

Re: Dual transformer power supply build

Actually the fan "controller" is a DC-DC converter that used to be a car cigarette socket charger for a phone...I modified its feedback loop with an NTC to lower/increase the output based on the temperature of a heatsink. Not sure how affected this guy is by the ripple on its input since it hasn't got too big a cap there and no diode on the input - its + is straight on the center tap and the - is the - of the bridge rectifier which also goes to the - of the module, so in this case I believe they DO share a common GND if I remember my circuit correctly.

EDIT: found my original post HERE and the second to last post by RadioFox is how I did it. In my case, the cap there is the one on the input of the DC-DC converter that used to be a phone charger - nothing too beefy there...

[Dannyx]

Re: Dual transformer power supply build

Still experimenting and not wanting to give up on the idea of making this as complex as possible, here's another design idea: 48v on the right, 96v in the middle...at least in theory. The only caveat I see is that it's impossible to have a common ground (- terminals of any of the rectifiers mustn't touch), as that would cause a short between the top and bottom windings...that's how I see it.

[R_J]

Re: Dual transformer power supply build

Why don't you keep the A/C secondaries isolated, Then just connect the DC in series for 96 volts dc. That way you can have a + and - 48 volts if you need.