Question about transformer (building a PSU)

[revtintin]

Hi all,

I'm following along Randy Slone's book and I'm at the stage where I'm looking to assemble the first part of the power supply. It's all scary to me, so I'm looking for help in checking my understanding.

My progress so far:

1. Drilled holes into chassis sides for switch, cord, fuse holder
2. Drilled holes on bottom for lug, transformers
3. "Tied" transformers with a soldering iron: wires grey to violet for 230VAC on primary, wires red and orange for series
4. Soldered ground wire of plug to lug

In the book, it says the 2 transformers (I bought 2x Triad Toroidal VPT48-1040) will be connected in series for the secondary - so I assumed to follow the tying for "series" instead of "parallel". I've included a picture of the wires of the transformer after soldering - am I on track? 

Another question is the bolt for the transformer is going to be mounted to the chassis bottom, where the lug with the ground wire will be connected as well. I saw a comment on Youtube where there might be concern for a "shorted loop" - is this the case in my setup? If so, how can I remedy the situation?

I bought the wrong fuse size and I realize I didn't have any high temp wires to hook everything up so I'm currently waiting on those before continuing.

Thanks!

[rstofer]

With no schematic and no drawing/datasheet for the transformer, it's going to be a little difficult to help.  Individual transformers may have their windings in series of parallel and then the overall transformers connected in series or parallel although series seems most likely.  But first you have to wire the transformers to get the right output for the given input.

Because of the toroidal nature of the core, I would avoid tying the ground to the center mounting bolt.  Connect it to the chassis somewhere.

You can imagine the situation where there is just a single piece of wire from the bolt to the chassis.  This would, in fact, be a single turn winding.  Part of the winding is the chassis, part is the bolt and the rest is the wire itself.  It would be an odd situation to create but it is possible.

[revtintin]

Thanks for the reply. Here's the datasheet I found on Triad's website. I will post back once I give the wiring a go.

[Ian.M]

Its a good policy to test any mains transformer you are unsure about with a current limiter in series with the feed to the primary side, and no load on the secondary side.  An incandescent bulb rated for your local mains voltage makes a good current limiter.

N.B. the bulb wattage shouldn't grossly exceed the transformer's VA rating nor should it be less than 10% of the VA rating.  DO NOT USE a LED or CCFL lamp. - It *MUST* be an incandescent tungsten filament bulb. 

Power the transformer up initially with a 40W or 60W incandescent bulb in series with the primary to limit the current, and no connections to the secondaries apart from how you have tied them.  The bulb should remain dark apart from possibly a very brief flash at powerup.  You can then check the secondary voltage to be certain you have got all the windings properly phased. 

Its unlikely you've got it wrong but if the bulb comes on at full brightness you either have a shorted winding or have a parallel pair of windings incorrectly phased or have misidentified the primary.  If it comes on dim, the most likely cause is you haven't got the primaries in series so you are applying double the nominal voltage to one or more of them and the core is saturating due to overvoltage. 

[revtintin]

Its a good policy to test any mains transformer you are unsure about with a current limiter in series with the feed to the primary side, and no load on the secondary side.  An incandescent bulb rated for your local mains voltage makes a good current limiter.

N.B. the bulb wattage shouldn't grossly exceed the transformer's VA rating nor should it be less than 10% of the VA rating.  DO NOT USE a LED or CCFL lamp. - It *MUST* be an incandescent tungsten filament bulb. 

Power the transformer up initially with a 40W or 60W incandescent bulb in series with the primary to limit the current, and no connections to the secondaries apart from how you have tied them.  The bulb should remain dark apart from possibly a very brief flash at powerup.  You can then check the secondary voltage to be certain you have got all the windings properly phased. 

That's new (to me)! I'll do that, thanks for the advice.

Its unlikely you've got it wrong but if the bulb comes on at full brightness you either have a shorted winding or have a parallel pair of windings incorrectly phased or have misidentified the primary.  If it comes on dim, the most likely cause is you haven't got the primaries in series so you are applying double the nominal voltage to one or more of them and the core is saturating due to overvoltage.

[HB9EVI]

Your primary wiring is ok for 230V; the secondary like that is giving you the 2 winding in series, so 48V; do you intend to build a 50-60V PSU?

And yes, mount the center tap on a single chassis mounting point, but not on the holder for the toroid