power supply
 

Copied from another thread:
I had another job for the laser cutter and it came in under their minimum order so i added one of these to make use of the few extra dollars i was going to be charged either way.

I calculated my power supply needed to be 600VA and it was easier to find two 300VA transformers. This bracket allows me to attach both, and their fuses and the bridge rectifier neatly on the one bracket.
Unnecessary, maybe, but it effectively didn't cost me anything.

I am a bit worried about coupling the 2 transformers in parallel, unless their voltages have been closely matched. Best to check with the transformer suppliers if they are happy that you do this.

How about if he adds a second bridge, and ties the D.C. in parallel instead of the A.C? The diodes should keep anything untoward from happening electrically. However, is there any specification on how close you can place two toroids together without interaction from magnetic fields?

It's good that you have separate fuses; I'd start with rather low fuse values and no load on the supply, and see if you have any heating, vibrations, or other untoward effects.

Stacking up transformers seems to be a fairly common practice, that part doesn't worry me.

I suppose the right thing to do is to power up both transformers on the same mains supply and then measure the differential voltage between their outputs. And then find a transformer expert to tell you whether you can have say a 2 Volt difference before "shorting" them to each other.

It probably is better to have 2 bridges and parallel after them, but I just can't get my head around what happens when that imaginary 2 Volt difference is present? Realising that the AC side has a completely "unstable" voltage that fluctuates high/low 60 times per second . . . .

I see that we discussed the paralleling aspect in this thread before:
Transformers

It can be done...
 

Stacking transformers and indeed paralleling transformers is quite common. As long as they are identical brand, model, voltage, and ideally the same batch then it should be ok. Toroidal transformers are not hand wound and machines are pretty reliable at putting the same number of turns on each transformer.

I would steer away from having to two rectifiers (one on each transformer). That won't solve anything. You need two matched transformers.

That said, i still took some precautions as i assembled the power supply. My windings are colour coded and even though i had good faith they were done the same it's good to be safe. I made this little test jig which is a must have for those paralleling up transformers. Its simply a 60W incadescent globe in series with a power cord.

Attachment 5122
Attachment 5123
(this is not quite complete, it needs grommets on the cable entries & insulating covers for the screws)

By connecting this in series with the transformers without a load you can feel assured you won't upset her indoors when you power it up. If there is short the globe will glow bright and quickly blow saving your household fuse from blowing. So your wife can continue watching TV without knowing you stuffed up :D If your transformers are badly matched the globe will glow dim. If its matched ok, it shouldn't glow at all.

My transformers had no measurable difference in voltage and there was no noticeable glow on the globe so i'm confident this power supply will be fine.

DISCLAIMER: This is a description of what i did and is not advice on how to make one of these. If you don't have the qualifications and/or understanding of mains electricity, don't attempt this.

Matt,
You are correct with your observations. The matching in from one transformer is the most important thing when paralleling transformers. Your test setup will tell the tale quite nicely. We have done the same sort of thing with more expensive test equipment when we have installed transformers in parallel as well. Although we have usually built the units in house, so it was really a check on our building. :)

Good luck.

Okay, the experts have stepped in and told us that it is okay if precautions are taken. Thanks!