Stupid transformer question?

[STURMJ]

I'm noticing that power transformers are rated for 115V, are not most homes getting 120VAC (mine is)? Does this extra voltage tax the transformer?  Also, i suppose this will affect the voltage coming off the secondaries, especially when wiring coils in serial (thus also doubling the extra voltage).  I'm trying to make a 25VDC power supply, I ended up with a 28V transformer which is a little too close to the maximum voltage allowed for what I'm trying to run.  I could always just put it together and check, but then I have to desolder it all.  Thanks for the info in advance!

[VoltSecond]

The 28V instead of 25V is more likely from the regulation of the transformer than the line voltage.  Regulation is the ratio of the no load voltage of the transformer vs the voltage into its rated (resistive) load.  I've see transformer regulation higher than 40%. 20% for decent mains transformers is common.

115V in some 115V transformer will make them vibrate and run hot. Other transformers run cool with 125V into them. You are more likely to get the 115V transformer that runs hot than the one that runs cool because the one that runs hot is less expensive and is more likely to get sold.

If you live in a 60Hz country (USA), if you buy a 50 Hz rated transformer, you'll get extra transformer margin.

Elliot Sounds Fig 8 DC block should help.
http://sound.westhost.com/articles/xfmr-dc.htm

Note: You could always add a linear regulator to drop the 28V down to 25V.

[Paul Joppa]

You are correct, that 115v transformers on 120v lines will make more voltage. And VoltSecond is correct (of course!), that if you do not use the full rated current of the transformer, the voltage will be higher still. The latter is pretty common, since most DIYers think that a transformer with excess current capability will run cooler (it will) and that is better (not always!).

Bottlehead power transformers are designed for 120v nominal input. Many of them have unused current capacity, but are designed to give the right voltages in the circuits we use. We can di this because they are built for the specific circuit.

VoltSecond also makes the point about magnetic headroom. Running a transformer at maximum flux density makes it possible to use a smaller transformer, with less iron and less copper - so it's cheaper. Many standard transformers run 15kGauss to 16.5kGauss, which is the nominal maximum for typical grain-oriented silicon steel. I try to stay under 10kGauss in our units, so they will operate properly even at 50Hz and even at 130v - a +/-10% variation in line voltage is not uncommon these days.

[STURMJ]

cool and thanks,I guess I'll look for a more suitable transformer then.

[2wo]

Thanks for the link V.S. I have a large toroid isolation transformer. At certain times of the day it will hum and growl. I need to try this...John     

[VoltSecond]

High line can make them buzz too. I've seen over 130V at my house.  It made my air conditioner blower motor hum quite loudly.