[sdiy] Transformer question

[Oakley Sound]

This is not as easy as it perhaps should be. There's a bunch of 
variables involved. Here in the UK I am finding that an 18V transformer 
produces too high a voltage for an already built 15V supply. I swap out 
quite a few 110V transformers (and old 240V buzzy ones) for new 230V 
ones in older gear. If I bought 18V transformers the heatsinks in said 
gear would normally not be able to cope and the device would overheat. 
So I often end using 15V output ones. This still gives me plenty of 
headroom. I always test them to see what they will work down to and most 
will be good down to below 210V.

The mains AC voltage in the UK is still 240V and not 230V. But these new 
transformers we are now getting here with 230V primaries seem to be 
producing more than x 1.04 than their older counterparts. Maybe the 
spec'manship has changed I don't know.

Either way, a 15V AC output transformer is spec'd to produce that under 
full load. With load regulation at around 8 to 20% the output voltage 
will rise under normal load conditions. Add this to the complications of 
the spiked load you get from a rectifier and smoothing cap and it 
doesn't make it easy to get a spot on choice.

If in doubt go with the 18V but be prepared to drop some voltage across 
the regulators and dissipate some heat.

Tony

www.oakleysound.com