# Transfomer question

Discussion in 'Electronic Basics' started by Harry Muscle, Dec 8, 2003.

1. ### Harry MuscleGuest

I have a large transforme at 250VA. It's got three windings, one for
the 120AC, one for 14.5AC, and one for 28AC. I hooked up the 120AC to it
with no load attached to any of the other windings, and they measured 14.4AC
and 28.4AC. What surprised me is that these readings were so close to the
printed value with no load attached. I was under the assumption that
usually the volts are a bit higher until you connect a load rated at what
the transformer can provide. Does this mean that I should expect the volts
to drop once I connect a load to it? Or is what I'm experiencing usual for
transformers this big. I'm just trying to estimate that vol I will
receive with a 100W load attached. Unfortunately I don't have a way of
testing right now since I don't have a 100W load handy and to make the load
that I will be using I need to find out the volts first. Catch 22.

Thank you for any help,
Harry

2. ### Ian StirlingGuest

A 15A 120V AC heater (or several smaller) will equal 100W at 28V.

3. ### John PopelishGuest

The regulation of transformers improves as they get larger. If the
regulation is limited by the temperature rise, and the volume and
power output go up approximately as the cube of the size, while the
surface (which dissipates the heat) goes up only by the square of
size, it is clear that they can't get away with the high losses on big
transformers.

Put some managable load on the transaformer and measure the regulation
with that load, and extrapolate the sag linearly to your expected

4. ### defaultGuest

Yes transformers are rated at the full load voltage as a rule. You
neglected to mention what voltage was applied to the 120 volt winding.
The nominal voltage is 120 but can vary by 10%.

Large transformers do have better load regulation than smaller ones.
Size matters.