In article , "Tom Coates"
writes:

How much voltage is the 110V primary winding insulated for, typically? Is
there a non-destructive way to find out? I've been thinking about achieving
a 4x stepup by feeding 24V into a 6V winding and getting 440V. Even more,
p-p.

The insulation MIGHT be figured as twice the maximum peak-to-peak
AC swing to all other windings and their twice peak-to-peak values and
the core and frame. No guarantee, just a rule of thumb.

A "non-destructive way to find out?" Absolutely. Get an old plate
transformer of about 250 VDC kind or higher, put a variable auto-
transformer on the input, and series the secondary through a very
high value resistor, say around a MegOhm, to the device under test.
Use a scope to measure the AC across the device under test.

Bring up the autotransformer (trade names Variac - General Radio -
or Powerstat - Superior Electric) voltage from zero and watch the
scope waveform. When it starts to break up, that's the breakdown
voltage, approximately related to the variable autotransformer setting
and the ratio of voltage step-up of the test transformer. The series
resistance value limits the short-circuit current flow; with 1 MegOhm
and, say, 400 VAC, the maximum current flow would be 0.4 mA AC.

Any "HiPot" tester (former Brewster Electronics) can do the same
thing.

The design and capabilities of transformers are covered in essence
in the old "Green Bible," or ITT Radio Handbook for Engineers, circa
1950s. Also the "Blue Bible" of the 1970s and later. There's a lot
more to power transformer design than realized. You CAN get a
voltage step-up as you described but you will NOT get the rated
currents into any finite load. I will let the "student" figure that one
out.

A coarse approximation of winding a small power transformer is to
figure the number of turns per volt per winding as the E-I core center
square area in square inches divided by 6.5. Choose wire size to
handle the current. That's only for estimation purposes since there are
lots of variables involved. I'm not saying to rewind anything, just that
you will probably get the increased NO-LOAD voltage, but that will
drop depending on the equivalent load resistance.

Len Anderson
retired (from regular hours) electronics engineer person