Re: Torroid Question

>
> If I can get some bigger numbers, maybe I can use these, otherwise

they are starting to look like landfill material. (Ok, recycleable
material.....) Maybe they are 2000VA !??

>
> Thanks,
>
> Chris L

If you have some time Chris, you can do what we used to do with
unmarked and military transformers.

Measure the wire diameter of each winding. You will need to find a
wire chart (type in "wire chart" in Google; use a copper wire chart)
that will give you circular mils and cross that over to a standard
wire gage. Using 750 circular mills per amp, calculate the suggested
amperage. That gives you some real rough ball park numbers to start
with. For example you can run .5 amps through a #24 wire (pretty
small). Now hook up the transformer to full AC (use insulated
connections). Using the secondary voltages, calculate a load
resistance to give you the amps you want to test (R = E/I) You will
have to have some pretty good sized load resistors to give you the
load. You may have to get creative and use Auto headlamps in series
or a similar type load. I actually used to use heating elements. An
AC ammeter on either side of the transformer can be a big help, but
you can do it all with a multimeter and ohm's law (E=I * R). Watts =
I*E =VA. If you design up a 3A load at 44VAC the dissipated power is
132VA(watts). At five lbs each and the size you listed I would
expect them to be at least 100 to 150 VA units....just a guess.

The objective is to run the unit long enough under load to determine
the temperature rise of the core. Once they get too hot to hold your
hand on for more than a few seconds, (80 deg C) it's time to crank
the amps back. The temp will rise slowly because of the transformer
mass and cool slowly as well. If it rises to a reasonable temp and
stays there you are probably operating within the range of the unit.
Your load WILL get hot and may have to be fan cooled. You may have
to make several runs at the test using different loads to get a
profile.

The wire diameter is usually a pretty good indicator. Look at this
way. A transformer designer wants to build the smallest most cost
efficient transformer he can. He starts out with the required input
voltage, input frequency (60HZ) and output voltages and the worst
case current draw. The secondary current dictates the primary
current which in turn dictates the size wire you have to use on
each. Then using another formula you calculate number of turns you
need to keep a given core area of a given core type for saturating.
Now you calculate the "fill" of the window area for the selected
core. Can you get all the wire on the core you selected? If not you
have to adjust the core and recalulate the primary and secondary
turns.
What all of this points to is that wire diameter predicts winding
losses and the size core you need to fit everything together. It is
wasteful to have too large a core by making the wire size larger than
it has to be and small wires will not carry enough current to dictate
using a larger core to stay out of saturation.

You can tell a lot about a transformer from the physical size and the
wire diameters.

Bottom line: If you burn up one of your surplus transformers below
the current you need then its no big loss.

One last note of caution. Make sure you have a piece of metal under
the transformer to protect your bench top and fuse the primary at 2X
the worst case predicted current draw. Make sure no voltages can
short to the metal.