<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<META content="MSHTML 6.00.2900.2180" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Arial size=2>There have been several questions about
transformers recently.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Transformer primer:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>V-out = V-in (N-out/N-in)</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The voltage ratio (V) corresponds directly to the
turns (N) ratio in a perfect transformer.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I-in = I-out (N-out/N-in)</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The current ratio( I) corresponds to the inverse of
the turns (N) ratio in a perfect transformer.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>P-out = P-in*eff.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The efficiency (eff.) for small transformers may
vary from 70% to 95%.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Z-out = Z-in (N-out/N-in)^2</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The Impedance (Z) ratio is the square of the turns
(N) ratio in a perfect transformer.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Many small transformers are wound approximately 4
(3 to 5) turns per volt. If you can accurately measure the DC resistance of the
wire and determine the gauge of the wire in the winding then you
can tell how long the wire is by using a wire table. From there the
number of turns can be determined by approximating the circumfrence of
each turn for the windings.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>A good approximation rule to remember: #10 wire has
a diameter of approximate 0.1 inches and has a resistance of approximately 1
milliohm per foot. Each three gauges up will double the resistance and halve the
cross sectional area (not the diameter). Each three gauges down will halve the
resistance and double the cross sectional area. Approximately.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Use a variable voltage transformer (Variac) for
testing unknown transformers. A common light dimmer will generate harmonics that
may contribute to additional transformer heating.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>If you measure the primary current while
increasing the primary voltage the current will approximately linearly
increase with the voltage. However, if the current sharply increases then the
transformer has reached saturation.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>It is a good idea to let an unknown used
transformer "cook" with no load. The transformer should get warm
but not hot. If the transformer gets hot there may be shorted turns. Time to
recycle the transformer.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Measure the size of the secondary winding
wire. Allow about 1 Ampere load for each 700 (500 to 1000) circular mils of wire
cross sectional area.</FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The Volt*Ampere rating of a transformer is related
to the transformer core cross sectional area: 0.4sq.in. = 6VA, 1sq.in. =
45VA, 2sq.in. = 120VA. The cross sectional area is the iron area directly inside
the winding only. I have not seen a simple formula for iron area versus VA
rating.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Transformers are rated in Volt-Amperes not Watts.
The transformer must supply full current when the load is reactive.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Avoid transformers with primary and secondary
bobbins on separate paths of the core. These transformers are probably,
self-regulating, ferroresonant type. If you look carefully you will find an air
gap in the magnetic path. There is a special secondary winding that
requires an AC capacitor.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I think the question was asked if a transformer
designed for 230 VAC will work on 120 VAC. The answer is yes, but. The copper
losses will be greater. The primary of seconday windings may not support
additional current. So, a transformer designed for 230 VAC to 24 VAC if used on
120 VAC will output 12 VAC but will not supply double the current.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Universal transformers have a split primary that is
used in series for 230 VAC or in parallel for 120 VAC.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I have successfully misused transformers. I have
used telephone hybrid transformers and repeating transformers as power
transformers. This is not efficient but it works.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I arranged for the utility company to supply
instrument transformers to power a cellular telephone base station directly
from 110kV high tension lines. An unusual approach but it did solve the power
problem.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Take care,<BR>John</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV></BODY></HTML>