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Th
For 100 W,
Is � � Np
�Ip � 387(0.83 A) � 321 A
Ns
Quite clearly, the secondary current is much higher for the 140-W setting.
The resulting current levels are probably higher than you might
have expected, but keep in mind that the above analysis does not
include the effect of the reflected impedance from the secondary to the
primary that will reduce the primary current level (to be discussed in
Chapter 21). In addition, as the soldering tip heats up, its resistance
increases, further reducing the resulting current levels. Using an
Amp-Clamp ® , the current in the secondary was found to exceed 300 A
when the power was first applied and the soldering tip was cold. However,
as the tip heated up because of the high current levels, the current
through the primary dropped to about 215 A for the 140-W setting and
to 180 A for the 100-W setting. These high currents are part of the reason
that the lifetime of most soldering tips on soldering guns is about
20 hours. Eventually, the tip will simply begin to melt. Using these
levels of current and the given power rating, the resistance of the secondary
can be approximated as follows:
For 140 W,
P 140W
R � ��2 � �
I (215
A)
2 � � 3 m�
For 100 W,
P 100W
R � ��2 � �
I (180
A)
2 � � 3 m�
which is as low as expected when you consider the cross-sectional
area of the secondary and the fact that the tip is a short section of
low-resistance, tin-plated copper.
One of the obvious advantages of the soldering gun versus the iron
is that the iron is off when you release the trigger, thus reducing energy
costs and extending the life of the tip. Applying dc current rather than
ac to develop a constant current would be impractical because the high
current demand would require a series of large batteries in parallel.
The above investigation was particularly interesting because of the
manner in which the constant current characteristic was established, the
levels of current established, and the excellent manner in which some of
the theory introduced in the text was verified.
Electronic Systems
One of the blessings in the analysis of electronic systems is that the
superposition theorem can be applied so that the dc analysis and ac
analysis can be performed separately. The analysis of the dc system will
affect the ac response, but the analysis of each is a distinct, separate
process. Even though electronic systems have not been investigated in
this text, a number of important points can be made in the description
to follow that support some of the theory presented in this and recent
chapters, so inclusion of this description is totally valid at this point.
Consider the network of Fig. 18.91 with a transistor power amplifier, an
8-� speaker as the load, and a source with an internal resistance of 800 �.
Note that each component of the design was isolated by a color box
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