Tab Electronics Guide to Understanding Electricity ... - Sciences Club

108 Chapter Three
“righthand” red lead of the other unit. This will stack them in series,
and with the correct phase. Insulate each of the center-tap leads from
each other, and from other circuit components, with wire nuts, or with
shrink tubing.
Testing the First Section of the Lab Power
Supply
There are two purposes for testing your work thus far. It is important, in
regard to function and safety, to be sure that the construction you have
completed to this point is correct. But it is just as important, in regard to
theory and practicality, to understand the testing procedure. If you
become confused, go back and review the area of your confusion. It will
be time well spent.
Do not apply power to the power supply yet. If you have installed
fuse F1 into the fuse block (or holder), remove it. Place the on/off switch
in the “off” position. Adjust your DVM to read resistance (“ohms” position),
and set it for the lowest resistance range available. Measure the resistance
across the two blade prongs of the power cord. Take the same type
of measurement from each blade to the round earth ground prong. All
three of these readings should be “infinite” (meaning an open circuit or
no continuity). Take the same measurement from the round earth
ground prong to any point on the metal chassis. This reading should be
very close to zero. This is because you connected the earth ground wire
directly to the chassis through the locking terminal solder lug.
Now place SW1 in the “on” position. Again, take a resistance reading
between the two blade prongs on the power cord. Again, this reading
should be infinite because the fuse is not installed. Install F1 into its
fuse block (or holder). Again, measure the resistance between the two
blade prongs. This time, you should get a very low resistance reading
(about 4 to 7 ohms). Referring back to Fig. 3-11, with SW1 closed and F1
installed, you are actually reading the parallel DC resistance of the
T1/T2 primaries.
The resistance of each of these primaries is actually twice what this
reading has indicated. To illustrate this fact, take two equal value resistors;
100 ohms each, for instance. Measure each resistor; they should each register
approximately 100 ohms. Now, twist the ends of the resistors together,
so that they are connected in parallel. The measured resistance should
now be about 50 ohms. The paralleled resistors represent the paralleled