______________________________________________________________ Solid State Power Supplies
4-137. If you find none of the defects listed in Table 4-2, go ahead with the signal tracing
procedure. The trouble is probably of such a nature that it cannot be seen directly with your
4-138. Tracing the AC signal through the equipment is the most rapid and accurate
method of locating a trouble that cannot be found by a visual check. It also serves as a
check on any repairs you may have made. The idea is to trace the AC voltage from the
transformer, to see it change to pulsating DC at the rectifier output, and then to see the
pulsations smoothed out by the filter. The point where the signal stops or becomes
distorted is the place to look for the trouble. If you have no DC output voltage, you should
look for an open or a short in your signal tracing. If you have a low DC voltage, you
should look for a defective part and keep your eyes open for the place where the signal
becomes distorted. Signal tracing is one method used to localize trouble in a circuit. This is
done by observing the waveform at the input and output of each part of a circuit.
4-139. Figure 4-51 shows what each part of a good power supply does to a signal. The AC
voltage is brought in from the power line by means of the line cord. This voltage is
connected to the primary of the transformer through the ON-OFF switch (S1). At the
secondary winding of the transformer (points 1 and 2), the scope shows you a picture of
the stepped-up voltage developed across each half of the secondary winding (the picture is
that of a complete sine wave). Each of the two stepped-up voltages is connected between
ground and one of the two anodes of the rectifier diodes. At the two rectifier anodes (points
4 and 5), there is still no change in the shape of the stepped-up voltage (the scope picture
still shows a complete sine wave).
4-140. When you look at the scope pattern for point 6 (the voltage at the rectifier
cathodes), you see the waveshape for pulsating DC. This pulsating DC is fed through the
first choke (L1) and filter capacitor (C1), which remove a large part of the ripple, or
"hum," (as shown by the waveform for point 7). Finally, the DC voltage is fed through the
second choke (L2) and filter capacitor (C2) that remove nearly the entire remaining ripple.
See the waveform for point 8, which shows almost no visible ripple. You now have almost
pure DC. No matter what power supply you use in the future, they all do the same thing
and that is that they change AC voltage into DC voltage.
Figure 4-51. Complete Power Supply (Without Regulator)
23 June 2005