386 Chapter 13
especially severe at PC-mount fuse holders. Wire links may also heat up and
consideration should be given to two links in parallel; this sounds crude but
actually works very effectively.Track heating can usually be detected simply by examining the state of the solder mask
after several hours of full-load operation; the green mask materials currently in use
discolor to brown on heating. If this occurs then as a very rough rule the track is too hot.
If the discoloration tends to dark brown or black then the heating is serious and must
defi nitely be reduced.
● If there are PCB tracks on the primary side of the mains transformer, and this has
multiple taps for multicountry operation, then remember that some of these tracks
will carry much greater currents at low voltage tappings; mains current drawn on
90 V input will be nearly three times that at 240 V.Be sure to observe the standard safety spacing of 60 thou between mains tracks and other
conductors for creepage and clearance. (This applies to all track-track, track-PCB edge,
and track-metal-fi xings spacings.)
In general, PCB tracks carrying mains voltages should be avoided, as presenting an
unacceptable safety risk to service personnel. If it must be done, then warnings must be
displayed very clearly on both sides of the PCB. Mains-carrying tracks are unacceptable
in equipment intended to meet UL regulations in the United States, unless they are fully
covered with insulating material that is nonfl ammable and can withstand at least 120°C
(e.g., polycarbonate).
13.1.6 Power Amplifi er Printed Circuit Board Layout Details
A simple unregulated supply is assumed.
● Power amplifi ers have heavy currents fl owing through the circuitry, and all of the
requirements for power supply design also apply here. Thick tracks are essential,
and 2-oz copper is highly desirable, especially if the layout is cramped.If attempting to thicken tracks by laying solder on top, remember that ordinary 60:40
solder has a resistivity of about six times that of copper, so even a thick layer may not be
very effective.
● The positive and negative rail reservoir caps will be joined together by a thick
earth connection; this is called reservoir ground (RG). Do not attempt to use any