Vectronics VEC-1500K - Manual (Page 39)

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Air-wound coils, toroids, and RF inductors belong to the same family of
components, but offer no standard method of color-coding or identification.
Check building instructions or part numbers for more specific identification.
Air-Wound Coil Toroid Coil
Slug-Tuned RF Coil
Diodes
Diodes allow current to flow in one direction, but not in the other. Like
electrolytic capacitors, diodes are polarized devices that must be installed
correctly. Always look for the banded--or cathode--end when installing, and
follow instructions carefully.
Cathode
Diode LED
(shorter Lead)
Transistors
Transistors are solid-state devices capable of amplifying or switching. If
transistors are installed incorrectly, damage may result when power is applied.
Transistors in metal cases have a small tab near the emitter lead to identify
correct positioning. Semiconductors housed in small plastic cases (TO-92) have
an easily-identified flat side to identify mounting orientation. Many specialized
diodes and low-current voltage regulators also use this type packaging. Larger
plastic transistors and voltage regulators use a case backed with a prominent
metal tab to dissipate heat (T-220). Here, orientation is indicated by the
positioning of the cooling tab.
Emitter
Flat Side
Metal Tab
Metal Can Device Plastic Device Tab-cooled Device