APPENDIX C. List of nonconforming class letters
NONCONFORMING | STANDARD | COMMENT Always check the “official” class letter list in Appendix B, Clause 0.4 |
---|---|---|
Binocular core | E | For a ferrite or iron powder binocular core use class letter E to mean miscellaneous electrical part. |
CF | FL | To mean crystal filter. All filters use the class letter FL. |
CN | C | Capacitor network? Whether a capacitor is a single fixed value, variable, adjustable, or of a multiple-element type, just use C. Why use two letters when one will do? |
D | DS | Light emitting diodes are diodes but they are listed under DS (for display). |
F | RT | Resettable fuse or polyfuse. There is no such thing as a resettable fuse or polyfuse, these are trade names. These are PPTC devices that go to a high resistance state when a high current trip point is exceeded, thus limiting the current to a miniscule amount. When the high current condition is removed the device reverts to its normal low resistance state. The schematic diagram should show the graphic symbol for a network protector (a circuit breaker) with the class letter of the part RT (see Appendix B, Clause 0.2.1). |
FB | E | To mean ferrite bead. The standard is to use E to mean ferrite bead ring [ferrite bead or ferrite core] or miscellaneous electrical part. |
FU | F | Fuse. |
FU | XF | Fuse holder. I saw ref des FU1 to mean a fuse and FU1 to mean fuse holder on the same PL—very confusing. |
GN | Z | General network. Why use two letters when one will do? |
HS | MP | To mean heat sink. Heat sinks are mechanical parts, use class letter MP. |
IC | U | Intergrated circuit. Should use U, why use two letters when one will do? |
JP | J | Meaning jumper. If the most fixed part is attached to a PCB use J. |
L | DS | To mean lamp or light bulb. These are considered displays. |
L | E | Ferrite beads do have inductance but the class letter to use is E. |
LED | DS | A light emitting diode (LED) is considered a display. |
M | B | M to mean motor. Use class letter B as M is the class letter for a meter or measuring instrument. |
MES | M | MES to mean measurement, instead use M for meter or measuring instrument. |
MH | NPH | Meaning mounting hole. Use NPH for non-plated through hole. NPH would never be on a PL as these would be part of the PCB and not a separate item. |
MH | PTH | Meaning mounting hole. Use PTH for plated through hole. PTH would never be on a PL as these would be part of the PCB and not a separate item. |
MX | Z | A frequency mixer or more properly a frequency converter. Use class letter Z to mean general network. |
PCB | U | For listing a printed circuit board on a PL use U (see Appendix B, Clause 0.2.5). Recommend using ref des U0 (U zero). |
R | U | The situation is a potentiometer with switch (volume control with power on-off function). Often times this part is listed on a parts list (PL) as R and the switch is listed as S with description of “part of R#”. This is a single part and would use the class letter U (see Appendix B, Clause 0.2.5). On a schematic diagram you would have U#R1 and U#S1, which may be shown in separate places on a schematic diagram, but for PCB mounting would have a single land pattern. |
RFC | E | If the RF choke is a ferrite bead then use class letter E to mean ferrite bead ring. |
RFC | L | If the RF choke is an inductor then use class letter L to mean inductor. |
RN | R | Resistor network? Whether a resistor is a single fixed value, variable, adjustable, or of a multiple-element type, just use R. Why use two letters when one will do? |
RV | R | Variable resistor. This would be a potentiometer (pot) or rheostat. Class letter RV is for a symmetrical varistor or voltage-sensitive resistor. |
S | U | The situation is a potentiometer with switch (volume control with power on-off function). Often times this part is listed on a parts list (PL) as R and the switch is listed as S with description of “part of R#”. This is a single part and would use the class letter U (see Appendix B, Clause 0.2.5). On a schematic diagram you would have U#R1 and U#S1, which may be shown in separate places on a schematic diagram, but for PCB mounting would have a single land pattern. |
SCR | Q | Meaning silicon controlled rectifier or semiconductor controlled rectifier. |
SKT | X | Socket. If the socket is for an IC then use XU, if for an op amp use XAR, if for a display use XDS. |
SP | LS | To mean speaker. Should use LS that means loudspeaker. |
SPKR | LS | To mean speaker. Should use LS that means loudspeaker. |
SW | S | Meaning a mechanical switch. Why use two letters when one will do? In my day SW meant shortwave. |
TP | E | TP, to mean test point, is a non-class letter, to be used on maintenance diagrams. Use E to mean terminal or miscellaneous electrical part. |
TR | Q | Transistor. Should use Q, why use two letters when one will do? |
TZ | D | Transzorb is a trade name. It is a type of breakdown diode or Zener diode. |
U | AR | Operational amplifier, audio amplifier, RF amplifier, or microwave amplifier should use class letter AR IAW the standard. (See Appendix B, Clause 0.2.4.) |
VR | R | To mean variable resistor. See R in the standard list of class letters. VR means voltage regulator. |
X | Y | To mean a crystal or crystal oscillator. Class letter X is for a socket (connector). |
XFMR | T | To mean a transformer. XFMR is an abbreviation but is not a class letter. Why use four letters when one will do? |
XFMR core | E | For a toroid or EI core use class letter E to mean miscellaneous electrical part. |
XTAL | Y | To mean a crystal. XTAL is an abbreviation (maybe even an acronym) but is not a class letter. Why use four letters when one will do? |
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1. Reference designators for electrical and electronics parts and equipment
2. Appendix A: A list of pertinent applicable standards
3. Appendix B: Class Designation Letters
4. Appendix C: List of nonconforming class letters
5. Appendix D: System subdivision
6. Appendix E: KiCad Anomalous Handling of Suffix Letters.
7. Appendix F: Scenario Questions and Answers.
About the author
The articles in this series are written by Lawrence W. Joy (Larry)
[email protected]
Michigan USA.