Appendix C: List of Nonconforming Class Letters

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  1. Reference Designations For Electrical And Electronics Parts And Equipment (this document)
  2. Appendix 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 (diagram)
  6. Appendix E: KiCad Anomalous Handling of Suffix Letters
  7. Appendix F: Scenario Questions and Answers
APPENDIX C. List of Nonconforming Class Letters
NONCONFORMINGSTANDARDCOMMENT

Always check the “official” class letter list in Appendix B, Clause 0.4

Binocular coreEFor a ferrite or iron powder binocular core use class letter E to mean miscellaneous electrical part.
CFFLTo mean crystal filter. All filters use the class letter FL.
CNCCapacitor 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?
DDSLight emitting diodes are diodes but they are listed under DS (for display).
FRTResettable 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).
FBETo mean ferrite bead. The standard is to use E to mean ferrite bead ring [ferrite bead or ferrite core] or miscellaneous electrical part.
FUFFuse.
FUXFFuse holder. I saw ref des FU1 to mean a fuse and FU1 to mean fuse holder on the same PL—very confusing.
GNZGeneral network. Why use two letters when one will do?
HSMPTo mean heat sink. Heat sinks are mechanical parts, use class letter MP.
ICUIntergrated circuit. Should use U, why use two letters when one will do?
JPJMeaning jumper. If the most fixed part is attached to a PCB use J.
JPPMeaning jumper. If the most movable part plugs to the connector on a PCB use P.
LDSTo mean lamp or light bulb. These are considered displays.
LEFerrite beads do have inductance but the class letter to use is E.
LEDDSA light emitting diode (LED) is considered a display.
MBM to mean motor. Use class letter B as M is the class letter for a meter or measuring instrument.
MESMMES to mean measurement, instead use M for meter or measuring instrument.
MHNPHMeaning 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.
MHPTHMeaning 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.
MXZA frequency mixer or more properly a frequency converter. Use class letter Z to mean general network.
PCBUFor listing a printed circuit board on a PL use U (see Appendix B, Clause 0.2.5). Recommend using ref des U0 (U zero).
RUThe 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.
RFCEIf the RF choke is a ferrite bead then use class letter E to mean ferrite bead ring.
RFCLIf the RF choke is an inductor then use class letter L to mean inductor.
RNRResistor 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?
RVRVariable resistor. This would be a potentiometer (pot) or rheostat. Class letter RV is for a symmetrical varistor or voltage-sensitive resistor.
SUThe 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.
SCRQMeaning silicon controlled rectifier or semiconductor controlled rectifier.
SKTXSocket. If the socket is for an IC then use XU, if for an op amp use XAR, if for a display use XDS.
SPLSTo mean speaker. Should use LS that means loudspeaker.
SPKRLSTo mean speaker. Should use LS that means loudspeaker.
SWSMeaning a mechanical switch. Why use two letters when one will do? In my day SW meant shortwave.
TPETP, to mean test point, is a non-class letter, to be used on maintenance diagrams. Use E to mean terminal or miscellaneous electrical part.
TRQTransistor. Should use Q, why use two letters when one will do?
TZDTranszorb is a trade name. It is a type of breakdown diode or Zener diode.
UAROperational amplifier, audio amplifier, RF amplifier, or microwave amplifier should use class letter AR IAW the standard. (See Appendix B, Clause 0.2.4.)
VRRTo mean variable resistor. See R in the standard list of class letters. VR means voltage regulator.
XYTo mean a crystal or crystal oscillator. Class letter X is for a socket (connector).
XFMRTTo mean a transformer. XFMR is an abbreviation but is not a class letter. Why use four letters when one will do?
XFMR coreEFor a toroid or EI core use class letter E to mean miscellaneous electrical part.
XTALYTo 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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