APPENDIX C: List of nonconforming class letters

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?