Reference designator

A reference designator unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board. The reference designator usually consists of one or two letters followed by a number, e.g. C3, D1, R4, U15. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, e.g. R17A, R17B. The IEEE 315 standard contains a list of Class Designation Letters to use for electrical and electronic assemblies. For example, the letter R is a reference prefix for the resistors of an assembly, C for capacitors, K for relays.

History

IEEE 200-1975 or "Standard Reference Designations for Electrical and Electronics Parts and Equipments" is a standard that was used to define referencing naming systems for collections of electronic equipment. IEEE 200 was ratified in 1975. The IEEE renewed the standard in the 1990s, but withdrew it from active support shortly thereafter. This document also has an ANSI document number, ANSI Y32.16-1975.

This standard codified information from, among other sources, a United States military standard MIL-STD-16 which dates back to at least the 1950s in American industry.

To replace IEEE 200-1975, ASME, a standards body for mechanical engineers, initiated the new standard ASME Y14.44-2008. This standard, along with IEEE 315-1975, provide the electrical designer with guidance on how to properly reference and annotate everything from a single circuit board to a collection of complete enclosures.

Definition

ASME Y14.44-2008^[1] and IEEE 315-1975^[2] define how to reference and annotate components of electronic devices.

It breaks down a system into units, and then any number of sub-assemblies. The unit is the highest level of demarcation in a system and is always a numeral. Subsequent demarcation are called assemblies and always have the Class Letter "A" as a prefix following by a sequential number starting with 1. Any number of sub-assemblies may be defined until finally reaching the component. Note that IEEE 315-1975^[2] defines separate class designation letters for separable assemblies (class designation 'A') and inseparable assemblies (class designation 'U'). Inseparable assemblies—i.e., "items which are ordinarily replaced as a single item of supply"^[2]—are typically treated as components in this referencing scheme.

Examples:

1A12A2R3 - Unit 1, Assembly 12, Sub-assembly 2, Resistor 3
1A12A2U3 - Unit 1, Assembly 12, Sub-assembly 2, Inseparable Assembly 3

Especially valuable is the method of referencing and annotating cables plus their connectors within and outside assemblies. Examples:

1A1A44J5 - Unit 1, Assembly 1, Sub-Assembly 44, Jack 5 (J5 is a connector on a box referenced as A44)
1A1A45J333 - Unit 1, Assembly 1, Sub-Assembly 45, Jack 333 (J333 is a connector on a box referenced as A45)

A cable connecting these two might be:

1A1W35 - In the assembly A1 is a cable called W35.

Connectors on this cable would be designated:

1A1W35P1
1A1W35P2

ASME Y14.44-2008 continues the convention of Plug P and Jack J when assigning references for electrical connectors in assemblies where a J (or jack) is the more fixed and P (or plug) is the less fixed of a connector pair, without regard to the gender of the connector contacts.

The construction of reference designators is covered by IEEE 200-1975/ANSI Y32.16-1975^[3] (replaced by ASME Y14.44-2008^[1]) and IEEE 315-1975.^[2]

Designators

The table below lists designators commonly used, and does not necessarily comply with standards. For modern use, designators are often simplified towards shorter designators, because it requires less space on silkscreens.

Designator	Component type	Modern use
A	Separable assembly or sub-assembly (e.g. printed circuit assembly)
AT	Attenuator or isolator
BR	Bridge rectifier (four diodes in a package)	often changed to "D" for diode
BT, BAT	Battery or battery holder	often shortened to "B"
C	Capacitor
CB	Circuit breaker
CN	Capacitor network	may be simplified to "C" for capacitor
D, CR	Diode (all types, including LED), thyristor	"D" is preferred for various types of diodes
DL	Delay line
DN	Diode network	may be simplified to "D" for diode
DS	Display, general light source, lamp, signal light
F	Fuse
FB	Ferrite bead	sometimes changed to "L" for inductor, though "E" was used in the currently inactive standard IEEE 315 (see Clause 22.4).
FD	Fiducial
FL	Filter
G, OSC	Generator or oscillator
GL	Graphical logo
GN	General network
H	Hardware, e.g., screws, nuts, washers, also used for drilled holes	sometimes hardware is expanded to "HW"
HY	Circulator or directional coupler
IR	Infrared diode	often changed to "D" for diode
J	Jack (least-movable connector of a connector pair), jack connector (connector may have "male" pin contacts and/or "female" socket contacts)	all types of connectors, including pin headers.
JP	Jumper (link)
K	Relay or contactor
L	Inductor or coil or ferrite bead
LD, LED	LED	often changed to "D" for diode
LS, SPK	Loudspeaker or buzzer
M	Motor
MK, MIC	Microphone
MP	Mechanical part (including screws and fasteners)
OP	Opto-isolator	often changed to "U" for IC
P	Plug (most-movable connector of a connector pair), plug connector (connector may have "male" pin contacts and/or "female" socket contacts)
PS	Power supply
Q	Transistor (all types)
R	Resistor
RN	Resistor network	sometimes simplified to "R" for resistor, or "N" for network
RT	Thermistor	sometimes simplified to "R" for resistor
RV	Varistor, variable resistor
S	Switch (all types, including buttons)	sometimes "SW" is erroneously used
SA	Spark arrester
T	Transistor	often changed to "Q", but sometimes "T" is used for bipolar transistors and "Q" for FETs.
TC	Thermocouple
TP	Test point
TR, T	Transformer	sometimes changed to "L" for inductor
TUN	Tuner
U, IC	Integrated circuit (IC)	shorter "U" (unit) is preferred instead of "IC"
V	Vacuum tube
VR	Voltage regulator (voltage reference), or variable resistor (potentiometer / trimmer / rheostat)	voltage regulators are often "U" for IC, pots and trimmers often "R" for resistor
X	Socket connector for another item not P or J, paired with the letter symbol for that item (XV for vacuum tube socket, XF for fuse holder, XA for printed circuit assembly connector, XU for integrated circuit connector, XDS for light socket, etc.)
X, XTAL, Y	Crystal, ceramic resonator, powered oscillator
ZD	Zener diode	often changed to "D" for diode

Other designators

AE: Aerial, antenna
ASSY: Separable assembly
B: Battery
CN: Connector
CRT: Cathode ray tube
DSP: Digital signal processor
FET: Field-effect transistor
GDT, SVP: Gas discharge tube, surge voltage protector
H: Pin header
J, JW: Wire link ("jumper")
JFET: Junction gate field-effect transistor
LA: Lightning arrester
LCD: Liquid crystal display
LDR: Light-dependent resistor
LED: Light-emitting diode
MCB: Miniature circuit breaker
MIC: Microphone
MOSFET: Metal-oxide-semiconductor field-effect transistor
MOV: Metal-oxide varistor
NE: Neon lamp
PCB: Printed circuit board
PLC: Programmable logic controller
PU: Pickup
RY, RLA: Relay
SCR: Silicon-controlled rectifier
SUS: Silicon unilateral switch
SW: Switch
TFT: Thin-film transistor (display)
TH: Thermistor
VC: Variable capacitor
VDR: Voltage-dependent resistor
VFD: Vacuum fluorescent display
VT: Voltage transformer
W: Wire

References

^ ^a ^b Reference Designations for Electrical and Electronics Parts and Equipment: ASME Y14.44-2008(R2014). Fairfield, New Jersey, USA: American Society of Mechanical Engineers. 2014 [2008]. (NB. Replaced IEEE 200-1975.)
^ ^a ^b ^c ^d "Chapter 22. Class Designation Letters". IEEE Standard 315-1975: Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters). New York, USA: IEEE / ANSI. 1993-12-02 [1975]. doi:10.1109/IEEESTD.1993.93397. ISBN 978-0-7381-0947-3. Retrieved 2024-07-16. (NB. Reaffirmed 1993.)
^ "Section 4.1.5.3 (2)". Standard Reference Designations for Electrical and Electronics Parts and Equipments: IEEE 200-1975. New York, USA: IEEE / ANSI. 1988 [1975]. (NB. Reaffirmed 1988.)