Reference designator


A reference designator unambiguously identifies 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. R13, C1002. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, e.g. R17A, R17B.
IEEE 315 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 and IEEE 315-1975 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 defines separate class designation letters for separable assemblies and inseparable assemblies. Inseparable assemblies—i.e., "items which are ordinarily replaced as a single item of supply"—are typically treated as components in this referencing scheme.
Examples:
Especially valuable is the method of referencing and annotating cables plus their connectors within and outside assemblies.
Examples:
A cable connecting these two might be:
Connectors on this cable would be designated:
ASME Y14.44-2008 continues the convention of Plug P and Jack J when assigning references for electrical connectors in assemblies where a J is the more fixed and P 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 and IEEE-315-1975.

Designators

The table below lists designators commonly used, and does not necessarily comply with standards.
DesignatorComponent type
ASeparable assembly or sub-assembly
ATAttenuator or isolator
BRBridge rectifier
BTBattery
CCapacitor
CNCapacitor network
DDiode, thyristor
DLDelay line
DSDisplay
FFuse
FBFerrite bead
FDFiducial
FLFilter
GGenerator or oscillator
GNGeneral network
HHardware, e.g., screws, nuts, washers
HYCirculator or directional coupler
IRInfrared Diode
JJack, Jack connector
JPJumper
KRelay or contactor
LInductor or coil or ferrite bead
LSLoudspeaker or buzzer
MMotor
MKMicrophone
MPMechanical part
PPlug, Plug connector
PSPower supply
QTransistor
RResistor
RNResistor network
RTThermistor
RVVaristor, Variable resistor
SSwitch
TTransformer
TCThermocouple
TPTest point
TUNTuner
UIntegrated circuit
VVacuum tube
VRVoltage regulator, Variable resistor
XSocket connector for another item not P or J, paired with the letter symbol for that item
XTALCrystal
YCrystal or oscillator

Other designators