For Beginners: Prefixes and Codes

Prefixes (SI standard) Usage conventions

• In Telecommunications and Electronics use of the correct SI definition of the unit is standard.
• Standard industry practice in RAM and ROM manufacture has been to use the Mb abbreviation incorrectly  in reference to the binary interpretation of the megabit. For example, a single discrete DDR3 chip specified at 512 Mb invariably contains 2²⁹ bits = 536870912bits = 512 Mibit of storage, or 671088648-bit bytes, variously referred to as either 64 mebibytes or 64 (binary) megabytes.

This why there is a disparity between Disk Sizes, File Sizes, Broadband Speeds and RAM. The Computer industry often incorrectly uses 1024 = K instead of 1000 = K, as RAM/ROM addressing is based on powers of  2, i.e.  10 address lines = 2¹⁰ = 1,024 values or locations.

The International System of Units specifies twenty SI prefixes:

Unit prefixes in italics are not commonly used in Electronics, Broadcast or Telecommunications.

( or __) indicates obsolete usage on capacitors or schematics

 

Example

A One nano Farad capacitor might be marked or on schematic as

0.001μF = 1nf = 1μμF = 1000pF = 0.001mfd

Often u is used for μ

Modern thousand micro Farad capacitors (since late 1970s) might be

0.001 F = 1mF = 1000uF

Vintage equipment never used more than maybe 200uF so using mfd = μF wasn't very confusing!

10 Farad (yes 10,000,000μF) parts are now a stock item as are 0.5pF, aslo Capacitors are in voltage ratings from 3V (small "super caps" of 1 Farad) to 50,000V as common stock items.

 

Colour Codes.

Similar schemes used for Inductors and Capacitors

Markings on Surface mount  Last digit is Multiplier as above

100 = 10 Ohms

101 = 100 Ohms

1R8  = 1.8 Ohms

103  = 10K Ohms = 10,000

185 = 1M8 = 1,800,000 Ohms

m  = 1/1000^th

M = 1,000,0000

k or K = 1,000

 

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Surface-mount components have been mentioned, even though they generally are not encountered in antique radios.  They are, however, increasingly encountered in other electronics and present some unique challenges and, due to their small size and according difficulty in marking them in conventional ways, often use some unique codings.

Surface-mount ceramic capacitors may or may not be marked.  Where they are marked, they are often marked with an alphanumeric code consisting of one letter and one number.  The letter refers to a base value (all letters except I and O are used) corresponding to a standard E24 value between 1.0 and 9.1:

A = 1.0          N = 3.3

B = 1.1          P = 3.6

C = 1.2          Q = 3.9

D = 1.3          R = 4.3

E = 1.5          S = 4.7

F = 1.6          T = 5.1

G = 1.8         U = 5.6

H = 2.0         V = 6.2 

J = 2.2          W = 6.8

K = 2.4          X = 7.5

L = 2.7          Y = 8.2

M = 3.0         Z = 9.1

The number represents the multiplier, as a power of 10 (0=x10⁰ or 1, 1=x10¹ or 10, 2=x10² or 100, etc.).  Values are always expressed in pF on these.

Thus one might encounter SMT capacitors marked, for example:

A3 (1.0x10³) = 1000pF/1.0nF/.001µF

S1 (4.7x10¹) = 47pF

J5 (2.2x10⁵) = 220 000pF/220nF/.22µF

NB:  A similar code, but in µF instead of pF, is occasionally found on SMT tantalum electrolytic capacitors (example: S0 = 4.7µF).

An alphanumeric code is also used with 1% surface mount resistors where marking them with a four-digit code (three significant digits plus multiplier) is not possible due to size.  This code consists of two digits and one letter, with the two digits (01-96) representing a base value and the letter representing the multiplier.  The values correspond to standard E96 values from 100 to 976 (ex: 01=100; 96=976).  Letters A-F are used for multipliers of 1 or greater, and letters X,Y and Z represent multipliers less than 1 (X = x0.1, Y = x0.01, Z = x0.001).  (A more detailed explation of this can be found at radio-electronics.com. This article includes a table of the 96 values and a further explanation of the system as a whole.)  Thus, you might encounter an SMT resistor marked '01C'; this is 'base value 01' (100) x 'multiplier C' (10² or 100) = 10 000 (10k) ohms.

 

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An older resistor colour coding scheme common on vintage radios is known as 'body-tip-spot'.

body was the first digit, tip or end  second digit,  spot on the body  is multiplier.

The other end is tolerance, same as "body" is 20%. Silver is 10% and Gold is 5%

The example is Grey Red Orange and  Silver 

 8   2    000   10%

82K Ohms 10% tolerance

 

Other Resistors.

A single black band on what looks like a resistor means 0 Ohms, i.e. a wire link. At one stage of automation actual wire couldn't be automatically inserted, so Zero Ohm resistors were used.

If there are four bands close together then there are 3 digits before the multiplier. This is common on 2% or better tolerance resistors.

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There are two schemes of Inductors (coils) using the same colour codes as above.

The multiplier is usually µ (micro Henry = 0.000 0001 Henry)

Brown Green Orange

1st digit, 2nd digit and coloured spot is multiplier: Sample is 15,000µH or 15mH

The other side has a silver or gold spot for 10% or 5% tolerance.

The next part can be a Resistor, Inductor or Capacitor!

Red Red Brown

It measures a few ohms so it can't be a capacitor or a 220 Ohm resistor. It's thus a 220µH coil. The Milli-Henry coils can read a hundred Ohms if they are very small. Ceramic Capacitors do exist in exactly this package. The multiplier is then pico-Farads pF

Here is a "Tropical Fish" Capacitor:

Brown Black Yellow = 100 000pf = 100nF = 0.1µF.

These (and the single colour cousins with printed digits) are metalised Polyester Foil. The end wires have very poor adhesion and any tension or heat tends to crack the coating. This part is essentially scrap.

These two are single layer high voltage Ceramic capacitors

The first ought to be 47 00pF = 4.7nF and the second 10 000pF = 10nF

Some of these have a band for voltage rating spaced away from the others.

Some have tolerance and temperature coefficient bands.

Tolerance %
Black 20
Orange 30
Yellow 40
Green 5
 

The voltage (if marked) is one or two bands and is 100s of volts i.e. Red = 200V. If its 2000V then Red Brown

More Ceramic types

 

Silver Mica Capacitors

Silver Mica Button capacitor

Multiplier gives pico Farads pF

Silver Mica or Paper "Stamp" capacitors

The tricky aspect is that there are various kinds!

1) Three chevrons (arrow heads) on three dots

World War II   American War Standard (AWS) Mica or Joint Army Navy (JAN)  Paper

Type code Dot:

black or white  =  mica

silver or none  = paper

Black or body color is 20% tolerance. Silver is 10%. Colour Brown to White is 1% to 9%

 

2) Non-WWII (not AWS or JAN) Silver Mica

 

In practice only the oldest bare construction clamped Silver Mica ever need  replaced. In over 30 years I've only had to replace physically damaged Silver Mica capacitors.

 

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