Ohms and resistance values
The ohm is the unit of resistance. Larger values are measured in kilo-ohms (1000 ohms) and mega-ohms (1,000,000 ohms). Resistors are marked, using a code specified in British Standard 1852, as follows:
- The letter R means ohm. Numbers coming before the R indicate a value more than one. So 1R (or 1R0) = 1 ohm; 47R = 47 ohms; and 4R7 = 4.7 ohms. Numbers coming after the R indicate a value less than one: so R56 = 0.56 ohms.
- The letter K means kilo-ohm. Numbers coming before the K indicate a value more than one, while numbers coming after the K indicate a value less than one. So 1K8 = 1.8 kilo-ohms and 5K6 = 5.6 kilo-ohms.
- The letter M means mega-ohm. Numbers coming before the M indicate a value of more than one, while numbers after the M indicate a value less than one. So 2M = 2 mega-ohms, and 2M2 = 2.2 mega-ohms.
Resistance value is shown by a series of coloured bands, read from left to right.
- The first band denotes tens, and the second band units. Each colour stands for a different unit: black is zero; brown is one; red is two; orange is three; yellow is four; green is five; blue is six; violet is seven; grey is eight; white is nine. So the sequence yellow through to violet denotes the value 47.
- The third band is the multiplier: black denotes a multiplier of one; brown 10; red 100; orange 1000 and so on. So the sequence yellow to violet to red denotes a value of 47 x 100, or 4.7 kilo-ohms.
- The fourth band is the tolerance. Manufacturers of resistors cannot guarantee the exact resistance figure shown by the first three bands, so they give a percentage value by which the resistance may be higher or lower than the resistance quoted. A red band denotes a tolerance of 2%; gold a tolerance of 5%; and silver a tolerance of 10%. So a 100 ohm resistor of 10% tolerance has an exact resistance value falling somewhere between 90 ohms and 110 ohms.