When in 1827 the German physicist Georg Ohm published his treatise The Galvanic Circuit investigated mathematically he could not possibly have imagined how resistor technology would evolve from the pieces of wire he used in his experiments to the huge range of devices available today. While this is good news for the designer it can be rather confusing for the purchasing professional especially when trying to source alternative devices. Paul Caston, Rhopoint Components, explains the terminology and the various associated pitfalls when specifying resistors. Resistance Value
The first thing that you need to know is the resistance value which is usually expressed in Ohms represented by the symbol which is sometimes replaced by the letter R.
However, even this most fundamental specification can cause problems: 1000 ohms is usually abbreviated to 1Kohms, K or KR, so 3600 ohms could be written 3K6, 3.6K, 3.6K, 3.6KR, or 3.6Kohms where the K means one thousand.
When we get to higher and lower resistance values the confusion can become even greater. For one million ohms the letter changes from K to M and for one thousandth of an ohm the letter changes to m.
1ohm, 1micro-ohm, 1, 1R = 1 millionth of an ohm (micro-ohm)
1mohm, 1milli-ohm, 1m , 1mR = 1 thousandth of an ohm (milli-ohm)
1 ohm, 1, 1R = 1 ohm
1K ohm, 1K, 1K, 1KR = 1 thousand ohms (kilo-ohm)
1Mohm, 1M, 1M, 1MR = 1 million ohms (meg-ohm)
1Tohm , 1T, 1T, 1TR = 1 million million ohms (Tera-ohm)
Tolerance
This is the variation of the resistance value that takes place during manufacture. This is expressed as a percentage: For example 100 ohms 1%. Typical tolerances for most resistors range from 0.5 to 10%, although some precision resistors can go down to 0.005%.
Many manufacturers indicate the tolerance with a letter which can change from manufacturer to manufacturer.
Lies, damn lies and specifications
The way of determining a resistor specification is not always the same from manufacturer to manufacturer. When specifying a resistor, especially when sourcing an alternative, there are a few pitfalls.
Power
This is the amount of power the resistor will handle. This is expressed in Watts, W. Smaller resistors may have their power rating expressed in milli-watts, mW (thousandths of a watt) much larger devices in KW (thousands of Watts).
Note: For certain devices, especially power resistors, the power specified is very much dependant on the ambient temperature. A device that is specified to handle a maximum of 25W at 25C ambient is not equivalent to one able to handle 25W at 100C ambient.
More and more resistors are now available in surface mount packages. Here again a little note of caution is needed: Some manufacturers quote a power figure which is only valid with a given PCB layout or material.
Temperature Coefficient (TC)
This is the variation of the resistance value that takes place with variation of temperature. This is usually expressed in parts per million per degree (ppm/C however the /C is often left out). In many applications the TC does not matter but in precision applications it can be critical.
Example: 100 ohms 0.1% 15ppm
(1ppm = 0.0001% to convert between the two multiply the percentage by 10,000)
Note: The TC is also subject to some interesting specification writing. Many manufacturers quote a TC figure over a limited temperature range. There are some good reasons for this. The temperature coefficient of many resistance materials is a curve which means that the temperature coefficient is different at different temperatures and it would be very confusing to specify an equation for the temperature coefficient rather than a single figure. On some resistor data sheets the temperature coefficient is shown as a typical figure and the maximum figure, invariably worse, is somewhere else.
Long term stability
This is the variation of resistance with time, expressed in parts per million per time. In many applications the long term stability of a resistor does not matter. The long term stability of a resistor is affected by the load, the time over which the measurement is made and environmental aspects such as temperature. Long term stability can also be known as shelf life (no load) and load life.
Example: 100ppm/10,000 hours
Note: There is no standard way of expressing long term stability so manufacturers tend to use their own method of measurement, which makes comparison difficult. Long term stability is frequently quoted as a typical figure.
Example: 100ppm/1000 hours is not as good as 100ppm/10,000 hours
In conclusion there are a few pitfalls associated with sourcing resistors. If in doubt a specialist distributor such as Rhopoint Components should be able to point you in the right direction.