RESISTOR

RESISTOR

The first and most common electronic component is the resistor.  There is virtually no working circuit I know of that doesn't use them, and a small number of practical circuits can be built using nothing else.  There are three main parameters for resistors, but only two of them are normally needed, especially for solid state electronics.

• Resistance - the value of resistance, measured in Ohms.  This is the primary parameter, and determines the current flow for any applied voltage. 
• Power - The amount of power the resistor can handle safely.  Large resistors (physically) generally have a higher power rating than small ones, and this is always specified by the manufacturer.  Excess power will cause the resistor to overheat and fail, often in a spectacular manner.
• Voltage - Rarely specified, but this is the maximum voltage that may appear across a resistor.  It has nothing to do with power rating, which may be exceeded at rated voltage.  It is a measure of the maximum voltage that may appear across any value of resistance for this style without breakdown.

The resistance value is specified in ohms, the standard symbol is "R" or Ω.  Resistor values are  often stated as "k" (kilo, or times 1,000) or "M", (meg, or times 1,000,000) for convenience.  There are a few conventions that are followed, and these can cause problems for the beginner.  To explain - a resistor has a value of 2,200 Ohms.  This may be shown as any of these ...

• 2,200 Ohms
• 2,200 Ω
• 2,200R
• 2.2k
• 2.2k Ω
• 2k2

The use of the symbol for Ohms (Omega, Ω is optional, and is most commonly left off, since it is irksome to add from most keyboards.  The letter "R" and the "2k2" conventions are European, and not commonly seen in the US and other backward countries :-)  Other variants are 0R1, for example, which means 0.1 Ohm

The schematic symbols for resistors are either of those shown below.  I use the Euro version of the symbol exclusively. 

Figure 1.1 - Resistor Symbols

The basic formula for resistance is Ohm's law, which states that ...

1.1.1   R = V / I   Where V is voltage, I is current, and R is resistance

The other formula you need with resistance is Power (P)

1.1.2   P = V² / R
1.1.3   P = I² * R

The easiest way to transpose any formula is what I call the "Transposition Triangle" - which can (and will) be applied to other formulae.  The resistance and power forms are shown below - just cover the value you want, and the correct formula is shown.  In case anyone ever wondered why they had to do algebra at school, now you know - it is primarily for the manipulation of a formula - they just don't teach the simple ways.  A blank between two values means they are multiplied, and the line means divide. 

Figure 1.2 - Transposition Triangles for Resistance

Needless to say, if the value you want is squared, then you need to take the square root to get the actual value.  For example, you have a 100 Ohm, 5W resistor, and want to know the maximum voltage that can be applied.  V² = P * R = 500, and the square root of 500 is 22.36, or 22V.  This is the maximum voltage across the resistor to remain within its power rating. 

Resistors have the same value for AC and DC - they are not frequency dependent within the normal audio range.  Even at radio frequencies, they will tend to provide the same value, but at very high frequencies other effects become influential.  These characteristics will not be covered, as they are outside the scope of this article. 

A useful thing to remember for a quick calculation is that 1V across a 1k resistor will have 1mA of current flow - therefore 10V across 1k will be 10mA (etc.).