Resistor

Resistors are components that just about every electronic device uses. A resistor is a component that resists the flow of current. They do this by either extending the length of wire that the electricity must flow through or forcing the current to pass through a poor conductor, such as carbon.

It can protect components that have a specific current rating

It can change the function of the circuit

It can create a "dummy load" for a circuit in order for testing purposes

It can create by products such as heat that can be utilized for special purposes

The protection is necessary in order to prevent destruction of certain components that have a maximum current that can be passed through them. This is particularly true in components such as LEDs, which can suffer permanent damage and/or destruction if excessive current is passed through them.

The resistance can change the function of certain circuits, such as oscillators. Certain circuits use the level of current as a control for specific functions. For example, the 555 timer IC outputs pulsed electricity, the frequency of which is determined by the current level sent to one of its leads. This is very important.

The "dummy load" is useful when testing circuits in the lab since the actual load can be impractical for testing (such as a very large antenna). The resistor duplicates the resistance of the real load and makes the circuit act as though it is connected as it normally would.

Resistance creates heat losses in electrical circuits, but that is not always a bad thing. Most electrical heaters utilize this by running electricity through resistors with very low resistance, producing a lot of heat.

On a circuit diagram normally a resistor will have a letter after the value, for values less than 1,000, an ‘R’ is used. So 100 ohms will read 100R. From 1,000 ohms a ‘K’ is used and the number is divided by 1,000. So 1,000 ohms is read as 1K, 22,000 as 22K and 100,000 ohms as 100K. Lastly, from 1,000,000 ohms a ‘M’ is used and the number is divided by 1,000,000. So 1,000,000 ohms is 1M of course. Resistors are too small to have these numbers printed on them, instead they have coloured bands, which is explained further down.

Resistor values with a decimal point in circuit diagrams are expressed in 2 ways, say the circuit requires a 1.2k ohm resistor. The diagram might have it as 1.2K or it might appear as 1K2. The ‘K’ is put in place of the decimal point to prevent the value from being misread as 12K ohms. For resistors below 1k ohm an ‘R’ is used in place of the ‘K’. So 5.6 ohm resistor on the diagram would appear as 5.6R or 5R6.



The above is a picture of a 4-band 1/4w carbon film resistor. These are the most commonly used in electronic circuits due to their low cost and versatility. They come in 1/4w, 1/2w and 1w. You can tell the difference in power handling by the physical size of the package. A 1/4w resistor 7mm long by 2mm diameter, a 1/2w is about 9mm long by 3mm diameter and a 1w is 11mm long and 4mm in diameter. They usually have a tolerance of 5%.



These are 5-band metal film resistors, they have a much smaller tolerance than carbon film resistors, these have a tolerance of 1%. These are used where you need an exact value, such as a high quality audio preamplifier.



This is a ceramic wire wound resistor. They usually come with a power rating of 5w and 10w. These are used where a lot of power is going to be dissipated, such as that of a dummy load. They will be used in a high power audio amplifier.



This is wire wound nichrome wire. The purpose of nichrome wire is to produce heat and hence it’s used in electric heaters and stoves. Nichrome wire normally has a resistance of about 13.8 Ohms per metre. This coil came from a 2400w, 240v fan heater, it had 8 lengths just like the one pictured and they were used in series and parallel combinations to achieve low, medium and high power. The wire actually had a faint red glow on full power. Nichrome wire in heaters should be protected so stray hands don’t touch them, because that stray hand will get burnt.

Because carbon resistors are so small it’s impractical to print the resistance on it so instead they have 4 or 5 coloured bands. The number of bands relates to the tolerance of the resistor, the tolerance is how much variation there is likely to be. At 5% a 100 ohm resistor can be as low as 95 ohms or as high as 105 ohms. 4 bands are used when the tolerance is 5% or 10% and 5 bands are used when the tolerance is 1% or 2%. The 5th band is used to achieve more precision.



The 1st, 2nd (3rd) and multiplier bands are bunched together so you can see where to start from. This is helpful especially with 5-band resistors, which is harder to tell because the tolerance band is brown or red.