Resistors and Resistance
Eric Edwards GW8LJJ goes back to basics with resistors and their uses.
There are many types of resistors that will affect the flow of current through a circuit. They can be used in controlling the speed of a motor, the pitch of a musical tone and the amplification of a signal. They can also be used for dividing voltages when some components in part of a circuit need to work at a lower voltage than the supplied input voltage by connecting resistors in series and tapping off at the junctions. As resistors can get hot because an electric current is passing through them, an advantage can be taken of this by using them in appliances that need heat. They are used in room heaters, toasters, electric cookers and many more heating appliances. Another well-known component is an incandescent light bulb. The metal filament glows white-hot due to the very high temperature produced from the resistance when current is passed through it. Resistors are used in series with LEDs and in transistor circuits for base bias, emitter and collector circuits. And a use for a large resistor close to home for radio amateurs is for a transmitter dummy load.
Resistor values range from a fraction of an ohm to tens of millions of ohms (megohms) and resistors are made from a variety of substances, including carbon, or more correctly a carbon composition, metal oxide and metal film. Higher wattage resistors are generally made from wire wound on a ceramic former.
The wire-wound resistors are inductive as they are, in effect, coils. The wattage rating is the amount of power that the resistor can handle without burning up! A one-watt resistor will be required to allow a current flow of 0.08 amp (near approximate) with a potential difference of 12 volts. What that means is, if a resistor of 150 ohms is used to drop 12 volts for a supply requirement of say 6 volts from a supply of 18 volts then the wattage that the resistor is expected to handle is found from the equation: 12 (volts) divided by 150 (ohms) which is 0.08 (amps). To convert this to wattage we must multiply 12 (the voltage) by 0.08 (the current worked out by the equation), which is 0.96 (Watt) almost 1W. If a ¼ Watt resistor were used in this location, it would burn up, reinforcing that resistor wattage is as important as resistor values.
Always replace faulty components with the same overall value (resistance and wattage) to maintain reliability.
Types of Resistors
There are two main types of resistors, linear and non-linear. Fixed (resistance value) resistors have a specific value and these values cannot be changed. There are several types of these as mentioned above, along with surface-mount types. The values of non-linear resistors change according to the temperature and voltage applied. Types of non-linear resistors include thermistors, varistors and light-dependent photo types.
Variable Resistors
Variable resistors do not have a specific fixed value and can be changed with the help of a control knob. These resistors find applications in radio receivers for controlling volume and tone. Rheostats are large power types for adjusting voltage or current usually in high power test circuits. Trimmers are those found in circuits we are more familiar with and are used to set a desired voltage or current in low power circuits.
Variable resistors fall into two groups, linear and logarithmic. Looking through component catalogues you will see values such as 10k lin. and 10k log. This, although it may be confusing to start with, will soon be understood when you realise that our own hearing is logarithmic (cramped on one end). If someone was to increase the volume of sound in a linear manner, it would not be detected in the same way in our ears as we would hear no difference at the start, then it would get louder but not at the same rate as the actual rising level. Sound level on an audio amplifier must be increased non-linearly (logarithmically) to correspond to our hearing, giving the impression of a smooth increase in volume. In contrast, our sight is linear. If a light bulb is connected to a variable power supply, our perception of brightness would match the increase in voltage. What all that means is, for audio a logarithmic (log) pot (potentiometer) is used and for video, voltage or current increase, a linear (lin) pot would fit the bill.
Resistor Colour Coding
Resistors are colour coded to identify their resistance and tolerance and reliability. Not all resistors will have this full information but they will all have the resistor value in resistance displayed as coloured bands. The reason for using coloured bands as opposed to the resistance value printed on the body in digits (numbers) is because the numbers can be erased by overheating of the resistor. The individual colours are painted all around the body so if any part has been obliterated the colours can still be seen somewhere on the body. The older
high wattage carbon types used a coding that was referred to as ‘body, tip, spot’. The body of the resistor was in one colour, one end of the resistor had another colour and there was a coloured ‘spot’ on the centre of the body, Fig. 1. The body represents the first significant figure (resistance value) and the tip, the second significant figure with the spot as the number of noughts (zeros), also called multipliers. As an example, the large resistor shown at the back in the photo has a brown body, green tip and a yellow spot. This shows the value as 150,000. The ‘Brown’ is the value one and is the first significant figure, ‘Green’ is represented as five and is the second significant figure and the ‘Spot’ is the multiplier (number of noughts), which is four. The resistor value as shown by the colours, is ONE, FIVE and FOUR zeros which is 150000Ω = 150kΩ. It is not often you will come across these resistors unless restoring vintage domestic electronic equipment. Most wire-wound resistors are as shown in Fig. 2. Modern resistors have coloured bands that are printed around the body and can be three, four, five or even more bands. There is an exception where the component is a surface-mount type. These use numbers printed on their body although there are some tubular (MELF) types that have the coloured bands. Component suppliers will show a complete range of resistors and provide the necessary information about them.
The Colour Code
There are many ways of learning the code and it’s not only resistors that use these because the code is universal. Brown will always mean ONE whether it’s for resistors, capacitors, coils or wire looms, so it is worth learning this code. There are only ten numbers (well, nine plus zero, which is coloured black). It is better to learn the colours as you see them rather than use any rhyming or mnemonics. How did you learn the days of the week or names of the months? It was simply by remembering and using them and no other names or colours were substituted for them. Learn the colours as you see then and soon they will become second nature and there will be no need to think about each individual colour and as soon as you see, say, yellow on a resistor, capacitor or any other component or wire you will know it as the number four. You have already learned that and you have not even started yet! The colours are as shown in Table 1. It will not take long before you will be able to look at a 22kΩ resistor (RED, RED, ORANGE) and know its resistance value without thinking about the colours. A single black band on a resistor designates a zero-ohm resistor and is a wire link used to connect tracks on a printed circuit board (PCB). It has the same format as a resistor and looks neater than a wire link.
Tolerance and Reliability
All resistors and indeed other components have a tolerance. In other words, the value marked (or coloured) on the component will not always be an exact value. A 10kΩ (BROWN, BLACK, ORANGE) resistor may have a value as low as 9.5kΩ or as high as 10.5kΩ. This is known as a ±5% resistor. The black ring can be used as a multiplier or as a decimal. If the colours are BLACK, BROWN, BLACK, the resistor value is 010Ω which is 0.1Ω. The zero before the one represents a decimal point and the zero after the one represents no multiplier or noughts. Better or close tolerance resistors are 1% or even 0.1% and it will depend how important it is to have the actual value (or very close) in the circuit. For amateur radio use, 5% is acceptable although as the cost of 1% is now affordable it can be useful to stock and use those.
The tolerance or % difference is also indicated with some of the same colours. Brown as we know is one so that will be 1%, Red is two and is 2%. Other colours used are GOLD for 5% and SILVER for 10%. These are four band resistors with the first three used for the resistance and the fourth for the tolerance. With some high voltage resistors, to prevent contaminations getting in the coating, the gold and silver bands are often replaced with yellow and grey bands. Resistors produced with military specifications will often include a band that indicates reliability and is specifically for failure rate.
More Bands
Early resistors were three colour band types but then came along four, five and six bands. The four band has the first three as the resistance value with the fourth band as the tolerance. The five band type has a third significant resistance value with the fourth being the multiplier and the fifth as the tolerance. The six band type has the first three as
the resistance values, the fourth as the multiplier, with the fifth as the tolerance and a sixth band indicating the temperature coefficient. The number of bands and hence the range of resistor values are allocated as ‘E’ types with E3 being the most limited in resistance values. These have values that will be used with circuits not requiring critical values of close tolerance. The E3, as the term suggests, have three values and they are 1.0, 2.2 and 4.7. These are the values used but the range is not just Ohms but can be multiples of the base numbers. 1.0 can be 1Ω, 10Ω, 100Ω, 1000Ω etc and 2.2Ω can also have the multiplier to provide 22Ω, 220Ω etc. So, it still has a useful range of values.
The next range is E6 and again the term suggests the number of base values which are 1.0, 1.5, 2.2, 3.3, 4.7 and 6.6. As with the E3 series the multipliers extend the resistors values. These resistors are =/-1% and =/-5% types. Others such as E12 are close % tolerances and have resistor base values slotted in between the ones of the E6 types. Yet more are E24 and E96, which I tend to stock as they have come down in price and are readily available. There is E192, which has a very large range as will be appreciated, and with a 0.5% tolerance. There are many places to look online along with component suppliers to see the full range of resistor values and tolerances. Another important point about choosing resistors is the voltage it will be used with.
Resistors in Series and Parallel
Resistors are said to be in series when the current flowing through all the resistors is the same. These resistors are connected from head to tail in series, Fig. 3. The overall resistance of the circuit is equal to the sum of individual resistance values. If we were to connect three 100Ω resistors in series the total resistance will be 300Ω. The total resistor value in a series arrangement is always greater than the largest resistor.
If we were to put them in parallel (resistors are said to be in parallel when the terminals of resistors are connected to the same two nodes) they share the same voltage at their terminals and the final resistance value will be very much lower,
Fig. 4.
The total resistance in a parallel arrangement is always smaller than the smallest resistor. If the resistor values were 10 ohms in parallel with 15 ohms and another 20 ohms in parallel, the total value will be less than 10 ohms (the smallest resistor). The formula,
Fig. 5, is 1 divided by the result of 1 divided by 10 and 1 divided by 15 along with 1 divided by 20 which is 0.1 + 0.066 + 0.05 = 0.216 divided into 1 = 4.63 ohms to the nearest decimal place. If each of the three resistors had been the same value, the total resistance value could have easily been found by simply dividing the number of resistors into one value. If the three resistors are 15 ohms each, then by dividing 15 (one resistor value) by 3 (the number of resistors) the total resistance is 5 ohms. This only applies for a network when all resistors are the same value and works for any number of resistors.
IncreasedWattage
One advantage of this method of using many same value resistors is to replace a single lower value wattage one when a higher wattage resistor is required and is not available or is expensive as a single resistor. If three one-Watt resistors are used in the above example, the total wattage capability of the total resistance is 3 Watts. This is true because the current flow is shared equally among the three resistors. If the three resistors in parallel are 150 ohms each and the voltage across the circuit (Potential Difference) is 12 Volts, the current through the combined resistance is 12 divided by (150 x 3) = 0.24 Amps (240mA). The total wattage dissipation is 12 x 0.24 = 2.88 watts, but the wattage dissipated through each individual resistor is calculated by multiplying the current (12 x 150 = 0.08 amp.) by the potential difference (PD) 12 which is 0.96 Watt, a third of the combined total wattage of 2.88 W. Although the total wattage dissipation is of the whole resistance circuit, each resistor only dissipated a third of the power. This is only true, however, for resistors of equal value. Having the same value resistors to increase the power dissipation is used by radio amateurs when making a dummy load for measuring or setting up their transmitters. Using ten 10-watt resistors in parallel of 500 ohms each will allow testing the output of the transmitter at 50 Ohms impedance to a power dissipation of 100 watts. The only other way is to obtain a 100 watt 50 ohm resistor! A dummy load is a non-radiating substitute for the transmitting antenna, or it should be, and is composed of carbon, noninductive resistors placed in an RF-sealed metal box, which may be filled with oil (to increase the power rating even further by cooling).
In other parallel resistor circuits where all the resistors are not of the same value the wattage dissipated by each resistor will be determined by its own resistor value. Placing equal value resistors in series also increases the overall wattage rating.
However, this is only advisable if the resistors are exactly the same value because each resistor will pass the same current but will have different voltages across their terminals. Let’s take an example. Three resistors of 10Ω are placed in series so the total resistance is 30Ω. If we placed 24V across this total resistance, the current through all three is equal and is 0.8A. (Remember Ohm’s law?). The total power dissipated in heat is (24 × 0.8) 19.2W. The voltage across each of the 10Ω resistors is 8V and as the current is the same though all three resistors the power dissipation of a 10Ω resistor (8 × 0.8) is 6.4W. This, in theory, means that three lower wattage resistors can be used to replace one larger wattage resistor. However, it is not good practice (for safety reasons) but it is explained here to complete the theory of resistors in series and parallel. Remember when replacing a component in a circuit, the correct replacement type must be used.
Other Arrangements
Where several of the same value resistors are used in a circuit, sometimes resistor networks are used. These multiple resistor networks can be used for pull-ups where it is an advantage to use them on gates of integrated circuits (IC) to hold them at a slightly positive potential until a signal comes along to reduce them to a low potential. This in digital terms is, the IC gate is held at logic one (high) until it is required to go to logic zero (low). The advantage of having these SILs (single-in-line) resistor networks is the saving of space in a complex printed circuit layout where size is all-important. Other resistor packages are In-Line Isolated resistors where there are four independent resistors in an 8-pin package, for example, similar to the SILs, and there are also DualIn-Line Isolated resistors that are packaged in a 16-pin arrangement, two sets of 8 pins opposite each other as a standard 16-pin integrated circuit configuration. These resistors are manufactured from a thick film material and consequently can be made very small (thin).
My thanks to Ray G7BHQ (retired college lecturer) for checking and advising on some of the points made.