A Guide to Resistors

Resistors are a very large subject, so in this and in other coming articles we will discover all their secrets. Let's see their basic principles and their classification (according use and structure) to start with.

Resistor symbol

Suppose to have a fluid flowing inside a tube. If the tube section reduces or an obstruction occurs the fluid, in those points, will flow more slowly. Every time the fluid must face some resistances on its path, its speed will decrease, the same phenomenon happens in the electric circuits. Higher is the voltage applied to the conductor, higher is the current that flows through it. The current, anyway, will be reduced from the faced resistances. Materials, generally, own a peculiarity, that is they present a "resistance" when a current flows through them. This is the origin of the resistance term in electronics. So, basically, a resistor is a component that opposes to the electrons passing through it. It is a conductive material, because it does not prevent the electrons from passing, but it is not a very good conductor anyway. A resistor is defined by the electric resistance value (expressed in Ohm - Ω) and by the dissipated power (expressed in Watt - W) in security conditions, that is the produced heat must not produce its degradation. A conductor resistance is proportional to its length, but it decreases if its section increases and, moreover, it depends from the conductor material. A resistance causes, always, an energy loss in terms of heat generation: this energy transformation is known as Joule effect. A resistor obeys to the Ohm's law:

where V is the electric voltage expressed in volts (V), R is the resistance expressed in Ohm (Ω), I is the electric current expressed in ampere (A). So the dissipated power is equal to:

as a consequence of Ohm's law.

Other resistors characteristic parameters are:

• Nominal resistance: it is the resistance value given, from the manufacturer, to a defined resistor at 25 °C.
• Tolerance: it is the maximum deviation of the actual resistive value from the nominal value. It is generally expressed in percentage.
• Nominal power: it is the maximum power, expressed in watt, that the resistor can dissipate in an environment with the temperature lower than 75 °C. In the graph below we can see a resistor derating curve, also called power/temperature diagram.
Dissipable power (%nom.)

Temperature - graph courtesy of Itis Omar
• Limit voltage: it is the maximum voltage value recommended.
• Critical resistance: it is the maximum resistive value that can be used, at full power, without exceeding the limit voltage.
• Stability: it is the resistance value drift due to the ageing, measured, for example, after 1000 working hours at full power and at 70°C.
• Temperature coefficient: it is the variation of the resistive value due to the temperature and it is expressed in ppm/°C.
• Maximum working voltage: it is the maximum voltage that can be applied to the resistor in DC (Direct Current). It does not depend only from the resistor dissipated power, but from the dielectric stiffness of the used material also.
• Noise: it shows the voltage floatings, at the resistor ends, due to the Johnson effect. This phenomenon is caused from the electrons chaotic movement, due to the thermal agitation.
• High frequency parameters: at high frequencies the parasitic effects must be considered. The equivalent circuit consider an inductance in series to the resistance and a capacity in parallel to the RL series.

Actual resistor with parasitic elements
• The parasitic impedance values depend on the construction technique. Generally it is the parasitic inductance to create problems at high frequencies. At low frequencies the capacity and the inductance can be ignored because C acts as an open circuit (very high resistance) and L acts as a short circuit (L is practically zero). If the frequency is increased, the R value is decreased from C and increased from L until the couple LC resonates at the frequency:
  
  

Increasing the frequency still further, L becomes much bigger than R (the RL impedance increases) while C tends to short-circuit the signal. As result of the above explanation the resistance can be used in frequencies lower than the LC resonance frequency.

Classification according to the use
The resistors can be classified, according to the used area, in the following categories:

• Power resistors : they can absorb power in the range ~ 5W to several KW and they have natural or forced air or water cooling. The smallest ones, until several hundreds of watts, are used both in power circuits and in electronics.
• Measurement instruments resistors : resistors, as shunts, are used for DC ammeters connection. These high precision resistors, with four ends, are usually designed in order to create a 60 mV voltage fall when they are passed through the nominal current. Resistors, with high precision and stability, are used to increase the DC/AC voltmeters and watt-meters range.
• Electronic circuits resistors: These resistors are mainly realized for electronic circuits. They have small power and size. They are often used in selective circuits with voltages that can reach some hundreds of volts, but with currents over 100mA rarely.

Classification according to the structure
Resistors can be also classified according their structure and their resistance, fixed or variable.

Fixed resistors
The fixed resistors are widely used, their values are determined during the circuit design phase. Here following the main fixed resistors will be described.

• Reference resistors: An extremely accuracy must be paid during their construction because they are used as reference in the resistance measurement. They are very stable during the time passing and during the thermal excursions.
• Carbon composition resistors: Once they were largely used, but not nowadays. They are made of solid bars of conductive composite material. The carbon mixture can be varied to get the wanted resistivity. When the temperature goes up, due to the carbon particles being relaxed, the resistance goes down. The opposite situation happens when the temperature goes down (the resistance goes up due to the carbon particles being compressed). They have a voltage coefficient, a voltage rating and, due to their construction, they generate noise.

CCR: Carbon Composition Resistors
• Carbon film resistors: They are similar to the CCR. These resistors are made by coating ceramic bars with a mixture of carbon materials. Their TCR (Trickle Charge Resistor) is around 100 ÷ 200 ppm and is generally negative. Their frequency response is much better than CCR, but not as good as wirewound resistors.

Carbon film resistor
• Metal film resistors: They are made by an evaporating/deposition process and are a good compromise among all the resistor types. They have a very good accuracy, a lower temperature coefficient than the carbon film ones, do not have a voltage coefficient, a very low noise if properly manufactured, a TCR around 50 ÷ 100 ppm and very good frequency characteristics.

Metal film resistors
• Precision wirewound resistors: They are made by a high resistance wire wounded, usually, around a ceramic tube and connected to electrodes at each end. Due to the high cost, they are generally used in highly precise DC applications and designed not to carry power, but for maximum accuracy. Moreover they have a very good stability and reliability.

Precision wirewound resistor
• Power wirewound resistors: They are used when a lot of power must be managed. Some of them are without wires, like heater elements, but, in this case, they need some form of cooling to manage any appreciable power quantity. Other power wirewound resistors are designed to be mounted to metal plates or a chassis to improve the heat conduction, for this reason they are called " Chassis Mounted Resistors "

Chassis Mounted Resistors
• Fuse resistors: They are both resistors and fuses. If a large surge current occurs they will open. The fusing current is calculated taking account of the energy required to melt the resistive material.

Fuse resistor
• Foil resistors: They are similar to the metal film ones, but they have better stability and lower TCR. Moreover they have a very good frequency response.

Foil resistors

Resistors with variable resistance

They consist of a resistance track with terminals at both ends and a slider that moves along the track when the spindle is turned. They can be mainly divided in rheostats and potentiometers, but there are also the thermistors and the varistors.

• Rheostats: They are constituted by two connections, the slider and just one end of the track. This device allows to change the resistance, existing between the rheostat in/out terminals, continuously or jerkily. In the market there are principally rheostats with continuous or discontinuous variation.
• Potentiometers: These devices have three connections, the two terminals and the slider. The slider can be a rubbing contact or a brush allowing the resistance regulation between the slider itself and each end. The potentiometers are classified by the total resistance, the acceptable losses in watt and the resistance variation law. A potentiometer can be also with continuous or discontinuous variation.
• Thermistors: These devices are used for the temperature measuring and, for this reason, their temperature resistance variation is used. So they are similar to the thermoresistances, but the thermistors are composed from semiconductor materials, the thermoresistances are composed from metallic conductor materials. The conductor property is to increase the resistance if the temperature increases, the semiconductor property is opposed.
• Varistors: These electronic devices must protect the other electronic devices from over-voltage transitory phenomena. Their functioning is similar to a non linear resistor that, once the designed characteristic voltage has been overcome, decreases its resistance roughly, so the disturbance is strongly attenuated.

Other resistors types

• Thick film resistors: These resistors have a power in the range 15÷200W. They are not inductive resistances, ideal for high frequencies. The resistive values are in the range 0,001Ω to 10 KΩ , with tolerance between ±1 and 5%.

Thick film resistor
• SMD resistors: The SMD (Surface mounting device) are miniaturized resistors, with thick film, in 0603 - 0805 - 1206 standard industrial forms. The connection process of three layers, with nickel barriers, prevents the dissolution and assures a very good soldering process.

SMD resistor
• Custom resistors: If we need, for a particular application, a resistor with accurate size, value and precision and it does not exist in the market, the manufacturer itself can create this resistor giving a code to it. Therefore this is a custom resistor because it cannot be bought in any electronic shop.
• Magnetic resistor: In this resistor the resistance value can be controlled by a magnetic field. It can have sub-layers in ferrous materials and/or plastic material. The ferrous sub-layers are in ferro-magnetic material which has a big permeability. The plastic sub-layers are in indium antimonyurum. The resistance values depend on the size and construction and can vary from a few Ω to several KΩ.

See all the Resistors from Farnell.

Read also:
http://dev.emcelettronica.com/base-electronics-resistors-application-and...
http://dev.emcelettronica.com/base-electronics-resistors-configuration