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An introduction to resistors

Main resistor properties and characteristics

Glossary of technical terms

An introduction to resistors

  1. What is resistance?
  2. What are resistors?
  3. What is Ohm's Law?
  4. What are the major design considerations for using resistors?
  1. What is resistance?
    Resistance is one of three basic quantities in electrical or electronic circuits:
    Current is the flow of electrons through a circuit. It is the principle quantity because it does work, and accomplishes the desired results. We measure current in Amperes.

    Voltage is the force that causes current to flow in a circuit. In fact, we sometimes call voltage "electromotive force" or "EMF." We measure it in Volts.

    Resistance controls the flow of current. We measure it in Ohms.

    These three quantities are so basic to electrical and electronic circuits, a simple equation called Ohms Law relates them.

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  2. What are resistors?
    Resistors are electronic devices that resist the flow of electrical current. A resistor obeys Ohm's Law, which states that the voltage (or potential) across a resistor is proportional to the current flow through the resistor. In formula form, V=IR, where V is the voltage across the resistor, R is the value of the resistor in ohms, and I is the current flow through the resistor in amperes (amps).

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  3. What is Ohm's Law?
    Ohm's Law states that the current flowing in a circuit is proportional to the voltage applied to the circuit, and inversely proportional to the resistance of the circuit. In other words, for a given voltage, the current in the circuit will decrease as the resistance increases. Mathematically, Ohm's Law is: I = V/R, or current equals voltage divided by resistance. This simple equation can be transformed to find voltage or resistance, given the other two quantities. (For example, if you know the voltage and current in a circuit, you can calculate the circuit resistance by dividing the voltage by the current.)

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  4. What are the major design considerations for using resistors?
    The designer / salesman must take into account the following parameters;
    1. Circuit minimum and maximum operational characteristics i.e. Power, voltage, current, other component values, duration of pulses (if any) and frequency.
    2. Environment into which the parts are to be fitted. Temperature, humidity, mounting type and relation to other heat sensitive/generating parts.
    3. Short term / long term requirement in terms of electrical performance. Stability / life etc.
    4. Design criteria, safety specifications and the level to which the part should be approval tested.
    5. Cost band of resistor versus other alternatives if there are any.
    6. Connection and terminal configuration.
    A decision made against each of the above questions will reduce the available product list until the most suitable is left. Unfortunately, a recommendation often has to be made balancing a number of undesirable parameters. This is where good product knowledge will enable the best practical decision for the Customer. To specification and budget.

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Main resistor properties and characteristics

  1. Resistance Tolerance
  2. Temperature Coefficient of Resistance (TCR)
  3. Power Rating
  4. Temperature Rating
  5. Frequency Response and Rise Time
  6. Aryton-Perry Windings
  7. Stability
  8. Voltage Coefficient
  9. Noise
  10. Thermocouple Effect
  11. Reliability
  1. Resistance Tolerance
    Resistor tolerance is the deviation from the nominal value in any production run. It is expressed as a ±%, measured at 25°C with no load applied. Some resistor designs have extremely tight tolerances. For example, precision wirewound resistors are made with tolerances as close as ±0.05%. Film resistors typically have tolerances of ±1% to ±5%, however, very close tolerance High Voltage resistors are available for special applications. In applications like precision voltage dividers and networks, the designer should consider resistor sets matched for resistance or ratio tolerances. Often, these matched sets save cost over buying individual resistors with very tight resistance tolerances.

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  2. Temperature Coefficient of Resistance (TCR)
    Temperature Coefficient of Resistance (TCR) specifies the maximum change in resistance in response to change in temperature, it is expressed as “parts per million per degree Centigrade” (ppm/°C). A wide range of TCRs are available to the designer (typically from ±5 ppm/°C to ±6700 ppm/°C in very low values) for specific applications.

    Specifying TCR is important in applications where the change in resistance with temperature changes must be small. Equally important may be applications where a specific TCR is required (temperature compensation circuits for example). Typically, there are two contributors to temperature-related resistance changes; the resistor’s temperature increases as it dissipates power and also, the resistor’s temperature is affected by the ambient temperature.

    Often matching TCRs for pairs or sets of resistors is more important than the actual TCR itself. In these cases, matched sets are available which assure that resistance values of the set track in the same magnitude and direction as operating temperature changes.

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  3. Power Rating
    Power ratings are normally specified the maximum dissipation rated for a device under reference conditions, normally +25°C. this will be derated as ambient temperature increases, a graph or curve is often used to represent this relationship. Since these parameters are application dependent, power derating curves should be considered general rather than absolute. Power ratings are based on many factors. The safest designs use the largest physical size operating at conservative temperatures and power ratings.

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  4. Temperature Rating
    Temperature rating is usually the maximum internal operating temperature of the resistor. An operating temperature range is often specified: for example, -55°C to +200°C. Internal resistance figure are available for calculation. However, it is recommended that reasonable safety margins are applied to maximise reliability and stability in application.

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  5. Frequency Response and Rise Time
    Frequency response relates to the change in impedance with frequency, caused by reactive components from the resistors inductance and capacitance. Rise time is an associated parameter, relating the resistors response to a step or pulse input. Wirewound designs use special winding techniques to minimize reactive components. Typical reactive values for these special designs are less than 1µh for a 500 ohm resistor, and less than 0.8 pf capacitance for a 1 megohm resistor. A typical fast rise time resistor has a rise time of 20 nsec or less.

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  6. Aryton-Perry Windings
    In Aryton-Perry windings, a layer of wire turns is first wound in one direction. After a layer of insulation is applied the next winding is wound in the opposite direction with the turns crossing every 180 degrees over the turns in the lower layer. The net effect of this is that the magnetic forces that are responsible for creating ‘reactance’ are equal and opposite – thereby cancelling each other.

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  7. Stability
    Stability is defined as the repeatability of resistance of a resistor when measured at a referenced temperature and subjected to a variety of operating and environmental conditions over time. Stability is difficult to specify and measure since it is application dependent. Experience with practical circuits has given us some guidelines; Wirewound and bulk metal designs are best, while designs using composition materials are less stable. For highest resistance stability, it is best to derate critical resistors to generate limited temperature rise.

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  8. Voltage Coefficient
    Voltage coefficient is the change in resistance with applied voltage. It is normally associated with carbon composition and carbon film resistors, and is a function of the resistor’s value and its composition. In terms of the our High Voltage thick film range of devices, whilst not of the above technologies, the voltage range is so great (up to 150kV), this should be considered as a design issue.

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  9. Noise
    Noise does not effect the resistor's value, but can generate circuit errors in high gain and sensitive circuits. Wirewound and metal film resistors are best: carbon composition and film have high noise potential. Consideration should be given in applications where there is an EMC (Electro-magnetic Compatibility) consideration.

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  10. Thermocouple Effect
    The thermocouple effect generates a thermal emf at the junction of two dissimilar metals. In resistors, it is caused by the materials used in leads and the resistive element. It is normally insignificant, but may be important in high gain or critically balanced circuits and low value resistors. Thermal EMF is minimised by keeping the resistor leads and body at the same temperature.

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  11. Reliability
    Reliability is the statistical probability that a resistor will perform its function. Normally, it is specified as Failure Rate per 1,000 Hours of Operation. Various statistical studies are used at arriving at these failure rates by testing large samples. Reliability is seldom defined for commercial products, but is a common requirement for critical designs such as in aerospace applications.

    Another measurement for reliability is MTBF or (Mean Time Between Failures) with estimated Service life. If you would like to discuss the implications of these measures and how they will affect your application please contact us.

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Glossary of technical terms

  1. Critical Resistance
  2. Rated Voltage
  3. Critical Voltage
  4. Rated Dissipation (Commercial Disspation Rating)
  5. Limiting Element Voltage
  6. Isolation Voltage
  7. Voltage Proof Test (Standard)
  8. Voltage Proof Test (Special)
  9. Partial Discharge Test (Special only)
  10. Insulation Resistance
  11. Derating Curve
  1. Critical Resistance
    The Critical Resistance is that resistance value at which the Rated Voltage is equal to the Limiting Element Voltage.

    At an ambient temperature of 25°C, the maximum voltage which may be applied across the termination’s of a resistor, is either the rated voltage, if the resistance is less than the critical resistance, or the limiting element voltage if the resistance is equal to or greater than the critical resistance.

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  2. Rated Voltage
    The DC or AC rms voltage calculated from the square root of the product of unit resistance and the rated dissipation (commercial dissipation rating). This is dependent on the Limiting Element Voltage & Critical Resistance.

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  3. Critical Voltage
    Please see Technical Information Sheet No.002. (Critical Resistance / Voltage Values).

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  4. Rated Dissipation (Commercial Disspation Rating)
    The Rated Dissipation of a resistor is the maximum allowable dissipation at an ambient temperature of 25°C when mounted in accordance with recommended conditions.

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  5. Limiting Element Voltage
    The Limiting Element Voltage is the maximum value of the voltage that may be applied continuously to the terminations of the resistor. (Limiting Element Voltage may become critical dependent on the value of the resistance).

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  6. Isolation Voltage
    The Isolation Voltage is the maximum peak voltage which may be applied under continuous operating conditions between any of the resistor termination's and any conducting mounting surface. The value of the Isolation Voltage shall be not less than 1.42 times the Limiting Element Voltage.

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  7. Voltage Proof Test (Standard)
    An alternating voltage of 40-60 Hz and with a peak value of not less than 1.42 times the Isolation Voltage shall be applied instantaneously between the termination's and the case of the resistor with no breakdown or flashover allowable.

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  8. Voltage Proof Test (Special)
    As for standard, in addition the voltage shall be applied for a period of 1 minute. The voltage shall be applied gradually at a rate of 100 Volts / Second with no breakdown or flashover allowable.

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  9. Partial Discharge Test (Special only)
    Standard test applied to 'High Voltage' resistors upon special request. An alternating voltage of 40-60 Hz applied for a duration of 1 minute between the terminations of the resistor and a conductive mounting plate.

    Maximum allowable discharge:- (a) 40 pico-Coulombs @ 2KV rms (b) 400 pico-Coulombs @ 2.8KV rms

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  10. Insulation Resistance
    500V DC applied to the terminations of the resistor connected together as one and a conductive mounting plate, duration is for sufficient period to achieve a stable reading. Standard for the HS resistor range:- Not less than 10,000 MOhms.

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  11. Derating Curve
    The Derating Curve shows the maximum allowable dissipation of a Heatsink Mounted resistor at ambient temperatures between 25°-200°C.

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