About resistors

PST Technology Co., Ltd is specialized in METAL ALLOY HIGH POWER LOW OHMIC CURRENT SENSING RESISTORs (0~1 Ω) in Taiwan and the factory with ISO14001, ISO9001, OHSAS18001 and ISO/TS16949 certificated. PST’s current sensing resistors are specifically in low resistance 0~1Ω with metal alloy materials (no ceramics) and particularly with high rating power. PST resistors are SMD resistors (fixed resistors) and compact in chip sizes. 
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Here is brief intro about resistors in general. 
　

Functions
There are various functions of resistors applied in different fields according to designs and types. The functions of resistors in limiting current and voltage in a circuit are according to Ohm's law. Its functions are as follows: 

(1) Current and voltage division
(2) Current limiting
(3) Signal reading
(4) Current sensing
(5) Dummy load
(6) Loading test
(7) Resistance matching
(8) Signal switch
(9) Signal comparison

Resistors are selected in concerns of features, power…etc. and of application, working conditions…etc. 


Resistors’ main applications:
Resistors are often found in electronic circuits as an important electrical element; therefore, is widely applied in various applications. Controlling the proportion of voltage and current in the certain section of the circuit: 
If voltage in a sectional circuit is fixed, it is capable for resistor to produce a constant current. If the sectional current is fixed, the resistor can provide fixed voltage.
a. Distributing the voltage ratio of different parts of the circuit
b. Limiting the current flowing through a part of the circuit
c. Heat releasing: Hotline is produced based on this feature of resistor.


Resistance unit
Resistance unit: mΩ, Ω (1Ω=1000mΩ), KΩ, MΩ (1MΩ=1000KΩ=1000000Ω) 
Power: WATT (W), e.g. 1/8W、1W、50W
Unit Conversion：1MΩ=1,000KΩ=1,000,000Ω

Nominal resistance of resistor: resistance labeled on a resistor is called the nominal value. The nominal value is defined by the state in accordance with the standard series marked but not by producers. Not all resistance value is available. Common resistors are E24 series (± 5%) and E96 series (± 1%). Acceptable tolerance is maximum permissible deviation of the actual resistance of resistors to the nominal value. The tolerance is coded B, D, F, G, J, ... (common tolerance is: 0.1%, 0.5%, 0.25%, 1%, 2%, 5%, etc.)


Types 
There are various types of resistors available to control current. Changeable resistance value is categorized as the variable resistor. Resistance value maintained constantly is categorized as the fixed resistor. 
By application, there are general type resistors, precision resistors, power type resistors, high frequency resistors, high voltage resistors, high resistance resistors…etc. 
By materials, there are wire-wound resistors, thin film resistors, thick film resistors, metal alloy resistor….etc. 
By resistance structure, there are cylindrical resistance, tubular-type resistor, disc-shaped resistor, aluminum shell-type resistors, chip resistors…etc.

Fixed resistors and variable resistor: 
A variable resistor, which adjusts resistance value by sliding contact, controls volume, frequency, brightness…etc. By materials, a variable resistor might be carbon film, metal film, half-closed type, hermetic. Contact point aging and poor contact are common drawbacks.

Fixed resistors are as follows:
(1) Carbon film resistor
no specific features; poor stability; unable to produce precision resistance, resistance range: 5Ω ~ 50MΩ; inexpensive; low-power; requiring medium voltage.

(2) Metal film resistor 
high stability; high precision; thin film; for low-power; low-cost; resistance range: 5Ω ~10MΩ; requiring medium voltage.

(3) Metal-oxide film resistor
moderate stability; thick film; withstand high temperatures; power range: 0.5W~5W; with non-flammable paint; sometimes buried in cement resistor for high power usage; resistance range: 10Ω ~ 200KΩ; requiring medium voltage.

(4) Wire-wound resistor 
high stability; wide range of power: 1W~1000W; for relatively large current but with problems of skin effect; not suitable for high frequency application.

(5) Carbon core resistor
high stability; poor in the application power; no cutting and no resistance in the form of processed without interlayer insulation problems (requiring high voltage).

(6) Jumper wire resistor 
resistance range: 5mΩ~40mΩ; simple low resistance resistor; simple machining but low accuracy; big resistance varies after soldering; skin effect for withstanding large surge

(7) Current sensing resistor (milli-ohm shunt resistor)
resistance range: 0.25mΩ~1Ω; small in sizes; for high current; metal alloy materials for high temperature and surge resistance. PST brand metal alloy current sensing resistors (0~1Ω) are compact (miniature) with low T.C.R. to be applied in wide range of electrical circuits and applications. 

(8) Other resistors
Thermistor: resistance value changes according to temperature
Voltage dependent resistor: resistance value changes according to voltage.
Photoresistor: resistance value changes according to brightness.


Ideal resistor
In an ideal resistor, the resistance value does not vary with voltage or current and will not change due to a sudden change of current as well which is still an unaccomplished ideal in reality. Nowadays design allows resistors to perform relatively small change in resistance at extreme voltage or current or even other environmental factors such as temperature…etc.


Reality 
Each resistor has an upper bound voltage or current which depends on volume of resistor. If the voltage or current is out of application range, the resistance value is changed or even with dramatic changes which might cause damaged resulted from overheating. Most resistors are with rated power; others are with rated current or voltage.

Furthermore, in addition to resistance, a resistor in reality has small amount of inductance or capacitance which makes difference from ideality.