Resistor – a double-ended, passive electronic component. It is a linear component meaning that the voltage drop is directly proportional to the current that flows through it. Resistor is used to reduce or achieve the desired value of the current that flows in the electrical circuit. It is used to work in direct, alternating or pulse current (a type of circuit has a significant impact on its properties). Memory resistor (that is controlled by current) is called memristor.
There are plenty of applications for resistors:
- in amplifiers – as elements of feedback,
- in transistors – to set the operating point,
- in signal-filtering devices – combined with capacitors,
- broadly defined electrical circuits.
Resistor with adjustable electrical resistance is called a potentiometer. It is a triple-ended element used as the voltage divider.
The main parameters that characterize resistor:
- nominal resistance (conductance is often used in Siemens) – given by the manufacturer on the housing element that can be measured using a multimeter,
- accuracy class (tolerance) – possible deviation of the actual value of the resistor from the nominal value (given in percentages),
- power rating – the maximum permissible power that can be spun off as heat from resistor under certain conditions,
- temperature coefficient of resistance (TCR) – it defines resistance variation under the influence of temperature (the smaller the TCR, the more stable resistor is),
- voltage limiting – the maximum value of the voltage item on direct current (or the highest effective value of alternating current), that won’t cause any damage. The value depends on the material from which resistor was made eg. for common low power range resistors it’s 150 to 500 V.
Formulas and issues associated with the element:
R = U / I
I. Ohm’s Law
R– the resistance (Ω – Ohm)
U(V) – the current between the element ends (V– Volt)
I – current (A – Amperes)
P = U * I
II. The power dissipated on the resistor
P – electric power (W – Watts)
After switching equation number one on the form: I = U/R it can be noticed that with determined voltage value, by changing resistor value, the value of the current flowing though the device is changed (and vice versa – in accordance to the “triangle” above). Therefore, we can risk saying that this is an element that is used to process voltage current (and vice versa).
III. Series resistors
In the series connection, the total resistance is the sum of the individual values.
R = R1 + R2 + …Rn
IV. Parallel resistors
For two, parallel connected resistors the total resistance pattern is written as:
R = R1,2 = R1 * R2 / R1 + R2
V. Voltage divider
Voltage divider – two resistors in series connection. It is used to separate the voltage supplied to its input, so its output voltage is part of the input voltage. Input voltage is supplied to the R1 and R2 resistors, while the output voltage is equal to the voltage drop on the resistors R2:
Uwy = Uwe * R2 / R1 + R2