Useful Tips

Resistor Resistance - Calculation Formula


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Formula for calculating current strength by voltage and resistance:

R is the resistance
U is the voltage
I is the current strength.

You can quickly perform this simple physical operation using our online program. To do this, enter the initial value in the corresponding field and click the button.

This page presents the simplest online calculator for converting currents in terms of power and voltage. Using this calculator, you can determine the current strength in one click if the voltage and resistance are known.

Types of Resistors

A resistor is an inert (passive) circuit element in which the resistance can be either constant or variable. It depends on its design. It is used to regulate current and voltage in circuits, power dissipation and other restrictions. Literal translation from the English word "resistor" - I resist.

Resistors can be classified according to the following criteria:

  • element assignment
  • type of resistance change,
  • manufacturing material
  • view of the conductor in the element,
  • CVC - current-voltage characteristic,
  • mounting method.

Devices are divided into elements of general and special purpose. Special parts have increased characteristics of resistance, frequency, operating voltage or special requirements for accuracy.

The type of change in resistance divides them into constants and variables. Variable resistors are structurally different not only from elements with constant resistance, but also among themselves. They are different in design: there are adjusting and tuning.

Variable-type adjusting elements are designed to change resistance frequently. This is part of the process of the device circuit.

The trimmer type is designed to fine-tune and adjust the circuit during initial startup. After that, the position of the regulator is not changed.

In the manufacture of resistive bodies (work surface), materials such as:

  • graphite mixtures,
  • metal film (oxide) tapes,
  • wire,
  • compositional components.

Integral elements occupy a special place in this series. These are resistors made in the form of a p-n junction, which is a zigzag channel integrated into the chip chip.

Attention! Integrated elements are always characterized by increased non-linearity of their I – V characteristics. Therefore, they are used where the use of other types is not possible.

The type of current-voltage characteristic divides the elements under consideration into linear and nonlinear. A feature of nonlinearity is that the component changes its resistance depending on the following characteristics:

  • voltage (varistors)
  • temperature (thermistors),
  • magnetic field level (magnetoresistors),
  • illumination values ​​(photoresistors),
  • strain coefficient (strain gages).

The nonlinearity of the current-voltage characteristics has expanded the possibilities of their application.

Installation method can be:

When printed wiring, the findings of the part are inserted into the hole on the board, and then soldered to the contact track of the panel. This installation method is automated, and soldering occurs by immersing the pads in a solder bath.

Mounted mounting, for the most part, manual. The findings of the connected parts are first twisted together, then soldered to improve contact. Soldering itself is not designed to withstand mechanical stress.

Integrated installation is carried out in the process of manufacturing chip crystals.

Resistor Parameters

When plotting a graphical designation of a resistance element, some of its parameters are indicated on it.

The main parameters and basic characteristics include:

  • nominal value of resistance
  • temperature coefficient
  • maximum power dissipation
  • permissible operating voltage
  • noise figure
  • relative deviation from the nominal
  • resistance of the element to high temperature and humidity.

In the drawings and diagrams, the resistor is indicated by the letter R, with the application of its serial number.

Formula for calculating resistance and power

Use Ohm's law for a section of the circuit to calculate the resistance of the resistor, the formula is:


  • U is the voltage at the terminals of the element, V,
  • I - amperage in the circuit, A.

This formula is applicable for direct currents. In the case of calculations for alternating current, the impedance of the circuit Rz is taken into account.

Important! The structure of the circuits is not limited to installing only one resistor. Usually there are many, they are interconnected in parallel and in series. To find a common indicator, individual methods and formulas are used.

Serial connection

With this connection, the "output" of one element is connected to the "input" of another, they go sequentially one after another. How to calculate the resistor in this case? You can use the electronic online calculator, you can apply the formula.

The total value will be the sum of the resistances of the components included in the series connection:

Each of them will have the same voltage drop: U1, U2, U3.

Parallel connection

When performing this type of connection, the same terminals are connected in pairs, the formula is:

R = (R1 x R2) / (R1 + R2).

Typically, the resulting value of R is less than the smaller of all the values ​​of the connected elements.

Information. In practice, parallel or serial connection is used when there is no part of the required rating. Elements for such cases are selected of the same power and of the same type, so as not to get a weak link.

Mixed compound

It is possible to calculate the total resistance of mixed compounds using the join rule. First, all parallel and serial connections are selected and equivalent equivalent circuits are composed. They begin to calculate using formulas for each case. From the obtained simpler circuit, parallel and serial links are again distinguished and calculations are again made. Do this until you get the most basic connection or one equivalent element. The calculated result will be the desired one.

Just searching for the resistance value is not enough to apply the part. You need to find out what power the element should be designed for. Otherwise, it will overheat and fail. Powerful parts for surface mounting are best installed on a radiator.

The calculation of the power of the resistor is performed according to the formula:


  • P - power, W,
  • I - current, A,
  • U is the voltage, V,
  • R is the resistance, Ohm.

After determining the power of the resistors according to the formula, components are selected based on the graphic designation in the diagrams.

Voltage divider

The most used ready-made power supplies are designed for output voltages: 9, 12 or 24 volts. At the same time, most electronic circuits and devices use a supply voltage in the range from 3 to 5 V. In this case, there is a need to reduce the value of Upit to the required value. This can be done using a voltage divider, which has many options. The simplest is the resistor divider.

Such voltage dividers are used exclusively in low-power circuits. This is due to their low efficiency. Part of the power of the power supply is dissipated on the divider, turning into heat. These losses are greater, the more you need to reduce the initial voltage. Connecting the load parallel to one arm requires that Rn be much larger than the resistor installed in this arm. Otherwise, the divider will produce unstable power.

With this scheme, the voltage across the arms of the divider is distributed according to the obtained relations between R1 and R2. The value of the resistance does not play a role in this. But it should be remembered that at low values ​​of R1 and R2, both the power at the load and the magnitude of the loss on heating the elements increase.

Attention! Before calculating the exact parameters, you need to remember how to choose resistors. With their equal value, the output voltage is divided in half. If the equality is not respected, it is necessary to remove the divided voltage from the element with a larger nominal value.

Temperature dependence of resistance

The use of resistors as thermometers is due to the almost linear dependence of their resistance on temperature. This applies to those resistors in which wire or metal is used as a resistive material. Dependency formula:

  • α is the temperature coefficient, K-1,
  • R0 is the resistance of the conductor at 00K,
  • t0 is the temperature of the conductor at 00K.

We are talking about the temperature in Kelvin. At temperatures approaching zero according to Kelvin (-273 ° С), when many metals are cooled, R drops abruptly to zero. In this case, we can talk about superconductivity.

Interesting. Metals having good conductivity at normal temperature may not be superconductors at a critical level of this physical quantity. Superconductors in normal condition have greater resistance than traditional current conductors: copper, silver or gold.

When the conductors are heated, a change in resistance occurs mainly due to a change in its specific value and has a linear relationship.

The voltage value provided by the resistor element

The ideal element that turns electricity into another form of energy is called resistive. Electricity can be converted into light, heat or mechanical types. The magnitude of the voltage on such an element depends on the potential difference at the ends of the resistor. This means, the greater the value of its resistance, the greater the value of voltage on it.

Changing a resistor characteristic such as resistance allows you to implement schematic solutions in various branches of radio engineering and electronics. When choosing elements, one should take into account the specific value of this quantity and the change in the current – ​​voltage characteristic under different operating conditions.