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The measurement of electrical resistance

By analogy with hydraulics, resistance represents flow restriction.

A flow restriction causes a pressure difference across the throttle and allows the flow to be reduced.

An electrical resistance causes a voltage difference across its terminals and reduces the flow of electrons (Intensity).

Electrical resistance is expressed in Ohms (Ω). An ohmmeter can measure it.

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1 - Component

The electric resistor is a dipole composed of a conductive material which opposes the passage of electric current.

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The resistor has no direction of connection, it is not polarized.

It allows for example to lower the voltage of an electrical circuit.

In Fig. A is represented the color code which allows to know the value of the resistors.

The first 3 colored rings of the resistor are the significant digits.

The next ring is the multiplier digit.

The next ring is the resistance tolerance.

The last ring is the temperature coefficient. (optional)

For example, the resistor shown in Fig. A has a value of 120 Ω with a tolerance of +/- 5% and a temperature coefficient of 100 ppm.

Starting from the left:

  • Ring N°1 brown: 1
  • Ring N°2 red: 2
  • Ring N°3 black: 0
  • Ring No. 4 black: * 1 Ω
  • Golden ring N°5: +/- 5%
  • Ring No. 6: 100 ppm

3 - Allocation table

 5% tolerance:

That is, the 120 Ω resistance can vary between 114 Ω and 126 Ω.

Temperature coefficient 100 ppm:

That is, the resistor has a thermal variation of 100ppm/°C (parts per million). The resistance will vary by 100/1,000,000 of its value as soon as the temperature changes by 1°C.

120 * 100 / 1 000 000 = 0.012 Ω /°C

Resistors heat up when an electric current passes through them. They are of different powers to dissipate calories.

Dissipated power:

P = R * I²

P : Power in Watts

R : Resistance in  Ω

I : Intensity in Ampere

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For example :

The 120 Ω resistor is crossed by a current of 0.1 A.

P = R *I²

P=120*0.1²

P = 1.2W.

The resistor used must be able to dissipate 1.2W.

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2- Measurement

It is possible to measure the resistance (R) of an electrical component using an Ohmmeter.

When the electrical component is composed of a winding, the resistance is called Inductance (Z)

A resistor does not cause a phase shift between the voltage and the current, whereas for the inductor (a winding) the voltage leads the current.

For reasons of simplification Z = R

The measurement is made with power off.

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3- Resistors in series

Resistance en serie 1
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Using a multimeter calibrated on Ohm, the equivalent resistance taken at the terminals of the resistors connected in series is added.

 

Re = R1 + R2

Re : Equivalent resistance in  Ω

R1 : Resistance 1 in   Ω

R2 : Resistance 2 in  Ω

Re = 120 + 120

Re = 240 Ω

 

4 - Resistors of the same value online

Resistance en parralele 1
 

Using a multimeter calibrated on Ohm, the equivalent resistance taken at the terminals of the resistors of the same ohmic value connected in parallel are divided.

Re = R / N

Re : Equivalent resistance in   Ω.

A : Resistance in   Ω.

N : Number of resistance.

Re = 120 / 2

Re = 60 Ω

 

5 - Resistors of different value online

Resistance en parralele valeur differente 1
 

Using a multimeter calibrated on Ohm, the equivalent resistance taken across the terminals of the resistors of different ohmic value connected in parallel is lower than the lowest resistance.  

 

1 / Re = 1 / R1 + 1 / R2

Re : Equivalent resistance in  Ω.

R1 : Resistance 1 in  Ω.

R2 : Resistance 2 in  Ω.

 

1/Re = 1/120 + 1/100

1/Re = 0.0083 + 0.01

1/Re = 0.0183

Re = 1/0.0183

Re = 54.64 Ω

 

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