Basic Instrumentation And Measurements - Resistor

Resistor is a 2 poles passive electronic components which apply the Ohmic Principal in an electrical or electronic circuit. Resistor is used :

1. to limit or to control the current to a specific value.
2. to create a warm energy.

Symbol of resistor

Usage

• To set a value of current with a given value of voltage. e.g multiplier , siries resistor
• To set a value of voltage with a given value of current e.g shunt.
• To control the behaviour of the voltage e.g potentiometer, potential divider by connecting 2 or more resistors in siries.
• To control the behaviour of the current e.g current divider by connecting 2 or more resistors in parallel.
• To act as a protector for an IC from high voltage.
• To transform an electrical energy to heat e.g lamp , solder, heater.

General

An electronic component which is used to control the resistance between 2 points is called Potentiometer. It has 3 connector which is connected to a resistor . This resistor is different than the normal one as this one has a slider which make the control of the resistance for each point on the resistor is possible.

Dimensions

Besides the value of resistance there are other dimensions which are related to resistor :

• tolerance of the resistance
• maximum power loss
• temperature coefficient
• electric strength
• parasitic inductivity
• parasitic capacitance
• noise (depend on material , resistance and voltage)
• impuls

Form and Material

Most of the resistors are in cylinderical form and they are coated by ceramic with 2 connectors at both ends. These connectors are put onto the PCB board and will be soldered. The maximum power loss is between 0.1W and 5W.

The resistor in axial form with quadratic cross section normally are for resistor which is coated with wire and filled with Quarz sand. This type of resistor is designed for high power loss.

The resistor in special form i.e in sinuosity which is used as a power or high power resistor.

The benefit of this form is a long wire can be compressed into a small form. It can be normally seen at the heater manganin-ceramic resistor.

Resistor is also manufactured in SMD (Self Mounted Device) version. This kind of resistor is very small i.e 1.2mm - 0.5mm long. At both end of the component there are flat metal plates which act as contacts and this contacts can be soldered directly on the PCB board.

Different type of materials are used to produce resistor depend on their tolerance and the stability of the temperature. Carbon has negative temperature coefficient and it is not so accurate. Metal resistor has a very high accuracy if it is made of alloy which has a very low temperature coefficient. Generally metal has a positive temperature coefficient. Metal resistor is normally used as fuse - to break the circuit when there is overload in the circuit.

Metaloxide resistor is normally used for a very high resistance and high voltage. It is because this type of resistor is very stable against high voltage.

A very small and high load resistor e.g shunt is made of mangan. It is normally used to measure the current e.g shunt and normally it has 2 extra contact (kelvin contact) to avoid error during measurement when the voltage drop at the contact.

All of the resistors can be differentiated by types :

• carbon layer resistor
• metal layer resistor
• metal oxide layer resistor
• cermet layer resistor
• photo resistor
• etc.....

Resistor which is in round shape normally can't be engraved or labelled with numbers. Therefore normally colour code is used to label the resistor. Nowadays even the small SMD resistor is engraved with the laser mark to label the component.

Colourcode of Resistors

The colour code is very important to label the value of each components.Each code represent the electrical value for the resistor.

There are resistors with 4 , 5 and 6 colour rings of colour code.

For resistor which has 4 colour rings , the first 2 colour rings represent the value numbers (refer to the table) , the 3rd ring represent the multiplicator (1=x1 , 2=x10 , 3= x100) and the 4th ring give the value of the tolerance class. Using this method we can have 8640 different values.

For 5 colour rings, the first 3 rings are the value numbers and the 4th ring is the multiplicator and the 5th ring is the tolerance class.

For the 6 colour rings , they are basically same as the 5 colour rings code but additionally the 6th ring represent the temperature coefficientof the resistor.

This is is how the rings are organized in order to avoid mistake while reading the colour code :

• There is some empty space after the tolerance colour ring.
• If read in the wrong direction we can't get the correct value.

Colour code of resistance for 4 rings
Colour	Resistance in Ohm			Tolerance
	1st Ring 1st Number	2nd Ring 2nd Number	3rd Ring Multiplicator	4th Ring
Nothing	—	—	—	±20 %
Silver	—	—	10-2 = 0,01	±10 %
Gold	—	—	10-1 = 0,1	±5 %
Black	—	0	100 = 1	—
Brown	1	1	101 = 10	±1 %
Red	2	2	102 = 100	±2 %
Orange	3	3	103 = 1.000	—
Yellow	4	4	104 = 10.000	—
Green	5	5	105 = 100.000	±0,5 %
Blue	6	6	106 = 1.000.000	±0,25 %
Violet	7	7	107 = 10.000.000	±0,1 %
Grey	8	8	108 = 100.000.000	—
White	9	9	109 = 1.000.000.000	—

Resistor with high accuracy (Metal layer resistor) normally have 5 or 6 rings. For the 5 rings normally the first 3 rings give the value of the resistance. The 4th ring is the multiplicator and the 5th ring is the tolerance. The 6th ring give us the value of the temperature coefficient.

Colour code of resistance for 5 and 6 rings
Colour	Resistance in Ohm				Tolerance	Temp Coefficient
	1st Ring 1st Number	2nd Ring 2nd Number	3rd Ring 3rd Number	4th Ring Multiplicator	5th Ring	6th Ring
Silver				10-2 = 0,01
Gold	—	—		10-1 = 0,1
Black		0	0	100 = 1		200 10-6 K-1
Brown	1	1	1	101 = 10	±1%	100 10-6 K-1
Red	2	2	2	102 = 100	±2%	50 10-6 K-1
Orange	3	3	3	103 = 1.000		15 10-6 K-1
Yellow	4	4	4	104 = 10.000		25 10-6 K-1
Green	5	5	5	105 = 100.000	±0,5%
Blue	6	6	6	106 = 1.000.000	±0,25%	10 10-6 K-1
Violet	7	7	7		±0,1%	5 10-6 K-1
Grey	8	8	8		±0,05%
White	9	9	9

Examples :

• For the ring colour yellow - violet - red - brown the reading is 4700 and the tolerance is ±1 %. Therefor the resistance value is 4.7kOhm and the tolerance is between 4.653 kOhm and 4.747 kOhm.
• For the resistor which have 5 rings ( green,brown , brown , red, brown ) - the reading is 511·10²Ω and the tolerance is 1%. So the value of the resistance is 51.1 kΩ ±1 %.

Calculation

Siries Resistor

Rq = R1 + R2 + .....Rn

The current which flow through the circuit is still the same but the potential between each resistor is different. The total of voltage across each resistor is actually the total voltage for that circuit.

Parallel Resistor

1/Rq = 1/R1 + 1/R2 + .......... 1/Rn

The voltage across each resistor is the same .