MEASURING RESISTANCE

MEASURING RESISTANCE
Turn a circuit off before measuring resistance.
If any voltage is present, the value of resistance will be incorrect.
In most cases you cannot measure a component while it is in-circuit. This is because
the meter is actually measuring a voltage across a component and calling it a
"resistance." The voltage comes from the battery inside the meter. If any other voltage
is present, the meter will produce a false reading.
If you are measuring the resistance of a component while still "in circuit," (with the

power off) the reading will be lower than the true reading.

Measuring resistance

Do not measure the "Resistance of a Battery"

Internal impedance) is not measured as shown in the diagrams above. It is measured

by creating a current-flow and measuring the voltage across the battery. Placing a

multimeter set to resistance (across a battery) will destroy the meter.

2. Do not try to measure the resistance of any voltage or any "supply."

Resistance is measured in OHMs.

The resistance of a 1cm x 1cm bar, one metre long is 1 ohm.

If the bar is thinner, the resistance is higher. If the bar is longer, the resistance is

higher.

If the material of the bar is changed, the resistance is higher.

When carbon is mixed with other elements, its resistance increases and this knowledge

is used to make RESISTORS.

Resistors have RESISTANCE and the main purpose of a resistor is to reduce the

CURRENT FLOW.

It's a bit like standing on a hose. The flow reduces.

When current flow is reduced, the output voltage is also reduced and that why the

water does not spray up so high. Resistors are simple devices but they produce many

different effects in a circuit.

A resistor of nearly pure carbon may be 1 ohm, but when non-conducting "impurities"

are added, the same-size resistor may be 100 ohms,

1,000 ohms or 1 million ohms.

Circuits use values of less than 1 ohm to more than 22

million ohms.

Resistors are identified on a circuit with numbers and

letters to show the exact value of resistance - such as 1k 2k2 4M7

The letter  (omega - a Greek symbol) is used to identify the word "Ohm."

but this symbol is not available on some word-processors, so the letter "R" is used.

The letter "E" is also sometimes used and both mean "Ohms."

A one-ohm resistor is written "1R" or "1E." It can also be written "1R0" or "1E0."

A resistor of one-tenth of an ohm is written "0R1" or "0E1." The letter takes the

place of the decimal point.

10 ohms = 10R

100 ohms = 100R

1,000 ohms = 1k (k= kilo = one thousand)

10,000 ohms = 10k

100,000 ohms = 100k

1,000,000 ohms = 1M (M = MEG = one million)

The size of a resistor has nothing to do with its resistance. The size determines the

wattage of the resistor - how much heat it can dissipate without getting too hot.

Every resistor is identified by colour bands on the body, but when the resistor is a

surface-mount device, numbers are used and sometimes letters.

You MUST learn the colour code for resistors and the following table shows all the

colours for the most common resistors from 1/10th of an ohm to 22 Meg ohms for

resistors with 5% and 10% tolerance.

If 3rd band is gold, Divide by 10

If 3rd band is silver, Divide by 100

(to get 0.22ohms etc)

resistor color coding table

MEASURING CURRENT

MEASURING CURRENT
You will rarely need to take current measurements, however most multimeters have
DC current ranges such as 0.5mA, 50mA, 500mA and 10Amp (via the extra banana
socket) and some meters have AC current ranges. Measuring the current of a circuit
will tell you a lot of things. If you know the normal current, a high or low current can
let you know if the circuit is overloaded or not fully operational.
Current is always measured when the circuit is working (i.e: with power applied).
It is measured IN SERIES with the circuit or component under test.
The easiest way to measure current is to remove the fuse and take a reading across
the fuse-holder. Or remove one lead of the battery or turn the project off, and
measure across the switch.
If this is not possible, you will need to remove one end of a component and measure
with the two probes in the "opening."
Resistors are the easiest things to desolder, but you may have to cut a track in some
circuits. You have to get an "opening" so that a current reading can be taken.
The following diagrams show how to connect the probes to take a CURRENT reading.
Do not measure the current ACROSS a component as this will create a "short-circuit."
The component is designed to drop a certain voltage and when you place the probes
across this component, you are effectively adding a "link" or "jumper" and the voltage
at the left-side of the component will appear on the right-side. This voltage may be too

high for the circuit being supplied and the result will be damage.

Measuring the current of a globe

MEASURING VOLTAGE

Most of the readings taken with a multimeter will be VOLTAGE readings.
Before taking a reading, you should select the highest range and if the needle does not
move up scale (to the right), you can select another range.
Always switch to the highest range before probing a circuit and keep your fingers away
from the component being tested.
If the meter is Digital, select the highest range or use the auto-ranging feature, by
selecting "V." The meter will automatically produce a result, even if the voltage is AC
or DC.
If the meter is not auto-ranging, you will have to select DC source or AC source.
DC means Direct Current and the voltage is coming from a battery or supply where the voltage is steady and not changing and AC means Alternating Current where the voltage is coming from a
voltage that is rising and falling.
You can measure the voltage at different points in a circuit by connecting the black
probe to chassis. This is the 0v reference and is commonly called "Chassis" or "Earth"
or "Ground" or "0v."
The red lead is called the "measuring lead" or "measuring probe" and it can measure
voltages at any point in a circuit. Sometimes there are "test points" on a circuit and
these are wires or loops designed to hold the tip of the red probe (or a red probe fitted
with a mini clip).
You can also measure voltages ACROSS A COMPONENT. In other words, the reading is
taken in PARALLEL with the component. It may be the voltage across a transistor,
resistor, capacitor, diode or coil. In most cases this voltage will be less than the supply
voltage.
If you are measuring the voltage in a circuit that has a HIGH IMPEDANCE, the reading
will be inaccurate, up to 90% !!!, if you use a cheap analogue meter.
Here's a simple case.
The circuit below consists of two 1M resistors in series. The voltage at the mid point
will be 5v when nothing is connected to the mid point. But if we use a cheap analogue
multimeter set to 10v, the resistance of the meter will be about 100k, if the meter has
a sensitivity of 10k/v and the reading will be incorrect.
Here how it works:
Every meter has a sensitivity. The sensitivity of the meter is the sensitivity of the
movement and is the amount of current required to deflect the needle FULL SCALE.
This current is very small, normally 1/10th of a milliamp and corresponds to a
sensitivity of 10k/volt (or 1/30th mA, for a sensitivity of 30k/v).
If an analogue meter is set to 10v, the internal resistance of the meter will be 100k for
a 10k/v movement.
If this multimeter is used to test the following circuit, the reading will be inaccurate.
The reading should be 5v as show in diagram A.
But the analogue multimeter has an internal resistance of 100k and it creates a circuit
shown in C.
The top 1M and 100k from the meter create a combined PARALLEL resistance of 90k.
This forms a series circuit with the lower 1M and the meter will read less than 1v
If we measure the voltage across the lower 1M, the 100k meter will form a value of
resistance with the lower 1M and it will read less than 1v
If the multimeter is 30k/v, the readings will be 2v. See how easy it is to get a totally
inaccurate reading.

This introduces two new terms:

HIGH IMPEDANCE CIRCUIT and "RESISTORS in SERIES and PARALLEL."

If the reading is taken with a Digital Meter, it will be more accurate as a DMM does not

take any current from the circuit (to activate the meter). In other words it has a very

HIGH input impedance. Most Digital Multimeters have a fixed input resistance

(impedance) of 10M - no matter what scale is selected. That's the reason for choosing

a DMM for high impedance circuits. It also gives a reading that is accurate to about

1%.

MEASURING VOLTAGES IN A CIRCUIT

You can take many voltage-measurements in a circuit. You can measure "across" a

component, or between any point in a circuit and either the positive rail or earth rail

(0v rail). In the following circuit, the 5 most important voltage-measurements are

shown. Voltage "A" is across the electret microphone. It should be between 20mV and

500mV. Voltage "B" should be about 0.6v. Voltage "C" should be about half-rail

voltage. This allows the transistor to amplify both the positive and negative parts of

the waveform. Voltage "D" should be about 1-3v. Voltage "E" should be the battery

voltage of 12v.

MEASURING VOLTAGES IN A CIRCUIT

USING A MULTIMETER

USING A MULTIMETER
Analogue and digital multimeters have either a rotary selector switch or push buttons
to select the appropriate function and range. Some Digital Multimeter (DMMs) are auto
ranging; they automatically select the correct range of voltage, resistance, or current
when doing a test. However you need to select the function.
Before making any measurement you need to know what you are checking. If you are
measuring voltage, select the AC range (10v, 50v, 250v, or 1000v) or DC range
(0.5v, 2.5v, 10v, 50v, 250v, or 1000v). If you are measuring resistance, select the
Ohms range (x1, x10, x100, x1k, x10k). If you are measuring current, select the
appropriate current range DCmA 0.5mA, 50mA, 500mA. Every multimeter is different
however the photo below shows a low cost meter with the basic ranges.

The most important point to remember is this:

You must select a voltage or current range that is bigger or HIGHER than the

maximum expected value, so the needle does not swing across the scale and hit the

"end stop."

If you are using a DMM (Digital Multi Meter), the meter will indicate if the voltage or

current is higher than the selected scale, by showing "OL" - this means "Overload." If

you are measuring resistance such as 1M on the x10 range the "OL" means "Open

Loop" and you will need to change the range. Some meters show "1' on the display

when the measurement is higher than the display will indicate and some flash a set of

digits to show over-voltage or over-current. A "-1" indicates the leads should be

reversed for a "positive reading."

If it is an AUTO RANGING meter, it will automatically produce a reading, otherwise the

selector switch must be changed to another range.

The black "test lead" plugs into the socket marked "-" "Common", or "Com," and the

red "test lead" plugs into meter socket marked "+" or "V-W-mA." The third banana

socket measures HIGH CURRENT and the positive (red lead) plugs into this. You DO

NOT move the negative "-" lead at any time.

The following two photos show the test leads fitted to a digital meter. The probes and

plugs have "guards" surrounding the probe tips and also the plugs so you can measure

high voltages without getting near the voltage-source.

Analogue meters have an "Ohms Adjustment" to allow for the change in voltage of the

battery inside the meter (as it gets old).

MULTIMETERS

MULTIMETERS
There are two types:
DIGITAL and ANALOGUE
A Digital Multimeter has a set of digits on the display and an Analogue Multimeter
has a scale with a pointer (or needle).
You really need both types to cover the number of tests needed for designing and
repair-work. We will discuss how they work, how to use them and some of the
differences between them.

DIGITAL AND ANALOGUE MULTIMETERS

BUYING A MULTIMETER

There are many different types on the market.

The cost is determined by the number of ranges and also the extra features such as

diode tester, buzzer (continuity), transistor tester, high DC current and others.

Since most multimeters are reliable and accurate, buy one with the greatest number of

ranges at the lowest cost.

This article explains the difference between a cheap analogue meter, an expensive

analogue meter and a digital meter. You will then be able to work out which two

meters you should buy.

Multimeters are sometimes called a "meter", a "VOM" (Volts-Ohms-Milliamps or Volt

Ohm Meter) or "multi-tester" or even "a tester" - they are all the same.

Introduction

This Blog shows you how to test electronic components and how you can start electronic repairing, testing of electronic components such as resistors, capacitors, diodes and transistors are widely used in any electronic devices and gadgets.To do this you need “Test Gear.” The best item of Test Gear is a Multimeter It can test almost 90% of all components. Knowing how to conduct a test on this components using a multimeter would give you an idea on how to trouble shoot and repair any electronic equipments at home.