TESTING COMPONENTS "IN-CIRCUIT"

You can test components while they are IN CIRCUIT, but the surrounding components
will have an effect on the results.
You can get all sorts of "In-Circuit" testers. They are expensive and offer little more
accuracy than a multimeter.
In-Circuit testing with a multimeter can give you the same results as a tester.
All you have to do is turn the project ON and use a multimeter (set to voltage) to
determine the voltage at various points. It is best to have a circuit of the equipment so
you can what to expect at each point.
Only major departures from the expected can be located in this way.
Obviously the first thing to look for is burnt-out components. Then feel components
such as transistors for overheating.
The look for electrolytics that may be dry. Sometimes these have changed colour or
are slightly swollen.
If they are near hot components, they will be dry.
For the cost of a few dollars I change ALL THE ELECTROLYTICS in some pieces of
equipment, as a dry electrolytic is very difficult to detect.
Testing a transistor "in-circuit" is firstly done with the supply ON. That's because it is
quicker.
Measure the voltage between ground and collector.
In most cases you should get a voltage of about half-rail. If it is zero, or close to rail
voltage, you may have a problem.
Turn off the supply and use the multimeter on low-ohms to measure all six resistances
between the leads.
A low resistance in both directions on two leads will indicate a fault.
Resistors almost NEVER go "HIGH." For instance, a 22k will never go to 50k. However
a low-value resistor will "burn-out" and you will read the value of the surrounding
components.
Don't forget, some low-value resistors are designed to burn-out (called fusible
resistors) and anytime you find a damaged low-value resistor, you will need to look for
the associated semiconductor.
You can replace the resistor quickly and turn the circuit ON to see it burn out again.
Alternatively you can trace though the circuit and find the shorted semiconductor.
It's always nice to "see the fault" then "fix the fault."
Sometimes a transistor will only break-down when a voltage is present, or it may be
influenced by other components.
When the piece of equipment is turned OFF, you can test for resistance values. The
main thing you are looking for is "dry joints" and continuity. Dry joints occur around
the termination of transformers and any components that get hot. Rather than wasting
time checking for dry joints, it is better to simply go over the connections with a hot
iron and fresh solder.
You may need to check the continuity of a track (trace) and it may go from one side of
the PC board to the other.
Use a multimeter set to low-ohms and make sure the needle reads "zero-ohms."
It is very dangerous to do any testing on a project using a multimeter set to "amps" or
"milliamps."
You cannot test "current flowing through a component" by placing the probes across a
component. You will simply over-load the rest of the circuit and create a problem.
To find out if current is flowing though a circuit or a low-value resistor, turn the project
ON and measure the voltage either across the component or the voltage on one end
then the other.
A voltage-drop indicates current is flowing.
That's about it for testing "in-circuit." Use the rest of this eBook to help you with
diagnosis.
Don't think an IN-CIRCUIT COMPONENT TESTER is going to find a fault any faster than
a multimeter. They all use a multimeter principle.

TESTING INTEGRATED CIRCUITS (IC's)

An Integrated Circuit is also called a "chip." It might have 8 pins or as many as 40.
Some chips are ANALOGUE. This means the input signal is rising and falling slowly and
the output produces a larger version of the input.
Other chips are classified as DIGITAL and the input starts at 0v and rises to rail voltage
very quickly. The output does exactly the same - it rises and falls very quickly.
You might think the chip performs no function, because the input and output voltage
has the same value, but you will find the chip may have more than one output and the
others only go high after a number of clock-pulses on the input, or the chip may be
outputting when a combination of inputs is recognised or the output may go HIGH
after a number of clock pulses.
Integrated Circuits can be tested with a LOGIC PROBE. A Logic Probe will tell you if a
line is HIGH, LOW or PULSING.
Most logic circuits operate on 5v and a Logic Probe is connected to the 5v supply so
the readings are accurate for the voltages being tested.
A Logic Probe can also be connected to a 12v CMOS circuit.

TESTING A CIRCUIT

Electronic circuits

Whenever you test a electronic circuits, the TEST EQUIPMENT puts "a load" or "a change" on it.
It does not matter if the test equipment is a multimeter, Logic Probe, CRO, Tone
Injector or simply a LED and resistor.
There are two things you need to know.
1. The IMPEDANCE of the electronics circuit at the location you are testing, and
2. The amount of load you are adding to the electronics circuits via the test equipment.
There is also one other hidden factor. The test equipment may be injecting "hum" due
to its leads or the effect of your body at absorbing hum from the surroundings or the
test equipment may be connected to the mains.
These will affect the reading on the test equipment and also any output of the circuit.
Sometimes the test equipment will prevent the circuit from working and sometimes it
will just change the operating conditions slightly. You have to be aware of this.
The point to note here is the fact that the equipment (and the reading) can be upset
by hum and resistance/capacitance effects of test equipment. This is particularly
critical in high impedance and high frequency circuits.

TESTING PIEZO DIAPHRAGMS and PIEZO BUZZERS

There are two types of piezo devices that produce a sound.
They are called PIEZO DIAPHRAGMS and PIEZO BUZZERS.
A piezo diaphragm consists of two metal plates with a ceramic material between. The
ceramic expands and contracts when an alternating voltage is placed on the two plates
and this causes the main plate to "dish" and "bow."
This creates a high-pitched sound. There are no other components inside the case and
it requires an AC voltage of the appropriate frequency to produce a sound.
A piezo buzzer has a transistor and coil enclosed and when supplied with a DC
voltage, the buzzer produces a sound.
Both devices can look exactly the same and the only way to tell them apart is by
connecting a 9v battery. One device may have "+' and "-" on the case to indicate it is
a piezo buzzer, but supplying 9v will make the buzzer produce a sound while the piezo
diaphragm will only produce a "click."

A piezo diaphragm will produce a click

when connected to 9v DC.

A piezo buzzer will produce a tone when

connected to a DC voltage.

TESTING CELLS AND BATTERIES

There is an enormous number of batteries and cells on the market and a number of
"battery testers." Instead of buying a battery tester that may give you a false reading,
here is a method of testing cells that is guaranteed to work.
There are two types of cell: a rechargeable cell and a non rechargeable cell.
The easiest way to test a rechargeable cell is to put a group of them in an appliance
and use them until the appliance "runs down" or fails to work. If you consider the cells
did not last very long, remove them and check the voltage of each cell. The cell or cells
with the lowest voltage will be faulty. You can replace them with new cells or good
cells you have in reserve.
There is no other simple way to test a rechargeable cell.
You cannot test the "current of a cell" by using an ammeter. A rechargeable cell can
deliver 10 amps or more, even when nearly discharged and you cannot determine a
good cell for a faulty cell.
Dry cells are classified as "non-rechargeable" cells.
DRY CELLS and MANGANESE CELLS are the same thing. These produce 1.5v per cell
(manganese means the Manganese Dioxide depolariser inside the cell. All "dry cells"
use manganese dioxide).
ALKALINE CELLS produce between 2 - 10 times more energy than a "dry cell" and
produce 1.5v per cell.
Alkaline cells can fail for no reason at any stage in their life and are not recommended
for emergency situations.
The output voltage of some Alkaline cells can fall to 0.7v or 0.9v for not apparent
reason.
There are lots of other cells including "button cells," hearing-aid cells, air cells, and
they produce from 1.2v to 3v per cell.
Note:
Lithium cells are also called "button cells" and they produce 3v per cell.
Lithium cells are non-rechargeable (they are generally called "button cells") but some
Lithium cells can be recharged. These are Lithium-ion cells and generally have a
voltage of 3.6v. Some Lithium-ion cells look exactly like 3v Lithium cells, so you have
to read the data on the cell before charging.
You cannot test the voltage of a cell and come to any conclusion as to the age of the
cell or how much energy remains. The voltage of a cell is characteristic to the
chemicals used and the actual voltage does not tell you its condition.
Some "dry cells" deliver 1.5v up to the end of their life whereas others drop to about
1.1v very quickly.
Once you know the name of the cell that drops to 1.1v, avoid them as the operation of
the equipment "drops off" very quickly.
However if you have a number of different cells and need to know which ones to keep,
here's the solution:
1. Check the voltage and use those with a voltage above 1.1v
2. Next, select 500mA or 10A range on a meter and place the probes on a cell. For a
AAA or AA cell, the current should be over 500mA and the needle will swing full scale
very quickly.
Keep the testing short as you are short-circuiting the cell but it is the only way to
determine the internal impedance of the cell and this has a lot to do with its stage-ofcharge.
This will give you a cell with a good terminal voltage and a good current capability.
This also applies to button cells, but the maximum current they will deliver will be less.
If you want to get the last of the energy out of a group of cells they can be used in the
following circuits:

Testing an SCR

An SCR can be tested with some multimeters but a minimum current Anode-to-
Cathode is needed to keep the device turned on. Some multimeters do not provide this
amount of current and the SCR Tester circuit above is the best way to test these
devices.
Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail
shorted rather than open).
Measure the anode-to-cathode resistance in both the forward and reverse direction; a
good SCR should measure near infinity in both directions.
Small and medium-size SCRs can also be gated ON with an ohmmeter (on a digital
meter use the Diode Check Function). Forward bias the SCR with the ohmmeter by
connecting the black ( - ) lead to the anode and the red ( + ) lead to the cathode
(because the + of the battery is connected to the negative lead, in most analogue
multimeters). Momentarily touch the gate lead to the anode while the probes are still
touching both leads; this will provide a small positive turn-on voltage to the gate and
the cathode-to-anode resistance reading will drop to a low value. Even after removing
the gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the
anode or cathode will cause the SCR to revert to its non-conducting state.
When making the above test, the meter impedance acts as the SCR load. On larger
SCRs, it may not latch ON because the test current is not above the SCR holding
current.
Using the SCR Tester
Connect an SCR and press Switch2. The lamp should not illuminate. If it illuminates,
the SCR is around the wrong way or it is faulty.
Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will turn ON and
remain ON. Release Sw 2 and press it again. The Lamp or motor will be OFF.

TESTING MOSFETs and FETs

MOSFETs and JFETs are all part of the FET family.
MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
FETs operate exactly the same as a "normal" transistor except they have different
names for the input and output leads and the voltage between the gate and the source
has to between 2v to 5v for the device to turn on fully. A FET requires almost NO
CURRENT into the Gate for it to turn on and when it does, the voltage between drain
and source is very low (only a few mV). This allows them to pass very high currents
without getting hot. There is a point where they start to turn on and the input voltage
must rise higher than this so the FET turns on FULLY and does not get hot.
Field Effect Transistors are difficult to test with a multimeter, but "fortunately" when
a power MOSFET blows, it is completely damaged. All the leads will show a short
circuit. 99% of bad MOSFETs will have GS, GD and DS shorted.
The following symbols show some of the different types of MOSFETs:

Most MOSFET transistors cannot be tested with a multimeter. This due to the fact that

the Gate needs 2v - 5v to turn on the device and this voltage is not present on the

probes of either meter set to any of the ohms ranges.

You need to build the following Test Circuit:

Touching the Gate will increase the voltage on the Gate and the MOSFET will turn on

and illuminate the LED. Removing your finger will turn the LED off.

SIMPLEST TRANSISTOR TEST

The simplest transistor tester uses a 9v battery, 1k resistor and a LED (any colour).
Keep trying a transistor in all different combinations until you get one of the circuits
below. When you push on the two leads, the LED will get brighter.
The transistor will be NPN or PNP and the leads will be identified:

The leads of some transistors will need to be bent so the pins are in the same positions

as shown in the diagrams. This helps you see how the transistor is being turned on.

This works with NPN, PNP transistors and Darlington transistors.

TESTING A TRANSISTOR ON A DIGITAL METER

Testing a transistor with a Digital Meter must be done on the "DIODE" setting as a
digital meter does not deliver a current through the probes on some of the resistance
settings and will not produce an accurate reading.
The "DIODE" setting must be used for diodes and transistors. It should also be called a
"TRANSISTOR" setting.
TESTING AN unknown TRANSISTOR
The first thing you may want to do is test an unknown transistor for COLLECTOR, BASE
AND EMITTER. You also want to perform a test to find out if it is NPN or PNP.
That's what this test will provide.
You need a cheap multimeter called an ANALOGUE METER - a multimeter with a scale
and pointer (needle).
It will measure resistance values (normally used to test resistors) - (you can also test
other components) and Voltage and Current. We use the resistance settings. It may
have ranges such as "x10" "x100" "x1k" "x10"
Look at the resistance scale on the meter. It will be the top scale.
The scale starts at zero on the right and the high values are on the left. This is
opposite to all the other scales.
When the two probes are touched together, the needle swings FULL SCALE and reads
"ZERO." Adjust the pot on the side of the meter to make the pointer read exactly zero.
How to read: "x10" "x100" "x1k" "x10"
Up-scale from the zero mark is "1"
When the needle swings to this position on the "x10" setting, the value is 10 ohms.
When the needle swings to "1" on the "x100" setting, the value is 100 ohms.
When the needle swings to "1" on the "x1k" setting, the value is 1,000 ohms = 1k.
When the needle swings to "1" on the "x10k" setting, the value is 10,000 ohms = 10k.
Use this to work out all the other values on the scale.
Resistance values get very close-together (and very inaccurate) at the high end of the
scale. [This is just a point to note and does not affect testing a transistor.]
Step 1 - FINDING THE BASE and determining NPN or PNP
Get an unknown transistor and test it with a multimeter set to "x10"
Try the 6 combinations and when you have the black probe on a pin and the red probe
touches the other pins and the meter swings nearly full scale, you have an NPN
transistor. The black probe is BASE
If the red probe touches a pin and the black probe produces a swing on the other two
pins, you have a PNP transistor. The red probe is BASE
If the needle swings FULL SCALE or if it swings for more than 2 readings, the transistor

is FAULTY.

Step 2 - FINDING THE COLLECTOR and EMITTER

Set the meter to "x10k."

For an NPN transistor, place the leads on the transistor and when you press hard on
the two leads shown in the diagram below, the needle will swing almost full scale.

For a PNP transistor, set the meter to "x10k" place the leads on the transistor and

when you press hard on the two leads shown in the diagram below, the needle will

swing almost full scale.

TESTING AN OPTO COUPLER

Most multimeters cannot test the LED on the input of an opto-coupler because the
ohms range does not have a voltage high enough to activate the LED with at least
2mA.
You need to set-up the test-circuit shown below with a 1k resistor on the input and
1k5 on the output. When the 1k is connected to 12v, the output LED will illuminate.
The opto-coupler should be removed from circuit to perform this test.

TESTING OF DIODES

Diodes can have 4 different faults.
1. Open circuit in both directions.
2. Low resistance in both directions.
3. Leaky.
4. Breakdown under load.

TESTING A DIODE ON AN ANALOGUE METER
Testing a diode with an Analogue Multimeter can be done on any of the resistance
ranges. [The high resistance range is best - it sometimes has a high voltage battery
for this range but this does not affect our testing]
There are two things you must remember.
1. When the diode is measured in one direction, the needle will not move at all. The
technical term for this is the diode is reverse biased. It will not allow any current to
flow. Thus the needle will not move.
When the diode is connected around the other way, the needle will swing to the right
(move up scale) to about 80% of the scale. This position represents the voltage drop
across the junction of the diode and is NOT a resistance value. If you change the
resistance range, the needle will move to a slightly different position due to the
resistances inside the meter. The technical term for this is the diode is forward
biased. This indicates the diode is not faulty.
The needle will swing to a slightly different position for a "normal diode" compared to a
Schottky diode. This is due to the different junction voltage drops.
However we are only testing the diode at very low voltage and it may break-down
when fitted to a circuit due to a higher voltage being present or due to a high current
flowing.
2. The leads of an Analogue Multimeter have the positive of the battery connected
to the black probe and the readings of a "good diode" are shown in the following two

diagrams:

The diode is REVERSE BIASED in the

diagram above and diodes not conduct.

The diode is FORWARD BIASED in the

diagram above and it conducts

TESTING A DIODE ON A DIGITAL METER

Testing a diode with a Digital Meter must be done on the "DIODE" setting as a digital

meter does not deliver a current through the probes on some of the resistance settings

and will not produce an accurate reading.

The best thing to do with a "suspect" diode is to replace it. This is because a diode has

a number of characteristics that cannot be tested with simple equipment. Some diodes

have a fast recovery for use in high frequency circuits. They conduct very quickly and

turn off very quickly so the waveform is processed accurately and efficiently.

If the diode is replaced with an ordinary diode, it will heat up as does not have the

high-speed characteristic.

Other diodes have a low drop across them and if an ordinary is used, it will heat up.

Most diodes fail by going: SHORT-CIRCUIT. This can be detected by a low resistance

(x1 or x10 Ohms range) in both directions.

A diode can also go OPEN CIRCUIT. To locate this fault, place an identical diode across

the diode being tested.

A leaky diode can be detected by a low reading in one direction and a slight reading

the other direction.

However this type of fault can only be detected when the circuit is working. The output

of the circuit will be low and sometimes the diode heats up (more than normal).

A diode can go open under full load conditions and perform intermittently.

Diodes come in pairs in surface-mount packages and 4 diodes can be found in a

bridge.

They are also available in pairs that look like a 3-leaded transistor.

The line on the end of the body of a diode indicates the cathode and you cannot say

"this is the positive lead." The correct way to describe the leads is to say the "cathode

lead." The other lead is the anode. The cathode is defined as the electrode (or lead)

through which an electric current flows out of a device.

The following diagrams show different types of diodes:

TESTING SWITCHES and RELAYS

Switches and relays have contacts that open and close mechanically and you can test
them for CONTINUITY. However these components can become intermittent due to dirt
or pitting of the surface of the contacts due to arcing as the switch is opened.
It is best to test these items when the operating voltage and current is present as they
quite often fail due to the arcing. A switch can work 49 times then fail on each 50th
operation. The same with a relay. It can fail one time in 50 due to CONTACT WEAR.
If the contacts do not touch each other with a large amount of force and with a large
amount of the metal touching, the current flowing through the contacts will create
HEAT and this will damage the metal and sometimes reduce the pressure holding the
contact together.
This causes more arcing and eventually the switch heats up and starts to burn.
Switches are the biggest causes of fire in electrical equipment and households.
A relay also has a set of contacts that can cause problems.
There are many different types of relays and basically they can be put into two groups.
1. An electromagnetic relay is a switch operated by magnetic force. This force is
generated by current through a coil. The relay opens and closes a set of contacts.
The contacts allow a current to flow and this current can damage the contacts. Connect
5v or 12v to the coil (or 24v) and listen for the "click" of the points closing. Measure
the resistance across the points to see if they are closing.
You really need to put a load on the points to see if they are clean and can carry a
current.
The coil will work in either direction.
If not, the relay is possibly a CMOS relay or Solid State relay.
2. An electronic relay (Solid State Relay) does not have a winding. It works on the
principle of an opto-coupler and uses a LED and Light Activated SCR or Opto-TRIAC to
produce a low resistance on the output. The two pins that energise the relay (the two
input pins) must be connected to 5v (or 12v) around the correct way as the voltage is
driving a LED (with series resistor). The LED illuminates and activates a light-sensitive
device.

MEASURING AND TESTING INDUCTORS

Inductors are measured with an INDUCTANCE METER but the value of some inductors
is very small and some Inductance Meters do not give an accurate reading.
The solution is to measure a larger inductor and note the reading. Now put the two
inductors in SERIES and the values ADD UP - just like resistors in SERIES. This way
you can measure very small inductors. VERY CLEVER!

TESTING COILS, INDUCTORS and YOKES

Coils inductors and yokes are just an extension of a length of wire. The wire may be
wrapped around a core made of iron or ferrite.
It is labeled "L" on a circuit board.
You can test this component for continuity between the ends of the winding and also
make sure there is no continuity between the winding and the core.
The winding can be less than one ohm, or greater than 100 ohms, however a coil of
wire is also called an INDUCTOR and it might look like a very simple component, but it
can operate in a very complex way.
The way it works is a discussion for another eBook. It is important to understand the
turns are insulated but a slight fracture in the insulation can cause two turns to touch
each other and this is called a "SHORTED TURN" or you can say the inductor has
"SHORTED TURNS."
When this happens, the inductor allows the circuit to draw MORE CURRENT. This
causes the fuse to "blow."
The quickest way to check an inductor is to replace it, but if you want to measure the
inductance, you can use an INDUCTANCE METER. You can then compare the
inductance with a known good component.
An inductor with a shorted turn will have a very low or zero inductance, however you
may not be able to detect the fault when it is not working in a circuit as the fault may
be created by a high voltage generated between two of the turns.
Faulty yokes (both horizontal and vertical windings) can cause the picture to reduce in
size and/or bend or produce a single horizontal line.
A TV or monitor screen is the best piece of Test Equipment as it has identified the
fault. It is pointless trying to test the windings further as you will not be able to test
them under full operating conditions.

TESTING FUSES, LEADS AND WIRES

All these components come under the heading TESTING for CONTINUITY. Turn off all

power to the equipment before testing for shorts and continuity. Use the low

resistance "Ohms Scale" or CONTINUITY range on your multimeter. All fuses, leads

and wires should have a low, very low or zero resistance. This proves they are

working.

A BLOWN FUSE

The appearance of a fuse after it has "blown" can tell you a lot about the fault in the

circuit.

If the inside of the glass tube (of the fuse) is totally blackened, the fuse has been

damaged very quickly. This indicates a very high current has passed through the fuse.

Depending on the rating of the fuse, (current rating) you will be able to look for

components that can pass a high current when damaged - such as high power

transistors, FETs, coils, electrolytics. Before re-connecting the supply, you should test

the "SUPPLY RAILS" for resistance. This is done by measuring them on a low OHMs

range in one direction then reverse the leads to see if the resistance is low in the other

direction.

A reading can be very low at the start because electrolytics need time to charge-up

and if the reading gradually increases, the power rail does not have a short. An

overload can occur when the supply voltage rises to nearly full voltage, so you

sometimes have to fit a fuse and see how long it takes to "blow."

If the fuse is just slightly damaged, you will need to read the next part of this eBook,

to see how and why this happens:

FAST AND SLOW BLOW FUSES

There are many different sizes, shapes and ratings of a fuse. They are all current

ratings as a fuse does not have a voltage rating. Some fuses are designed for cars as

they fit into the special fuse holders. A fuse can be designed for 50mA, 100mA,

250mA, 315mA, 500mA, 1Amp, 1.5amp, 2amp, 3amp, 3.15amp 5amp, 10amp,

15amp, 20amp, 25amp, 30amp, 35amp, 50amp and higher.

Some fuses are fast-blow and some are slow-blow.

A "normal" fuse consists of a length of thin wire. Or it may be a loop of wire that is thin

near the middle of the fuse. This is the section that will "burn-out."

A "normal" fuse is a fast-blow fuse. For instance, a 1amp fuse will remain intact when

up to 1.25 amp flows. When a circuit is turned on, it may take 2-3 amps for a very

short period of time and a normal 1 amp fuse will get very hot and the wire will stretch

but not "burn-out." You can see the wire move when the supply turns on.

If the current increases to 2amps, the fuse will still remain intact. It needs about 3

amp to heat up the wire to red-hot and burn out.

If the current increases to 5 amp, the wire VOLATILISES (burns-out) and deposits

carbon-black on the inside of the glass tube.

A slow-blow fuse uses a slightly thicker piece of wire and the fuse is made of two

pieces of wire joined in the middle with a dob of low-temperature solder. Sometimes

one of the pieces of wire is a spring and when the current rises to 2.5 amp, the heat

generated in the wire melts the solder and the two pieces of wire "spring apart."

A slow-blow fuse will allow a higher current-surge to pass through the fuse and the

wire will not heat up and sag.

Thus the fuse is not gradually being damaged and it will remain in a perfect state for a

long period of time.

A fuse does not protect electronic equipment from failing. It acts AFTER the equipment

has failed.

It will then protect a power supply from delivering a high current to a circuit that has

failed.

If a slow-blow fuse has melted the solder, it could be due to a slight overload, slight

weakening of the fuse over a period of time or the current-rating may be too low.

You can try another fuse to see what happens.

You can replace a fast-acting fuse (normal fuse) with a slow blow if the fast-acting fuse

has been replaced a few times due to deterioration when the equipment is turned on.

But you cannot replace a slow-blow fuse with a fast acting fuse as it will be damaged

slightly each time the equipment is turned on and eventually fail.

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.