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.
Sunday, 3 May 2015
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.
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."
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:
"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.
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:
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.
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