HIGH IMPEDANCE AND LOW IMPEDANCE
SECTION FOURTEEN
Every point in a
circuit has a characteristic called "IMPEDANCE." This has never been discussed before in any text book.
That's why it will be new to you.
In other words,
every point will be "sensitive to outside noise."
An audio
amplifier is a good example. If you put your finger on the active input, it
will produce hum or buzz in
the speaker. This is because it is a HIGH IMPEDANCE line or high impedance section of the
circuit.
The same applies
to every part in a circuit and when you place Test Equipment on a line for testing purposes; the
equipment will "upset" the line. It may be very slight but it can also alter the volt age on
the point CONSIDERABLY.
We have already
mentioned (above) how a cheap multimeter can produce a false reading when measuring across a 1M resistor. That's why you need high impedance test Equipment so you do not
"load" the point you are testing and create an inaccurate reading.
The word Impedance really means resistance, but when
you have surrounding components such as diodes, capacitors, transistors, coils, Integrated
Circuits, supply- voltages and resistors, the combined effect is very difficult to work out
as a "resistance"
and that's why we call it "Impedance."
The term "High
and Low Impedance" is a relative term and does not have any absolute values but we can mention
a few points to help you decide.
In general; the
base of a transistor, FET input of an IC are classified as HIGH IMPEDANCE. The outputs of these devices are
LOW IMPEDANCE. Power rails are LOW IMPEDANCE.
An oscillator
circuit and timing circuit are HIGH IMPEDANCE. A LOAD is low impedance. And it gets tricky: An input can be designed to accept a
low-impedance device (called a transducer or pick-up) and when the device is connected, the circuit
becomes LOW impedance, but the input circuitry is actually high impedance.
The impedance of
a diode or LED is HIGH before the device sees a voltage higher than the junction voltage and then it
becomes LOW Impedance.
Impedance is one
of the most complex topics however it all comes down to testing a circuit without loading it.
That's why test
equipment should have input impedance higher than 1M.
The first
circuit we will investigate is the Mini Bug Detector, shown above and below.
Points on the
circuit have been labeled A, B, C etc:
Point A- The first transistor is "self-biased"
and will have 0.6v on the base. The antenna is connected to a 20 turn coil and you might think the coil will
"short" the signals to earth.
But the coil and
470p capacitor form a circuit that oscillates at a high frequency when the antenna wire picks up stray signals.
The coil and capacitor actually amplify the
signals (see Talking Electronics website: Spy Circuits to see how a TANK CIRCUIT works) and these signals enter the
base of the first transistor.
This is
classified as a HIGH Impedance section because the signals are small and delicate and any loading via test
equipment will kill them. The first transistor amplifies the signals about 70 times and they
appear at Point B.
The signal
passes though a 22n to Point C and the transistor amplifies the signal
about 70 times to point D.
Point C is classified as high impedance as any voltage measurement at this point will
upset the biasing of the stage as a few millivolts change in base-voltage will alter the
voltage on the collector considerably.
Point D is classified as low impedance as any voltage-testing will not alter the voltage
appreciably.
The output of
the second stage passes through a capacitor to the join of two diodes.
These two diodes
are not turned on because the voltage at Point E can never rise above 0.7v as this is the voltage produced by the base-emitter of the
third transistor.
The purpose of
the two diodes is to remove background noise. Background noise is low amplitude waveforms and even though
the transistor is turned on via the 220k, low amplitude signals will not be received. The third
transistor works like this: It cannot be turned ON anymore because any waveform from the 22n will be
"clipped" by the bottom diode and it will never rise above 0.6v.
So, the only
signal to affect the transistor is a negative signal - to turn it OFF.
Firstly we have
to understand the voltage on the 22n. When the second transistor is sitting at mid-rail voltage, the
22n gets charged via the 2k2 and lower diode. When the transistor gets tuned ON, the
collector voltage falls and the left side of the 22n drops.
The right side
of the 22n also drops and when it drops 0.6v, the top diode starts to conduct and when the voltage on the
22n drops more than 0.6v the third transistor starts to turn OFF. This effect is amplified by the
transistor at least 100 times and appears at Point F.
All the voltages
around the two diodes are classified as HIGH
Impedance as any
piece of test equipment will upset the voltage and change the output.
There are some
losses in amplitude of the signal as it passes through the 22n coupling capacitors but the end result is a
very high strength signal at point G.
The 4th transistor drives a 10mH choke and
the mini piezo is effectively a 20n capacitor that detects the "ringing" of
the inductor to produce a very loud output.
The 22n
capacitor on the collector eliminates some of the background noise. The choke and piezo form an oscillatory circuit that can produce volt ages
above 15v, even though the supply is 3v.
The 47n
capacitor at Point J is to keep the supply rails "tight" (to create a LOW Impedance) to allow weak cells to
operate the circuit. The "Power-ON" LED tells you to turn the device off when not
being used and Point L is the power supply - a low impedance line due to the 47u electrolytic.
Testing the Mini Bug Detector
To test the Mini
Bug Detector, you will need a Signal
Injector. Place the Injector on Point G and you will hear a tone. Then go to E, C and A. The tone will increase in volume. If it does not increase, you have
pin-pointed the faulty stage.
The next circuit
is a combination of digital and analogue signals. It is a Logic Probe:
The voltage on a
circuit (to be tested) is detected by the probe at Point A of the circuit above and the
"tip" is classified as "reasonably high impedance" as it
has a 220k resistor between
the tip and 0v rail. The 1M reduces the impedance by about 20% but the inputs of the two inverters
have no effect on the "tip" impedance as they are extremely high input-impedance
devices.
The 1M trim pot
is designed to put a voltage on point B that is slightly higher than mid-rail so the green LED is turned off.
Point A will see a voltage below mid-rail and
point C will be HIGH.
Point C and F are low-impedance outputs.
When the tip of
the probe is connected to a LOW voltage, Point B sees a LOW and
Point F goes LOW
to illuminate the green LED. At the same time it removes the "jamming voltage"
produced by the diode between pin 4 of the 4049 and pin 3 of the 74C14 and the oscillator between points H and J produces a low-tone via the 100k resistor and 22n to indicate a LOW.
When the probe
tip sees a HIGH, a lot more things happen.
Point C goes LOW and turns on the red LED.
At the same time the 100p is in an
uncharged state and the right lead goes LOW. This takes
the left lead LOW as the left lead connects to a HIGH Impedance line and pin 9 goes LOW. This makes point E
HIGH and since the 1u is in an uncharged
state, pin 11 goes HIGH. This makes point G
LOW and the diode between pins 9 and 12
keeps pin 9 LOW and takes over from the pulse from the 100p. The yellow LED is illuminated. The 1u
starts to charge via the 470k and when it is approx half-charged, pin 11 sees a HIGH and point G goes low. This creates the length of pulse for the yellow
LED.
At the same
time, Point L goes LOW because the "jamming
diode" from pin 2 of the 4049 goes low and allows the inverter between point L and N to produce a
tone for the piezo.
In addition, Point I goes HIGH and quickly charges a 1u
electrolytic. This removes the effect of the jamming diode on pin 5 of the 74C14 and a low frequency oscillator
made up of 68k and 1u
between pins 5&6 turns on and off an oscillator between points O and R to get a beep. The mini piezo is driven n bridge mode via the two gates
between points QT and PS.
Point U is a 1u electrolytic to reduce the impedance of the power rail and
Point V is a protection diode
to prevent damage if the probe is connected to the supply around the wrong way.
Testing the Logic Probe You can test the Logic Probe with the simple
Logic Probe with
Pulse project described above. It will let you know if each
point in the circuit is HIGH or LOW. You will also find out the difficulty in testing the
point s that are HIGH Impedance, as the Probe will upset the voltage levels and the reading
may be inaccurate. More circuits will be added here in the future.
SOLDERING
We will study
soldering more in details.
1. TOOLS
2. Soldering
components and methods.
3. Soldering SURFACE MOUNT components
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.
They 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.
THE
END
This is not the
full story to learning about servicing. It is just the beginning.
We have only
covered the simplest tests and shown how 90% of faults can be found by checking voltages, waveforms and
looking for obvious things such as burnt out
components, cracks in PC boards. The author has fixed over 35,000
TV's, radios, stereos, VCRs and all those things that were on the market 30 years ago.
Things have not
changed. It's just that some repairs cost nearly as much as buying a new product and half the customers
opt for dumping a faulty item and buying the
latest "flat screen" version. That is why you
have to get things through the workshop as
fast and as cheaply as possible, to make a living.
If you want any
more devices added to this list, email
Colin
Mitchell or supernaturalsgift@gmail.com
To help with
understanding how a circuit works, we have produced an
E books on that
too: scroll next pages, we cover a whole range of circuits.
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