TESTING A CIRCUIT

SECTION THIRTEEN 

                             TESTING A CIRCUIT

Whenever you test a circuit, 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 t hings you need to know.

1. The IMPEDANCE of the circuit at the location you are testing, and

2. The amount of load you are adding to the circuit 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 last section of this eBook covers

High and Low Impedance and understanding impedance is something you need to know. 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 INTEGRATED CIRCUITS   (IC's)

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. You can make your own Logic Probe and learn how to use it from the following link:

LOGIC PROBE with PULSE

This is a very simple transistor circuit to provide HIGH-LOW-PULSE indication for digital circuits. It can be built for less than $5.00 on a piece of matrix board or on a small strip of copper clad board if you are using surface mount components. The probe will detect a HIGH at 3v and thus the project can be used for 3v, 5v and CMOS circuits.

LOGIC PROBE using CD4001 and CD4011

Here is a simple Logic Probe using a single chip. The circuits have been designed for the CD4001 CMOS quad NOR gate and CD4011 CMOS NAND gate.  The output has an active buzzer that produces a beep when the pulse LED illuminates (the buzzer is not a piezo-diaphragm but an active buzzer containing components)

SUPER PROBE MkII has 20 different features including a Logic Probe, capacitance tester, Inductance tester, and more.

SUPER PROBE MkII Circuit

SUPER PROBE MkII

To test an IC, you need a circuit diagram with waveforms. These diagrams will show the signals and are very handy if a CRO (cathode ray Oscilloscope) is used to diagnose the problem. The CRO will reproduce the waveform and prove the circuit is functioning correctly.

A Logic Probe will just show activity and if an output is not producing a "pulse" or "activity," you should check the power to the IC and test the input line.

It is beyond the  scope of this eBook t o explain how to diagnose waveforms, however it is important to know if signals are entering and exiting an IC and a Logic Probe is designed for this.

SIGNAL INJECTOR

This circuit is rich in harmonics and is ideal for testing amplifier circuits. To find a fault in an amplifier, connect the earth clip to the 0v rail and move through each stage, starting at the speaker. An increase in volume should be heard at each preceding stage. This Injector will also go through the IF stages of radios and FM sound sections in TV's

TESTING AUDIO AMPLIFIERS and AUDIO IC's

The Super Probe MII described above has a "noise" function and a tone function that allows you to inject a signal into an audio stage, amplifier (made from discrete components) or an audio chip, and detect the output on a speaker.

Audio stages are very difficult to work-with if you don't have a TONE GENERATOR or SIGNAL INJECTOR.

The signals are very small and not detected by a multimeter.

You can start anywhere in an amplifier and when a tone is heard, you can keep probing until the signal is not present or louder. From this you can work out which way the signal is travelling.

A Signal Injector is very handy for finding shorts and broken wires in switches, plugs, sockets and especially leads to headphones.

You can determine the gain of a stage (amplification) by probing before and after a chip or transistor and listen for the relative increase in volume from the speaker.

You can also use your finger to produce "hum" or "buzz" if a Signal Injector is not available.

Nearly all audio problems are plugs, sockets and cracks in the PC board, but finding them takes a lot of time and skill.

TESTING IC's - also called "CHIPS"

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 recognized or the output ma y go HIGH after a number of clock pulses.

AN ALOGUE CHIPS (also see above)

Analogue chips are AUDIO chips or AMPLIFIER chips.

To test these chips you will need three pieces of test equipment:

1. A multimeter - this can be digital or analogue.

2. A Signal Injector

3. A Mini Bench Amplifier.

The Mini Bench Amplifier is available as a kit.

 

MINI BENCH AMPLIFIER

MINI BENCH AMPLIFIER CIRCUIT

Start by locating the power pin with a multimeter. If the chip is receiving a voltage, you can use the Mini Bench Amplifier to detect an output.

Connect the Ground Lead of the Mini Bench Amplifier to 0v and touch the Probe tip on each of the pins.

You will hear faint audio on the Input pin and very loud audio on the Output pin.

If no input is detected, you can use a Signal Injector to produce a tone. Connect the clip of the Signal Injector to 0v and the probe to the input pin of the amplifier chip. At the same time, connect the Mini Bench Amplifier to the output pin and you will hear a very loud tone.

These pieces of test equipment can also be used to diagnose an amplifier circuit constructed with individual components.

Amplifier circuits using discrete components are very hard to trouble-shoot and these pieces of test equipment make it very easy.

DIGITAL CHIPS

It is always best to have data on the chip you are testing, but if this is not available, you will need three pieces of equipment:

1. A multimeter - this can be digital or analogue.

2. A Logic Probe,

3. A logic Pulser.

Firstly test the chip to see if power is being delivered. This might be anything from 3v3 to 15v.

Place the negative lead of the multimeter on the earth rail of the project - this might be the chassis, or the track around the edge of the board or some point that is obviously 0v.

Try all the pins of the chip and if you get a reading, the chip will have "supply."

Identify pin 1 of the chip by looking for the "cut-out" at the end of the chip and you may find a small dimple below the cut-out (or notch). This is pin 1 and the "power pin" can be directly above or any of the other pins.

Next you need to know if a signal is entering the chip. For this you will need a LOGIC PROBE.

A Logic Probe is connected to the same voltage as the chip, so it will detect a HIGH and illuminate a red LED.

Connect the Logic Probe and touch the tip of the probe on each pin.

You will not know if a signal is an input or output, however if you get two or more active pins, you can assume one is input and the other is output. If none of the pins are active, you can assume the signal is not reaching this IC.

If only one pin is active, you can assume the chip is called a CLOCK (or Clock Generator). This type of chip produces pulses. If more than two pins are active, you can assume the chip is performing its function and unless you can monitor all the pins at the same time, you don't know what is happening.

This is about all you can do without any data on the chip.

If you have data on the chip, you can identify the input(s) and output(s).

A Logic Probe on each of these pins will identify activity.

A Logic Probe has 3 LEDs. Red LED indicates a HIGH, Green indicates a LOW and Orange LED indicates a PULSE (activity).

Some Logic Probes include a piezo and you can hear what is happening, so you don't take your eyes off the probe-tip.

It is important not to let the probe tip slip between the pins and create a short circuit.

LOGIC PULSER

If you have a board or a single chip and want to create activity (clock pulses), you can use a Logic Pulser. This piece of test equipment will produce a stream of pulses that can be injected into the clock-line (clock input) of a chip.

You can then use a Logic Probe at the same time on the outputs to observe the operation of the chip.

You can also use the Mini Bench Amplifier to detect "noise" or activity on the inputs and outputs of digital chips.

This only applies if the frequency is in the audio range such as scanning a keyboard or switches or a display.

This is how to approach servicing/testing in a general way. There are thousands of digital chips and if you want to test a specific chip for its exact performance, you will need to set-up a "test-bed."

REMOTE CONTROLS

There are two types of remote control - Infrared and RF. Infrared is used for short-range, line-of-sight for TV's DVD's etc.

A few faults can be fixed, but anything complex needs a new remote control.

Check the batteries and battery-contacts. See if the IR LED is illuminating by focusing it into a digital camera and looking on the screen for illumination.

The only other things are a sticky button, a worn-out button or a crack in the PC board. Water damage is generally too much work to repair.

RF remote controls for cars, garage doors etc need a second working unit to check the power output.

Here is a simple circuit that can be connected to an analog multimeter to detect the signal strength at a very close range:

To hear the tone from a transmitter, the Mini Bug Detector circuit can be used:

 

Any further investigation requires a circuit diagram so you can work out what is actually being sent from the transmitter. Most of the time, it is a faulty switch, battery or contacts. Make sure the setting is correct on the "dip switches" and use a working unit to compare all you are testing.

TESTING VOLTAGES ON A CIRCUIT

There are basically two different types of circuit.

1.     ANALOGUE CIRCUIT

An analogue circuit can also be called an AUDIO CIRCUIT and the voltages at different points in a circuit can be measured with a multimeter but the changes (the waveforms) will be quite small or changing at a rapid rate and cannot be detected by a multimeter. You need a CRO to "see" the signals or a Signal Injector to inject a waveform into the circuit and hear the result on the circuit's speaker.

2.     DIGITAL CIRCUIT

A digital circuit can also be called a "Computer Circuit" or "Logic Circuit" and some of the voltages can be measured with a multimeter (such as supply voltages) but the "signal lines" will be be changing from HIGH to LOW to HIGH very quickl y and these signals are detected with a Logic Probe.

Here are some circuits with details of how to test the voltages. Most circuits do not show voltages at various different points and we will explain what to expect on each "stage."

A "STAGE"

A stage is a set of components with an input and output. A "stage" can also be called a "Building Block." Sometimes it has a capacitor on the input and one on the output.

This means the stage is completely isolated as far as DC is concerned.

The stage has a supply (a DC supply) and it is producing its own voltages on various points on the "stage." It can only process (amplify) "AC." (signals).

Sometimes the stage can be given a name, such as small-signal amplifier, push-pull amplifier or output.

If the stage has a link or resistor connected to a previous stage, the previous stage will have a "DC effect" on the stage. In other words it will be biasing or controlling the voltages on the stage. The stage may be called a "timer" or "delay" or "DC amplifier."

It is important to break every circuit into sections. This makes testing easy. If you have a capacitor at the input and output, you know all the problems lie within the two capacitors.

In a digital circuit (no capacitors) you need to work on each IC (integrated Circuit) and test the input for activity and all the outputs.

Once you have determined if the circuit is Analogue or Digit al, or a combination of both, you have to look at the rail voltage and work out the size or amplitude of the voltage or waveform.

This is done before making a test, so your predictions are confirmed.

You will need a multimeter (either Digital or Analogue) a

Logic Probe and a Signal Injector (Tone Generator).

An analogue meter has the advantage that it will detect slight fluctuations of voltage at a test-point and its readings are faster than a digital meter. A digital meter will produce an accurate voltage-reading - so you should have both available.