12. Conductivity probe

12. Conductivity probe

Conductivity tester is a simple, but very important instrument, which is able to test for faults many components like: diodes, transistors, coils, transformers, speakers and headphones, capacitors, switches, jumpers, cables and many other different electronic components. This method is a lot faster and straightforward than it is using some “off the shelf” instrument.
Schematic for this device is on 12.1a. It is called a relaxation audio oscillator. When you connect points A and B using a piece of copper wire, a variable current flows through the transistors as sequences of impulses. This means that immediately upon connecting the points A and B, current level rapidly rises to some destined maximum value, and then drops to zero. For certain amount of time there is no current, after which it rises again rapidly, and whole cycle repeats itself. Since relation of times when current is flowing and when it is not is highly in favor of the later, this kind of current is called the spike impulse current. Collector current of a T2 transistor flows through the speaker which generates sound, whose base frequency could be calculated using this approximate equation.

In our case R=47 kOhm and C=47 nF, which means:

From the equation above, it is clear that varying of the frequency is possible by varying the resistor or capacitor value. Frequency rise is achieved by lowering the resistance or capacitance of the circuit, and vice versa, rising the values of the resistor or capacitor, lowers the oscillator frequency. Active variation of the frequency base is possible by replacing the resistor with a several hundred kiloohm trimmer potentiometer. If such modification of the circuit was needed, special care must be taken not to set the trimmer into it's lowest position since this means zero resistance, and that could burn the transistors. To avoid unnecessary care and further complicating the operation of this straightforward device, low value resistor could be connected to the trimmer in series. This resistor would act as a protection for transistors inside of the circuit since it facilitates a minimum resistance, and thus doesn't leave transistors bare in the frying pan when the trimmer is in it's lowest position.
In this example we used an 1.5V battery for supply, but it is possible to plug this instrument on any battery between 1.5V and 9V.
Current flowing through the component that is being tested is lower than I=U/R, where U is the voltage of the supply battery, and R is the resistance of the resistor in the base circuit. In our example, these values are U=1,5 V i R=47 kW, which means that current flow is I=32 micro amperes, which is very low, so tested component is safe from harm from this device.
Oscillator's printed board design is on 12.2. This is viewed from the copper plated side of the board, components are placed on the other side, so their positions are marked in dotted lines. Component side of the board is on 12.2.
Printed board, battery and the speaker are placed in a small box, as shown on 12.3. Miniature speaker is fixed to the upper pane of the box using two wood screws. It is connected to the circuit board using two threaded isolated wires. Same wires are used for all other connections as well. Battery is connected to the board using these wires, for example. In our example, wires are soldered directly to the poles of the battery, and the board fixed inside of the box using wood screws and two rectangular wooden pads glued to the bottom of the box, leaving just enough space to squeeze the battery in. These are not proper solutions, they are cheap “hacks” used when other options are limited. But these are functional for people who always have their trusty soldering iron at hand. What would be a proper solution? Buying a battery holder (with enough battery slots as needed) or battery clips (for those square 9V batteries) would simplify the process of changing the battery, although this circuit is very low in power consumption. Other thing is plastic or metal mounts for boards, these are pretty cheap and you should keep them at hand in your “junk box” when experimenting with electronics.On the front side of the box, we drilled two holes, one for the switch and the other for wires which hold probes on their ends. Probes are cheap components and come in various shapes and sizes with various purposes in mind. Since we've been applying dirty methods, like soldering the battery, there is no reason why we should back from building our own probes now. Any old marker-pen will do, just slip thicker copper wire through it's center, and sand/grind/cut protruding ends into a pointy tip. It is advisable to make probes in different colors, red and black are dominant standards for distinguishing them. Positive probe (red) is connected to point A, and negative one (black) is connected to the point B. You could use alligator clips instead of probes, for example, this would leave your hands free for other purposes, but for some precise testing of the on-board components, go with the more precise probes we already mentioned.
Give your new instrument the initial self-test (battery might be empty, or some other unexpected thing happened) by connecting the probe tips together. If sound is heard from the speaker, everything is fine and ready for work.
Ok, everything is working, now you want to play with your new toy. Check, for example, conductivity of your own body. Hold probe tips between thumb and index finger of your left and right hand. What you hear is a sound whose level and especially frequency depend on your skin moistness. Wow, now instrument could be used as a very crude an inaccurate lie detector. This probably wouldn't be accepted in a court of law, but may be an interesting game you play on your friends. “Suspect” holds in his/hers hands probes which could be made of a metal pipe for this occasion. Pipe should be wide enough so that a large portion of palm surface is actually in contact with metal. When the suspect starts dodging questions or lying, his palms start sweating more than usual, and the tone produced by our “lie detector” is higher than usual.

12.1 Semiconductors check

To test diodes using this circuit, we fall back to the diode theory of operation: when anode is positive comparing to the cathode (red probe on anode, black on cathode), whole diode acts as a low value resistor, which means that speaker sound is higher than usual. On the other hand, in the opposite direction, sound is lower because in that direction diode acts as a high value resistor. Testing process is shown on 12.4.

DC transistor acts in the same fashion as two connected diodes (11.4a). If both diodes are functional, transistor is functional as well as shown on 12.5. As you can see, probe A is connected to the base, and then probe B is connected first to the emitter, and then to the collector. In both cases, if the transistor is ok, “music” would have been heard. We then switch probe connections, A goes where B was connected to and vice versa, if there is no music now, everything is in order. So, transistor is faulty if speaker remains silent in the first two measurements, or if it “plays” in one of the second two measurements.
FET testing is done in similar fashion as testing the bipolar transistors, which is shown on 12.6.

One principle that is applicable when testing the photo resistors, photo transistors and diodes is NL-NM (or, No Light – No Music). Probe A is connected to the collector of the transistor, or diode's anode or one side of the photo resistor, and the other one is connected to transistor's emitter or diode's cathode or the other resistor's side and some kind of sound should be heard from the speaker. If this continues when the component is shadowed using your palm, everything is in functional order. We displayed graphically the method of testing photo sensitive components on 12.7.

12.2 Checking other components

Many other components may be tested using this instrument. Base rule is: if component is intended to conduct electricity, sound will be heard. This is the case with resistors, coils, transformers, fuses, closed switches. If component doesn't conduct electricity, like capacitors, or open switches, or two copper wires on the circuit board which shouldn't be connected, then music would have not been heard.
When testing different resistors, it is apparent that different resistance values give different output sound. So with some experience using this instrument on various resistors it will be possible to tell the resistance of the resistor in question from only the generated sound. This may be easier and more accurately done using regular ohmmeter on your multimeter, but your nerd level will certainly rise sky high if you are able to tell resistor's value from bare sound.
Components which have coils in them, like different electro motors, headphones, speakers, transformers and such conduct electricity, so absence of sound while testing tells of some coil connection failure. With transformers with several secondary coils there is a possibility to find beginning and the end of each of them. And from the sound frequency one is possible to tell which coil is primary and which is secondary.
Functional capacitor will generate no music. An exception are electrolithic and block capacitors, especially the larger ones. Tone generated by connecting these capacitors to the instrument will change in level and frequency and fade until completely off when capacitor is discharged. Length of playing depends on the capacitance of the component, where higher values give longer sound time, which allows for a crude approximation of the component's capacitance.

11. Checking Components

11. Checking Components

So you've put a circuit together and as far as you know everything appears to be ok, but it doesn't work as expected. Even worse, it refuses to give any signs of life. What do you do? First, check the circuit for mechanical failures, like non-connected wires, broken vias on the board (these are holes on the printed circuit board that have a metal coating down the length of the hole to connect one side of the board to the other), bad battery contacts inside the case, broken pins on a component, cold solder joints, etc.
If this doesn't come up with a result, you should compare values of components with the schematic.
You may have put a component in the wrong place, or read values the wrong way. Maybe you forgot k in front of Ohms. Maybe you connected the supply to the wrong pin of an IC.
The next step is to test each component on the board.
Start troubleshooting by measuring DC voltages at certain points of the board, and comparing these values to the schematic. So, by knowing the operation of the circuit you start the process of elimination to find the “suspect” component.
If there are several “suspects”, and this is not a rare occurrence in complex devices, the testing is divided into groups of components. You start checking in reverse soldering order, this means you start with components last soldered, because those are the most sensitive components on the circuit like integrated circuits, transistors, diodes, etc.
The fastest and simplest method to troubleshoot is to use an “ohm-meter.”
In most cases you don't have an ohm-meter by itself as it is usually aded to an ammeter and voltmeter in one instrument, called AVO meter or multimeter.
The safest and most accurate method is to desolder the component from the board when testing it, because other components could lead to a wrong diagnosis, so you have to be very careful when testing in-circuit.
Ok, you should know something about multimeters now. There are two kinds: analog and digital. Analog ones are items of the past, and since they use a needle to tell you values, it can be difficult determining the right value. Digital meters, on the other hand have a display. You should go for this type, although both come in different sizes and with different ranges. Their price is from several dollars, to several hundreds of dollars for really good professional types.
Two instruments are shown in 11.1.

11.1 Diodes and Transistors

When using an analog instrument to test a diode, the needle will swing almost fully across the scale when the diode is placed in one direction and hardly move when the diode is reversed.
The needle does not measure the resistance of the diode but rather the flow of current in one direction and no current-flow in the other direction.
If the value is equal to or near equal, either low or high in both directions, the diode is faulty, and should be replaced.

Digital instruments have a position on the dial to measure diodes, as shown in 11.1b. When we connect probes to each other, the multimeter should buzz, which signals a short circuit, and display tells 0. When we separate the probes the buzzing stops, and a symbol for open circuit is displayed (this can be either 0L or 1). Now we connect probes to the diode (11.3a). Then we reverse the diode and connect it again (11.3b). If the measured diode was ok, one of the two measurements would have shown a value which represents a minimum voltage that could be conducted through the diode (between 400mV and 800mV), and the anode is the end of the diode which is connected to probe A (red one). The diode is faulty if you hear a buzz (closed circuit) or some value which represents infinity.
Transistors are tested in a similar fashion, since they act as two connected diodes. According to 11.4b, the positive probe is connected to the base, and the negative probe is first connected to the collector and then the emitter. In both cases the resistance should be low. After that, you do the same thing, only with switched probes. The negative probe is connected to the base and you test the collector and emitter with a positive probe.
Both cases should produce a high value on the meter.
When testing PNP transistors, all steps are the same, but the measurements should be opposite: on 11.4a they are high, and on 11.4c they are low.
If you test transistors using a digital instrument, the process remains similar to the one with diodes. Each diode should produce a value between 400mV and 800mV. Many modern digital multimeters have a socket for testing transistors. There is, as displayed on 11.5, a special socket where low and medium power transistors fit. If you need to test high power transistors, thin wires (0.8mm) should be soldered to transistor's pins and then plugged into the socket. As displayed on 11.5, a transistor is plugged into the socket according to its type (PNP or NPN) and the switch with a hFE marking is brought into position. If the transistor works, the display shows a value which represents the current amplification coefficient. If, for example, a transistor is tested, and the display shows 74, this means the collector current is 74 times higher than the base current.

11.2 Transformers and coils

Transformers are tested by measuring the resistance of the copper wire on the primary and secondary. Since the primary has more turns than the secondary, and is wound using a thinner wire, its resistance is higher, and its value is in range of tens of ohms (in high power transformers) to several hundreds of ohms.
Secondary resistance is lower and is in range between several ohms to several tens of ohms, where the principle of inverse relations is still in place, high power means low resistance.
If the multimeter shows an infinite value, it means the coil is either poorly connected or the turns are disconnected at some point.
Coils can be tested in the same way as transformers – through their resistance. All principles remain the same as with transformers. Infinite resistance means an open winding.

11.3 Capacitors

Capacitors should produce an infinite reading on a multimeter. Exceptions are electrolytics and very high value block capacitors. When the positive end of an electrolytic capacitor is connected to the positive probe of an analog instrument, and a negative end to a negative probe, the needle moves slightly and gradually comes back towards infinity. This is proof the capacitor is ok, and the needle's movement is charge being stored in the capacitor. (Even small capacitors get charged while testing.)
Variable capacitors are tested by connecting an ohm-meter to them, and turning the rotor. The needle should point to infinity at all times, because any other value means the plates of the rotor and stator are touching at some point.
There are digital meters that have the ability to measure capacitance, which simplifies the process. With this said, it is worth mentioning that capacitors have considerably wider tolerance than resistors, (about 20%).

11.4 Potentiometers

To test a potentiometer, (pot), or a variable resistor, the process is rather simple – you connect the component to the probes of a meter set to ohms and turn the shaft.
(A “noisy” pot can be repaired using a special spray.)

11.5 Speakers and headphones

When testing speakers, their voice-coil can be between 1.5 and 32 Ohms. The value marked on the speaker is an impedance value and the actual DC resistance will be lower. When measuring a speaker with an analogue meter, you should hear a click when the probes are connected.

10. Other components

10. Other components

The following table covers almost every circuit symbol you will need. This is the English/American version of each symbol. The European version of some symbols is slightly different and are shown further down the page.

L2-8-2007

NOTES
Here are a few notes on the symbols above.

Fuses (10.1a) have single role in a circuit - to detect excess current and protect the device. In most cases the excess current flows when a higher voltage is present but a fuse cannot detect the voltage - it can only detect when a higher current flows. The higher voltage causes the higher current to flow and this triggers the action of "blowing the fuse." Of course, when a component fails, a higher current can flow and this will also "blow the fuse."
Fuses come in all sizes and ratings (current flow) and it is important to know that the size of the wire inside a fuse does not necessarily indicate the current rating.
The wire inside can be made from copper and plated to protect it from oxidizing or it can be a low temperature material that needs to be a larger diameter.
The wire can also be wound in a spiral and formed into a spring. The end of the spring sits in a dob of solder and when the spring heats up, the solder melts and the spring separates from the other end.
This is called a DELAY FUSE.
Other forms of delay fuse consist of a wire joined at the centre by a dob of solder and others are made of low-temperature-melting material.
Some pieces of equipment use expensive fuses and whenever a fuse is damaged, you must decide if the problem is a major or minor fault.
Sometimes a fuse can go open-circuit for no apparent reason. It can "wear-out."
For instance, some equipment takes a very high current when it is turned on and you will see the fuse heat up and stretch and dip in the middle. This causes strain on the fuse and eventually the wire oxidizes to a point where it finally "burns out."
The equipment is not faulty and it is just a matter of replacing the fuse.
Sometimes the fuse completely explodes and the glass is thrown all over the chassis. This indicates a short-circuit in the power supply and most often one or more of the diodes must be replaced.
The fuse can also go off with a "bang" and the inside of the glass is coated with "silver." This also indicates a diode is damaged in the power supply. Generally 2 or 4 diodes are damaged.
If the fuse is damaged beyond recognition, you will not know if it is a delay fuse or a normal fuse.
The current-rating on the end-cap can sometimes help you.
For instance, if a fuse is rated at 4A, you will need to replace it with a 4 amp normal fuse or 3.15 amp slow-blow.
When fuses are rating at 100mA to 250mA, they are very delicate and will not accept the slightest overload.
When replacing this type of fuse, it is necessary to determine if the equipment is drawing a heavy current when turning on or if a fault exists in the power supply. Sometimes the switch can cause the problem if it is not making contact fast enough.
Replace the fuse and watch it as someone else turns on the equipment. If the fuse burns out immediately, a short exists. If the fuse glows red and burns out, the equipment is drawing too much current during turn-on. This may be due to devices you have added to the equipment or operation on a slightly higher voltage. You can try a fuse with a slightly higher rating to see if the fault is fixed.
Never replace a 100mA fuse with a 1 amp fuse. The 1 amp fuse will never "blow" and if the transformer is being overloaded, the transformer will simply "cook."

Lamps (10.1b) Ordinary electric light globes heat a coil of tungsten wire inside a glass bulb that has an inert gas such as argon. The resistance of the filament depends on the temperature it is heated to. It can be ten to twenty times higher than when it is cold.
A neon lamp (10.1c) contains a gas (such as neon) and this gas gives off a glow when a high voltage is applied to two plates. This glow occurs at about 70v to 90v and a resistor must be used in series to prevent the voltage rising higher than required by the lamp. To put this more accurately, the resistance of the neon lamp reduces when it "strikes" and a high current will flow. To limit this current a "current limit" resistor is needed.

VDR (10.1d) The resistance of a VDR depends on the voltage across it. A VDR is also called a VARISTOR. Its resistance is high until a critical value of voltage and the resistance suddenly drops. They are used as voltage protection devices. If they, for example, see a voltage higher than 220V, their resistance decreases and this “soaks” the excess voltage. Their response time is only a few 10's of nanoseconds.

The symbol for a single DC cell is shown in 10.1e.
A Quartz crystal is shown in 10.1f. It is a thin sheet of quarts material between two metal plates and packaged in a metal case. Quartz crystals are commonly used as the reference for an oscillator circuit, such as a clock source in microprocessor designs.
An instrument for measuring current (A) and voltage (V) is shown in 10.1g. This symbol dates from the time when analog instruments with a needle were used. The symbol remains the same, although digital instruments have replaced analog devices.
AC voltage symbol is shown in 10.1h. The shape of the wave is shown in the symbol. It can be sine-wave or saw-tooth or square-wave.
The simplest form of switch device is displayed in 10.1i. Because of the wide range of switches, there are many different types in use. For example, a two pole switch (10.1j) has two operating positions, in one position it connects points 1 and 2, and in the other it connects points 1 and 3.
There are switches with more operating positions. 10.1k is an example of a rotary switch with four positions.

Momentary switches, or push buttons have a built-in spring, which makes the switch conduct only while it is being pressed (your standard doorbell has this kind of switch).
Four diodes in a single case is called a BRIDGE. Two pins are marked with sine waves, used to connect to the AC voltage and two marked with "+" and "-"

RELAY When an electromagnet receives sufficient voltage on points 4 and 5, connection between points 2 and 3 is opened, and at the same time points 3 and 1 are closed. A relay is actually an electromagnetic switch.

Symbols for a receiving and transmitting antenna are shown.
Grounding symbols Grounding and common ground aren't the same thing, but if both exist in a circuit, they are always connected to each other. With electronic devices housed in a metal case, grounding is connected to the metal housing.

Schematic symbols representing logic gates and different digital integrated circuits are shown above. It should be kept in mind that basic logic gates (AND, OR, XOR, Inverter, etc.) aren't manufactured as single standalone components. They are always integrated in groups in an IC, but for the sake of clarity, they are represented as separate blocks. These components require a DC voltage, which may or may not be represented on the schematic. These voltages might be different depending on the internal structure and technology used between different family types. Detailed info on this can be found in the component's datasheet provided by the manufacturer.

10.1 Relays

A relay is an electro mechanical device which is commonly used to connect two different circuits. It can connect a low voltage circuit to a high voltage circuit or a low current circuit to a high current circuit or simply to isolate two circuits.
The simplest relay has one set of contacts (commonly called "change-over" contacts). Inside the relay is a coil (called a solenoid) and when the coil is energised, the centre core of the solenoid becomes magnetised and moves an arm closer to the coil. A "contact" is connected to this arm and the contact touches another contact to complete a circuit. The contacts are labeled "common" for the moving contact, "normally open" and "normally closed." This can be seen in diagram 10.2 a:

A relay can be connected as the collector load of a transistor, as shown on 10.3. When sufficient collector current flows in the transistor, the relay is activated and any device connected to the contacts will be operational.
Since a relay is an electro mechanic component which is consisted of moving parts, it has a limited operational life span, and cannot be used for rapid switching. It would not be very effective using it in a, for example, light show which has frequent switching frequency (several hundreds or thousands times per hour). Each opening and closing of the contact is followed by sparks which would dramatically shorten the life of such device.

Coil values are “input values” or voltage and resistance values at which relay draws the lever and switches. Usual coil voltages are 3V, 5V, 6V, 12V and 24V. They can be found printed on the relay's housing. These are all DC voltages, but there are AC voltage designed relays with 230V/250V. The current taken by the relay depends on the resistance of the coil. The coil resistance can be measured with a multimeter. Current flowing through the coil is calculated using Ohm's law, by dividing the relay's voltage by its resistance. For example a 12v relay has a coil resistance of 300 Ohm, which means the current flow is:
I=U/R=12/300=40mA.

2. Voltage on relay's contacts, also marked on the housing, is the maximum value allowed. Over-voltage will cause sparks inside the relay and possibly damage the contacts.

The maximum current rating for a relay is marked on the housing with all the other information. It is usually higher than 1A.