Component Testing - Transistor

Transistor is a three terminal semiconductor device that used to amplify electric current. This device is said to be one of the most greatest invention of man-kind in electronics because most of electronic gadgets and digital circuitry were composed of transistors. You can see transistor in a simple hand held radio, security system and even computers. There are two general types of transistor in used today - Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET).

fig1. Transistor in different packaging

Bipolar Junction Transistor (BJT)

A Bipolar Junction Transistor (BJT) is an active semiconductor device that is widely used as amplifier and a key component in most of high speed integrated circuit. It is composed of two coupled P-N junction. As shown in fig2, one P-N junction is in between collector and base while the other one is in between emitter and base. It is also a current controlled device because it can delivered a change in output voltage in response to a change in input current.

fig2. A typical representation of Bipolar Junction Transistor (BJT)

Classification of Bipolar Junction Transistor

There are two classification of Bipolar Junction Transistor, NPN and PNP. NPN transistor is formed by introducing a thin region of P-type material between two regions of N-type material. Same thing with PNP transistor, it is formed by introducing a thin region of N-type material between two regions P-type material.

fig3. NPN and PNP transistor

Determining NPN and PNP transistor using Multimeter

Note: We use Red probe as negative (-) and Black probe as positive (+)

1. Set multimeter to minimum resistance range.
2. Connect the positive (+) probe to base terminal and connect negative (-) probe to collector or emitter terminal.
   
3. If the multimeter needle deflects on either connection it is NPN transistor.

To determine PNP transistor reverse the polarity of multimeter probe connection.

Transistor Testing
Note: We use Red probe as negative (-) and Black probe as positive (+). Please set multimeter to minimum resistance. Remove transistor from its circuit.

There are three indications of bad transistor Open, Shorted and Leaky.

Open transistor will give you infinity resistance reading when measured all resistance in base-emitter terminal and base-collector terminal. The needle remains stationary and will not deflect at all.

fig4. base-collector resistance reading for open transistor

fig5. base-emitter resistance reading for open transistor

Shorted transistor will give you zero ohm resistance reading when measured all resistance in base-emitter terminal, base-collector terminal and collector-terminal terminal.

fig6. collector-emitter resistance reading for shorted transistor

fig7. base-collector resistance reading for shorted transistor

fig8. base-emitter resistance reading for shorted transistor

Leaky transistor will give you resistance reading when collector-emitter terminal resistance is measured. Please take note that resistance between emitter and collector should be infinity or needle should not deflect at all because transistor should not conduct at all.

fig9. collector-emitter resistance reading for leaky transistor

As shown in fig10, 11 and 12, good transistor should have the following resistance reading:

fig10. collector-emitter resistance reading for good transistor

fig11. base-emitter resistance reading for good transistor

fig12. base-collector resistance reading for good transistor

For PNP Transistors reverse the polarity of your multimeter test probe.

Component Testing - Diode

Diode is a two terminal semiconductor device that allows current to flow in only one direction. Its terminal was called Anode and Cathode.

fig1. diode terminals Cathode and Anode

DIODE TESTING
Diode testing is very easy if you know some basic things about it such as biasing and meter polarity. As you can see in fig1, when a diode is connected in forward biased the flow of current will allow while on the other hand it oppose the flow of current. This only means that diode should be connected in forward biased so that the flow of current is possible. This biasing plays vital role in diode testing.

fig2. Forward and Reverse Biased

Multimeter polarity is another factor that we need to consider in testing diodes. Most of the analog multimeters have a leds that was reversed in polarity. The black led is positive (+) and the red led is negative (-). In this article we treat the red led as positive (+) and black led as negative (-) to show how diode biasing play its role in diode testing.

To test diodes please follow the steps below:

1. Set your multimeter in low resistance range.
2. Connect the positive probe to anode and negative probe to cathode. The diode will conduct and the meter will display a low resistance.
3. Reverse the connection. The meter needle should not deflects at all because the diode is not conducting.

fig3. Meter display a low resistance value because the diode is conducting

fig4. Meter needle is not deflecting because the diode is not conducting

Indication of Bad Diode

There are two possible indications of bad diode, shorted and open. A Shorted Diode when test using a multimeter will give a zero ohms resistance reading even the diode was connected in both forward and reverse biased.

fig5. Diode is connected in Forward biased but the meter display zero ohms resistance reading

fig6. Diode is connected in Reverse Biased and the meter still displaying zero ohms resistance reading

An Open Diode gives infinity resistance reading even the diode is connected in both Forward and Reverse Biased.

fig7. Diode is connected in Forward biased but the meter display infinity resistance reading

fig8. Normally, when diode is in reverse biased the meter will not deflect at all.

Component Testing - Resistor

Resistors are one of the most common component that you will find in electronic circuitry. The main function of resistor is to limit the flow of electric current. It can also be used in a circuit that requires low current because of its characteristic.

Before testing resistors please make sure that you take it out from the circuit so that you will have the most effective way of testing.

RESISTOR COLOR CODING

Resistor color coding is one of the basic things that we need to know about resistor. If you take a look on a resistor - you can see some color lines on its body. Those lines of colors correspond to its ohmic value and tolerance. Below on fig1 are the lines of colors that you will find in resistor and its value.

fig1. Resistor Color Coding

Pretend that you have a resistor now and let say that the colors around it are Red, Black, Red, Gold. The first two colors, Red and Black, are first and second significant numbers, the third color, red, is multiplier and the last color, gold, is tolerance. Based on fig1, the value of those color are Red = 2, Black = 0, Red = 100 and Gold = 5%. To get the ohmic value we need to combine the value of first and second color then multiply it with third color.

20 x 100 = 2000 or 2K ohms

To get the ohmic range, we need to get the 5% of 2000 or the tolerance value.

2000 x .05 = 100

After getting the tolerance value, add the tolerance value to ohmic value for maximum range and for minimum range subtract the tolerance value from ohmic value.

Max: 2000 + 100 = 2100 ohms
Minimum: 2000 - 100 = 1900 ohms
Range: 1900 to 2100 ohms

This color coding is very important in testing resistor because we will based our test on the computed value specially if we are testing its reliability. If the measured value is outside the ohmic range, your resistor is bad and should be change.

Another way of testing the resistor is checking its continuity. Always remember that a good resistor when measured will always give correct resistance value. If the needle of you multimeter does not deflect at all it only means that you have a bad resistor.

fig2 - Indication of bad resistor.
Meter needle does not deflect at all and remain stationary in infinity position.

Technician's Basic Tools

There are lots of tools that a technician can use on his daily troubleshooting activities. Even a simple paper clip can be use as improvised tool in opening CD-ROM Drives. Below are the list of my favorite tools.


1. Fliers
2. Wire Cutter
3. Screw Drivers
4. Crimping Tool
5. Multimeter


The first four tools that I've said is very common and most of us know how to use it. The biggest challenge among those tools for a newbie technician is Multimeter.

Multimeter, also known as a volt/ohm meter or VOM is a combination of several measurement device in one unit - where you can measure voltage, resistance and current. This tool is very essential in diagnosing electrical and electronic components such as diodes, resistor and transistor. It can also determine whether a circuit is shorted and open. As show below is the example of Multimeter.

Analog Multimeter - courtesy of http://www.thomasnet.com/

Multimeter Basic

Using multimeter is easy as ABC.

A - Adjust meter range to correct settings
B - Be sure to stick the test probe to correct position
C - Check Meter Reading

Below are some tips regarding Voltage, Current and Resistance measurement:

Voltage Measurement

Before checking Voltage rating make sure you identify which type of voltage you will measured. Always remember that there is an Alternating Current(AC) and Direct Current(DC). To measure Voltage make sure you adjust the selector knob in appropriate voltage rating. i.e. If you want to measure 30 Vdc. Set the knob to DC section 50V. Please take note that it is better if you set the meter range one step higher that the actual rating. Another consideration that you need to remember is the meter should be connected in parallel.

Current Measurement

To measure current, Adjust the selector knob to Ammeter Section then connect the meter series to the circuit. Make sure that all current in circuit must pass through the meter and should not alter the behavior of the circuit.

Resistance Measurement

To measure resistance, Adjust the selector knob in the ohmmeter section. Select the appropriate multiplier. The component must be removed from the circuit.

Universal Troubleshooting Technique

Troubleshooting is a divine technique where you need to identify the cause of issue and come out with desirable solution. A good troubleshooter must know his boundaries and should think outside the box. It is a matter of analyzing things with combination of good technical skills. One technique that I always practice in troubleshooting is Universal Troubleshooting Technique. This technique can make your troubleshooting easily because it deals with logical reasoning and exploding complex issue into simplest form. Universal Troubleshooting technique consist of the following steps:

1. Identifying problems.
2. Identifying solutions.
3. Limit your Liabilities.
4. Testing.
5. Follow up.

Identifying problems

On this step you need identify the real caused of issue and need get a clear picture on the things that happen to your device. Most troubleshooter follow the golden rule in identifying problems.

DEVICE - CONNECTION - DRIVER - APPLICATION (D-C-D-A)

This rule will shorten your time exploding complex issue into simplest form. If we follow the DCDA rule we will deal first with the Device. If we test the device and found that it is physically functioning then we will go to the next field which is connection then Driver and then Application. On this way we can identify the real caused of issue easily without wasting our time.

Another way identifying problem is POST or Power ON self Test. POST is one of the most essential part of troubleshooting. This will help you identifying if the issue was cause by the device it self or by certain components that may affect its working condition such as application, connection, computer and other devices.

Identifying Solution

Identifying solution is an art of repairing and resolving issues. Solution should be logical and reasonable enough to resolve issues. I'm not saying that you should always stick your nose to the book. We don't want you to become a bookish troubleshooter. What we want is to become more competitive and more reliable technician. A good troubleshooter must know the logical troubleshooting steps. If we take a look back on DCDA rule, DCDA is one best example of a logical troubleshooting. It follows the step by step troubleshooting approach that leads you to resolve issue at less time. Another example of logical troubleshooting step is Modular Technique. Modular technique is a troubleshooting approach that deals with issue in a manner of module by module or branch by branch. DCDA and Modular technique are similar on many ways because both of them always deals with issue stage by stage, step by step, module by module, branch by branch until the issue will resolve.

i.e.

When you try print from your computer your printer is not responding. On this issue, what do you think will the best approach to start with? It is better if we start first with the printer. Identify first if your printer is working physically or not. if it is working then we go to the next step which connection. If no problem with connection, go to driver then application.

Limit your Liabilities

As troubleshooter we don't need to extend our liabilities - if the issue is more on application focus on application then resolve the issue one at a time. Extending your liabilities will waste your time and sometimes can cause more serious problem. Focus only on the current issue that you need to resolve. If there is another issue that occurred unexpectedly deal with it logically.

Testing

On this step, you will identify if the Troubleshooting approach that you have tried is effective or not. If you test your product and it works - congratulation you resolved the issue. If not then better come up with another solution. If you feel that you already exhausted all the troubleshooting steps that you know - take a break, we are not god, there are still tomorrow. Rest and think for a better solution or analyze more and seek advice from experts.

Follow up

Troubleshooting will not end after you resolved the issue. Sometimes, there is another issue that may occur or the same issue may appear again. Always remember that prevention is better than cure. Maintain your device well and it will prolong its life.

Welcome to my first technical support blog

Welcome to my first technical support blog.

This is the first time that I create a blog concerning technical issue and anything under the sun. I'm hoping that this will be helpful for those people seeking technical help.

Thank you so much and enjoy reading my articles.

--Eric